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Compact Antenna Test Ranges (CATR) are one of the workhorses of antenna measurements. They have been recently chosen as the preferred method for testing 5G antenna systems. Ideally, a plane wave is generated at the quiet zone (QZ) by the parabolic reflector. The purity of the plane wave is affected by the pattern of the feed and the termination of the reflector edges. The feed pattern must be broad enough to minimize the amplitude taper across the QZ. However, a broad beam antenna will cause direct illumination of the QZ from the feed, creating an interference ripple on the QZ fields. Recently a paper was presented where a CATR was analyzed using a high order MoM approach. This method provides the field distribution across the QZ, not only from the reflector but also from the source spillover. In this paper the absorber fence that is used to minimize the direct illumination of the QZ from the feed is analyzed. Traditional fences are analyzed for different absorber loadings, from a heavily loaded to lighter loaded absorber. In addition to the analysis of the absorber loading, a differently shaped absorber fence is analyzed. The results show that absorber loading has little effect on the QZ metrics. Shaping the fence is far more important. The specially shaped fence shows that the diffraction is reduced resulting in an improved ripple level and phase variation in the QZ.
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Low back-scattering pylons are a typical feature of RCS measurement ranges [1], these can achieve levels between -25 and -45 dBsm [2]. The measurement of the RCS level from a pylon is a difficult task. The Measurement requires adding a low RCS target to hide the top edge of the pylon. In this paper a numerical technique is explored to estimate the RCS of a pylon using a high order basis function method of moments technique (HOBF-MoM). The paper explores a technique to hide the top and bottom features of the pylon using a non-physical lossy material to get the RCS from the main pylon structure. The effects of the structure on the RCS of standardized targets like spheres is also explored.
Copyright 2023 IEEE. Reprinted from EuCAP 2023 Conference.
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Abstract— Recent papers have addressed making far-field measurements at much less than the traditional far-field distance, particularly for 5G MIMO test articles. These papers have focused on main beam measurements only, such as Total Radiated Power (TRP) and have stated that other normal antenna pattern metrics, such side lobe level measurements are not appropriate for this shortened distance. These papers have addressed fixed error levels acceptable for this quasi far-field technique. This paper will present main beam error from two other perspectives, looking at agreement with the previous efforts. In addition, the paper will present a trade study in terms of chamber size, measurement durations and measurement methods between the quasi far-field, compact range, and spherical near-field approaches. This trade study will cover five representative test articles in the C, Ka, and V frequency bands for 5G applications.
View The PaperRecently the author presented two papers on the use of full wave numerical methods for the prediction of anechoic chamber performance. In the first paper [1] the author argued that full wave analysis while accurate is still dependent on the accuracy of the input parameters, one of which may be the material properties of the RF absorber. On the second paper [2] presented the author stated that full wave analysis should be reserved to evaluating the potential effects of defective absorber of location of lights in different areas of the range. In this paper the author compares the full wave analysis of a taper anechoic range with the measured performance of the implementation of said tapered range.
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Abstract — A 2021 AMTA paper introduced a 3D holographic filtering algorithm optimized for the planar near-field (PNF) geometry. This filter has been shown to have an excellent combination of AUT-signal preservation, stray-signal rejection, and processing speed. It requires only the sampling of a conventional PNF measurement, along with a specified 3D boundary surrounding all the AUT’s possible radiating sources.
The 2021 paper suggested some topics for further investigation, specifically the optimal Z spacing through the 3D hologram and the X- and Y-widths of the blanking window’s tapered extension, and those are investigated here. This paper also explores the combination of filtering and probe correction, since the measured convolution of probe and AUT spatial distributions will be wider than that of the AUT by itself. Finally, filter effectiveness is evaluated for different stray-signal locations with respect to the AUT volume.
View The PaperAs wireless technology has evolved, the industry has integrated more “domains of control” to make useful measurements. The power meter can be triggered to measure at will; time is a controlled parameter. The spectrum analyzer integrates both time and frequency control. The SNA incorporates time and frequency control for both source and receiver, but also includes the added dimension of multiple measurement channels. Finally, the VNA includes all these as well as the ability to make coherent (amplitude/phase) measurements. There is one common factor among these measurement devices: They all center on measurement of microwave signals at ports, fixed connection points on the test article.
In keeping with the evolution of wireless technology, AMETEK NSI-MI has recently introduced a new instrument type, the Vector Field Analyzer (VFA). Like the other instruments, the VFA can measure signals at fixed ports, but its unique strength is its ability to make accurate electromagnetic (EM) field measurements. The VFA seamlessly blends multi-channel vector (amplitude/phase) electrical measurements with wide-band agile frequency control, 10-nanosecond precise timing, and convenient integration of complex device control schemes within the measurement flow. More importantly, the VFA precisely coordinates electrical measurements with spatial (position) measurements for complete understanding of three-dimensional EM fields.
View The PaperAbstract — Accurate antenna characterization is important in any wireless communication system. Traditionally, electrically large antenna ranges are not equipped to perform return loss measurements and thus a separate benchtop vector network analyzer (VNA) setup is required for measuring reflection coefficient or VSWR of an antenna under test (AUT).
In this paper, we demonstrate the two-port S-parameter measurement capability of the NSI-MI Vector Field Analyzer™ (VFA) and how it can be used to integrate return loss measurements in an antenna range. We selected three known millimeter-wave components as devices-under-test (DUT) and measured the S-parameter matrix for each. These WR-10 band measurements were made using the VFA with Virginia Diodes VNAX frequency extension modules. Results are compared with Keysight’s N5225A PNA network analyzer using the same set of extension modules for verification.
S-parameter measurements taken on VFA and PNA setups are compared based on three factors: Repeatability, reproducibility, and measurement comparison. The variations between successive measurements are presented in graphical form to compare repeatability of both instrument setups. Error distribution comparison is presented for reproducibility test data to show the difference between independent repeat measurements taken on both instrument setups. Measurement comparison result shows the total difference between independent VFA and PNA measurements taken for each DUT.
View The PaperAbstract — The IEEE Std 1720™ “Recommended Practice for Near-Field Antenna Measurements” is specifically dedicated to near-field (NF) antenna measurements [1]. It therefore complements the IEEE Std 149-2021™ “IEEE Recommended Practice for Antenna Measurements” which describes general antenna measurement procedures [2]. IEEE Std 1720™ was originally approved in 2012 as a completely new standard by the IEEE Standards Association Standards Board (SASB). It is highly relevant for users performing NF antenna measurements but also the design and evaluation of the antenna measurement facilities. After ten successful years, the standard expires in 2022 and will no longer be an active standard under the IEEE SASB. A “minor revision” of the current standard is ongoing. This paper gives an update on the running activities and discusses the suggested changes to the standard.
View The PaperAbstract — Several methods for measuring over-the-air metrics for communication base stations are being used. One is a recent development in measuring at less than the traditional far-field distance. This method, together with compact antenna range and spherical near-field approaches, are discussed in a trade study. Each approach has its advantages and disadvantages, which are also presented here.
View The PaperAbstract — Full wave electromagnetic simulation of a Compact Antenna Test Range (CATR) is not trivial given its electrical size. Typically, the reflector geometry is simulated using asymptotic methods using an assumed feed pattern, while RF absorber and its effects are ignored. A boundary element method of moments (MoM) implementation, using higher-order basis functions (HOBFs) is a good numerical technique for analyzing these ranges since the equations are only solved at the interfaces between different homogeneous regions. There is therefore no need to discretize and solve equations for the fields in the large empty volume portion of the CATR, unlike when using Finite-Difference Time-Domain (FDTD) or Finite Element Methods (FEM). Using HOBFs allows for the mesh size of the discretized CATR geometry to be as large as two wavelengths, reducing the number of unknowns while enabling fast, efficient solutions.
In this paper, a commercial software package that incorporates MoM with HOBFs is used to model a CATR that consists of a blended rolled edge reflector. The results for the reflector and feed model are compared with asymptotic analysis results to show agreement. A realistic feed horn, support structure, and RF absorber is then introduced to the model and its performance is also included to predict field distribution in the CATR test zone. Using this field solution, the Poynting vector is calculated to visualize the flow of energy in the range and from these results proper RF absorber layout can be designed to ensure optimum test zone performance. It is also shown how feed structure absorber treatment impacts CATR test zone performance.
View The PaperA set of Dual-Polarized Antennas with a 3:1 operating bandwidth has been developed for use in near-field ranges as the probe or range antenna and for use as a Compact Antenna Test Range (CATR) feed. Key development parameters of the antenna are: a wideband impedance match to the coaxial feed line, E and H-plane 1 dB beam widths in excess of 30 degrees, -30 dB on axis cross-polarization, minimum polarization tilt and a phase center that varies over a small region near the aperture. To accomplish these design parameters, a family of range antennas has been developed and previously introduced. Two versions of the antenna have been manufactured and tested for performance. A 2-6 GHz version has been developed using traditional machining techniques and a 6-18 GHz version has been produced using additive manufacturing (3D printing) techniques.
These antennas provide proper illumination of the quiet zone for compact ranges used for antenna measurements as well as radar cross section (RCS) measurements. For RCS measurements, an additional requirement for time-based energy storage performance is considered. Energy storage in the feed can result in a pulse spreading or additional copies of the pulse in time, resulting in poor performances of the target characterization. This effect is called ‘ringdown’.
In this paper, we focus on the RCS ringdown performance of the 6-18 GHz antenna produced using additive manufacturing. The measured performance of the antenna will be presented and discussed. Finally, the applicability of the antenna as a CATR feed for RCS measurements will be discussed.
View The PaperEarlier works have shown the benefits of imaging stray signals in a range with planar-scanner data. This paper discusses some additional tools that can be employed for stray-signal identification. Related range diagnostics are presented that employ Fourier spectral and holographic processing of 1D linear scans through the quiet zone. For the special case of a compact range, the interpretation of arrival angles from the paraboloidal reflector surface is explored. Measured data from multiple facilities are presented that were used to locate, quantify, and remedy the unwanted signals.
View The PaperModern antenna range design is often a careful balance of several competing objectives. Some of these design parameters are defined by the antenna under test (AUT), i.e. millimeter wave (Ka-band) test frequencies, frequencyconverting and non-converting AUTs, high-power radiation requirements, pulsed RF requirements, and interleaved transmit and receive (T/R) requirements. Other parameters are driven by the AUT’s application, like requirements for providing accurate pattern, gain, EIRP, and G/T predictions based on the measurement data. Yet other parameters are driven by cost and risk considerations, like the need for all-at-once acquisitions incorporating multi-frequency, multi-port, dual-pol, and multistate measurements. Also included in the “cost and risk” category is the need to collect all these measurements in the least amount of time.
A planar near-field antenna range designed with all these parameters in mind has been realized and is currently operational. This 1 m x 1 m planar near-field range incorporates several novel electrical and mechanical features, and we illustrate these features in terms of their driving requirements and their limitations. Included in our discussion: modular T/R range “front ends,” reconfigurable probe networks, absorber cooling strategies, near-field probes for high-power measurements, interleaved single-port transmit and multi-port receive measurements, and distributed pulse mode range architectures.
View The PaperThere has been much discussion in the last few decades regarding redundancy in conventional near-field sampling, and that redundancy is most pronounced in the planar geometry. There has also been much discussion regarding modal filtering of near-field data to attenuate the effects of stray signals. Both discussions revolve around the limited local spatial bandwidth that can be produced at each probe location when the antenna under test’s (AUT’s) radiating sources are all contained within a known geometric boundary.
This paper discusses a novel filtering technique that exploits the inherent sampling redundancy in conventional planar near-field acquisitions. The filtering is based solely on the known location and shape in the scanner’s coordinate system of a closed 3D boundary around the radiators of interest. The paper describes the algorithm and presents results from both measured and synthesized input. The new filter is also compared to other available filters in terms of speed, attenuation of stray signals, and preservation of AUT signals.
View The PaperPerforming End-to-End testing of satellite payloads on planar near-field test ranges can greatly reduce the cost and real estate required compared to conventional far-field systems. Previous work has shown that this is theoretically possible, with limited test data showing viability. This paper provides additional validation of the technique’s ability to characterize various system-level parameters, including the equivalent isotropically radiated power , group delay, saturating flux density, system noise temperature and the gain vs. frequency response. Details of a new software satellite payload test suite is presented, along with the accompanying simulated payload that was developed for system verification and facility-to-facility comparison.
View The PaperA hemispherical near-field test system that allows for the AUT to remain stationary is described. Typical structural test data is presented to illustrate the applicability of this type of test system for high frequency (mmWave) cases. Digital data rates for a typical AUT are also presented and it is shown to become restrictive in terms of the maximum throughput achievable during testing.
View The PaperTapered chambers use the reflections from the surfaces adjacent to the range antenna to illuminate the quiet zone (QZ). Polyurethane substrate is the preferred and most widely used radio frequency (RF) absorber in these chambers, due to its ability to be cut into complex shapes to conform to the tapered sections. Unfortunately, this type of absorber always presents slight differences in permittivity related to the manufacturing process.
To analyze the effects of the permittivity of the lossy foam on the QZ illumination in a tapered chamber, a series of numerical experiments using a full wave analysis technique are executed. The results are mainly obtained for frequencies under 1 GHz. The upper frequency of the simulation is limited by the electrical size of the problem and by the available information on the material permittivity. However, frequencies below 1 GHz is where the tapered chambers are superior to other methods for indoor antenna measurements.
Magnitude and phase are recorded over a 1.82m diameter spherical QZ to show the effects of the different absorber on the illumination. Results show that a variation on the absorber around the range antenna will deviate the illumination and skew the amplitude taper across the QZ. The amplitude distribution peak can be shifted by as much as 3.5 degrees from boresight. The effect on the phase taper is smaller with a negligible change in phase.
View The PaperA family of 3:1 Dual-Polarized Antennas has been developed for use in near-field ranges as the probe or range antenna and for use as a Compact Antenna Test Range (CATR) feed. Key development parameters of the antenna are: a wideband impedance match to the coaxial feed line, E and H-plane 1 dB beam widths in excess of 30 degrees, -25 dB on axis cross-polarization, minimum polarization tilt and a phase center that varies over a small region near the aperture. To accomplish these design parameters, a family of range antennas has been developed and previously introduced. Two versions of the antenna have been manufactured and tested for performance. A 2-6 GHz version has been developed using traditional machining techniques and a 6-18 GHz version has been produced using additive manufacturing (3D printing) techniques.
In this paper, we focus on the performance of the 6-18 GHz antenna produced using additive manufacturing. The measured performance of the antenna will be presented and compared to previous simulation. The advantages of additive manufacturing for this type antenna will be discussed. Finally, the applicability of the antenna as a CATR feed and its use in near-field scanning will be discussed.
View The PaperCharacterizing the performance of automobile-mounted antennas has been an ongoing and evolving challenge for the antenna measurement community. Today, the automotive test environment poses unique challenges with its diversity and complexity of wireless on-board systems and the large electrical size of the test article. The evolution of cellular technologies over the past decade means that the basic mobile handset has now become a smartphone with significantly increased capability; this exact same trend has been mirrored by the automotive industry where we have witnessed the basic car radio and cassette player evolve into a multi-function infotainment unit. Modern vehicles include a multitude of wireless technologies, including cellular (2G, 3G, LTE), Bluetooth, WiFi, Global Navigation Satellite System (GNSS), collision avoidance radar, and more. Testing the complete vehicle is currently the only method available that certifies the correct mode of operation for each technology (including co-existence and interference) and also assures the manufacturer that the various sub-systems are performing as expected in the presence of all other sub-systems and the vehicle itself.
While modern vehicles now function like large mobile devices, the conventional Over-the-Air (OTA) measurement systems and techniques available for small form factor devices (e.g. mobile phones) are ill-suited to testing such large devices. In this paper, we will highlight some of the unique challenges encountered in the automotive test environment. We will start by looking into existing methods of measuring radiation patterns of automobile-mounted antennas and providing a qualitative assessment of the various techniques with a focus on near-field solutions. A brief description of OTA testing will follow, coupled with an in-depth look into how techniques that are proven for handset type OTA measurements are being translated to automotive measurements. This section will provide a breakdown of key OTA test metrics, the measurement hardware typically required and key assumptions about the device under test. Finally, some performance tradeoffs and challenges associated with designing a multi-purpose antenna/OTA measurement system will be described.
View The PaperThe evolution of cellular communication technologies has been replicated by the automotive industry with modern vehicles being almost universally fitted, as a bare minimum, with a radio system, a cellular communication system and Bluetooth capability. Higher end vehicles have additional capabilities such as WiFi, GNSS, TPMS, smart keyless entry and smart start/stop feature. All these systems are highly integrated as part of the vehicle’s infotainment unit and they must operate satisfactorily in a co-existing manner.
Automotive wireless testing is currently facing several challenging aspects with one such aspect being MIMO OTA (Multiple-Input-Multiple-Output Over-the-Air) testing of the terrestrial cellular communication system of the vehicle. In this paper, we will examine the current approach for MIMO OTA testing in the 4G and 5G cellular environments and discuss various scenarios on how existing techniques can be adapted to support MIMO OTA testing in the automotive industry.
View The PaperBuilding on the theory of spiral near-field acquisitions, the authors present a novel spiral acquisition implemented in a spherical near-field (SNF) chamber for a large automotive application. This new spiral permits the relaxation of certain restrictions associated with the standard spiral. Specifically, it allows us to eliminate extra or redundant rings beyond the poles, allows for greater control of the angular velocity ratio (i.e. gear ratio) between the theta and phi physical positioning axes, and does not require that the theta axis retrace between acquisitions.
In this paper, we describe the new spiral’s motivations, implementation, advantages, and measurement results. We first discuss the new spiral sampling, its mathematical definition, and its comparison to a standard spiral. Next, we describe the practical considerations and implementation of the coordinated motion between theta and phi for spiral sampling over a spherical surface. Next, we present the results showing good pattern agreement between conventional SNF and the new spiral method. We also discuss the reductions in near-field acquisition time and total test time that were achieved using the new spiral.
View The PaperFinancial impacts often drive decisions to repurpose existing ranges instead of procuring new measurement facilities. These existing ranges have fixed geometries (height, width and length) that were set when the range was originally constructed and often are designed for a different purpose. The inability to set the geometry precludes the range designer from using the range geometry to improve measurement performance. Thus, the performance of the range is mostly dependent on the Radio Frequency (RF) absorber and the range antenna directivity. In rectangular-shaped ranges for example, the lateral surfaces, side walls, ceiling and floor, are the critical surfaces to address in RF absorber arrangement.
In this paper, numerical analyses of Chebyshev arrangements as well as dragon tail or tilted absorber are studied. This paper also analyzes the performance of Chebyshev absorber for normal incidence and for oblique incidence along with the proper arrangement of the Chebyshev period. While certainly these have been discussed previously in the literature, this paper consolidates the previous information and illustrates it with numerical examples to help the reader understand the best approach to use when repurposing a range.
View The PaperDriven by economics, it is common to repurpose existing indoor antenna ranges for different applications such as hardware-in-loop (HWiL) testing of systems. If the range was originally intended to have a centered line-of-sight, using it for a different use may create reflected paths with high angles on incidence onto the lateral walls. These reflected paths have angles of incidence onto the absorber that are very large and cause for the absorber to perform poorly. Two different approaches are possible to improve the range performance. One of them is to use a Chebyshev approach. The second approach is to tilt the absorber blocks to change the angle of incidence to the incoming wave. In this paper numerical methods are used to study the difference between the two approaches to see their advantages and disadvantages.
Copyright 2020 IEEE. Reprinted from EuCAP 2020 Conference.
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There have been a number of numerical analyses of RF absorber presented in the literature. These analyses however, tend to focus on the reflectivity of the material and not on the radar cross section (RCS) that it presents. Brumley studied the RCS of RF absorbers for the purpose of estimating the background RCS of anechoic ranges. The study was done empirically, obtaining measurements of the RF absorber and looking at the RCS of different pyramids and wedges, with and without paint. Brumley presents some potential methods to improving the RCS signature of the range, thus reducing the background RCS of the site.
In this paper, the suggestions presented by Brumley are revisited. Specifically, his recommendation for the twisted pyramid configuration which he was unable to measure due to the lack of absorber samples available for use in the test. In addition to the twisted pyramid, Brumley’s approach of inserting smaller pyramids in the valleys of a larger pyramidal arrangement to reduce the edges parallel to the incoming wave are also presented. Different carbon loadings are modeled for the inserted pyramids. One is the standard loading of the inserted pyramid, and the other is the same loading as the main larger pyramidal arrangement such that all the absorber on the wall has the same material properties. Numerical studies are performed using time domain techniques as well as frequency domain techniques. The model is validated by comparing the RCS of a flat square plate with the theoretical solution. The results validate the data and the suggestions presented in and present ways of improving some of the solutions by adjusting the material properties of the absorber.
View The PaperA signal source can introduce phase-measurement errors when its output crosses through internal frequency-band breaks. The source phase lock circuits in this band-break region sometimes report approximate phase lock before complete phase lock occurs. The result of this approximate phase lock is a minor error in the output frequency, which can lead to phasemeasurement errors at the system level. The magnitude of the phase errors depends on the amount of frequency offset and the difference in electrical lengths between the measurement system’s signal and phase-reference paths.
If this behavior were deterministic, then the resulting phase errors might be tolerable. Unfortunately, it was found that the final settling time (measured in many hundreds of milliseconds) was not consistent, depended in part on the two specific frequencies surrounding the band break, became more confused if a second sweep encountered the band break before the first break had settled, and of course changed behavior if the frequencies were sequenced in reverse order or measured one at a time.
The design approach described herein reduced to negligible the phase-measurement errors due to frequency errors in two large multioctave test systems. The approach relies on managing range transmission line lengths so that propagation time is sufficiently equal among the various signal and reference paths. Measured data are presented that show the advantage of the optimized system design.
View The PaperGraphical representations of the polarization state of an antenna or an electromagnetic wave propagating through space are useful tools to supplement rigorous mathematical analyses. One such example, the polarization ellipse, is frequently used in combination with the mathematical development of polarization theory.
The Poincaré sphere is another graphical representation but is much less widely used. Since each possible polarization state appears as a point on the surface of the sphere, it has limited value in representing a single polarization state. However, it can be quite useful for visualizing the relationships between multiple polarization states.
In this paper, we show a different way of presenting the Poincaré sphere using a Mercator projection and elliptical parameters. We also describe a tool that implements this technique and provides a real-time display of polarization state as a function of frequency.
Copyright 2020 IEEE. Reprinted from EuCAP 2020 Conference.
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Spherical Near-Field (SNF) systems using a swing arm gantry configuration have been the “go to” solution for automotive measurement systems. Recent advances in the automotive industry have warranted a need for SNF systems with high mechanical positioning accuracy supporting measurements up to 40 GHz and beyond. This paper presents the design and implementation of a new swing arm gantry positioner having an 8-meter radius and a radial axis to support high frequency SNF measurements.
We first define the relation of the gantry axis to the global coordinate system and discuss primary sources of error. Next, a robust mechanical design is presented including design considerations and implementation. We then present errors measured using a tracking laser interferometer for probe position through the range of gantry axis travel. Static corrections for probe positioning errors are implemented in the control system using the radial axis. The resultant residual error for the swing arm gantry is then shown to have the accuracy required for high frequency SNF measurements.
View The PaperThe authors have developed a new family of dualpolarized compact range feeds with a bandwidth ratio of 3:1. These feeds are also suitable for use as near-field probes.
Copyright 2019 IEEE. Reprinted from 2019 IEEE AP-S Symposium.
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Indoor antenna ranges must have the walls, floor and ceiling treated with RF absorber. Pyramidal absorber is shaped to create an impedance transform from free space to that of the absorbing material. The pyramidal shape provides this very effectively for normal incidence, but performance typically gets worse as the angle of incidence deviates from normal. Unfortunately, it is difficult to measure reflectivity at large oblique angles because of difficulty differentiating the reflected signal from that of the direct path. In this paper, a novel approach for performing such measurements is introduced. Preliminary measurements are compared to RF simulations. The comparison appears to indicate that this approach is a valid way to perform RF absorber measurements at wide angles.
Copyright 2019 IEEE. Reprinted from 2019 IEEE AP-S Symposium.
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Accurately measuring the polarization of an antenna is a topic that has garnered much interest over many years. Methods abound including phase-referenced measurements using two orthogonal polarizations, phase-less measurements using two or three pairs of orthogonal polarizations, spinning linear probe measurements, and the rigorous three-antenna polarization method. In spite of the many publications on the topic, polarization measurements are very challenging and can easily lead to confusion, particularly in accurately determining the sense of polarization.
In this paper, we describe a method of accurately and rapidly measuring the polarization of an antenna with the aid of a multi-channel measurement receiver and a dual-polarized probe. The method acquires phase-referenced measurements of two orthogonal polarizations. To enable such measurements, we describe a methodology for calibrating the probe. We also describe a tool for automating the polarization measurement and display of the polarization state. By automating the process, it is hoped that the common challenges and confusions associated with polarization measurements may be largely obviated.
View The PaperThis paper provides an overview of a planar near-field test methodology for measuring typical system level characteristics of transceiver payloads. Measuring parameters such antenna gain, equivalent isotropic radiated power, saturating flux density, group delay and channel frequency response is the objective. We describe how transfer functions are derived for the antennas in question, allowing one to compensate for the fact that measurements are being performed in the near-field of both uplink and downlink antennas. Practical implementation aspects like near-field probe selection, probe positioning and RF sub-system modification are addressed. We also present a concept simulated payload, since this is critical to system verification and facility-to-facility comparison.
Copyright 2019 EurAAP. Reprinted from EuCAP 2019
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The IEEE Standard 149, Standard Test Procedures for Antennas, has not been revised since 1979. Over the years the Standard was reaffirmed, that is, its validity was re-established by the IEEE APS Standards Committee, without any changes. Recently however, the IEEE Standards Association stopped the practice of reaffirming standards. This change in policy by the IEEE has been the “medicine” that this Standard needed. A working group was organized and a project authorization request (PAR) was approved by IEEE for the document to be updated. In this paper, the expected changes to the document are described and commented. The main change is to convert the Standard to a recommended practice document. Additionally, some new techniques to measure antennas, such as the use of reverberation chambers for efficiency measurements and more information on compact ranges, is discussed. Other topics inserted are more guidance on indoor ranges and an updated section on instrumentation. Most importantly, a discussion on uncertainty is included. The result will be a very useful document for those designing and evaluating antenna test facilities, and those performing the antenna measurements.
View The PaperTechniques for measuring G/T have been previously presented at AMTA; however, there are very few papers that discuss how to measure G/T in a near-field antenna range. One recent paper discussed such a method and gave a brief description within the larger context of satellite payload testing [1]. The paper’s treatment of G/T was necessarily brief and gives rise to several questions in relation to the proposed method. Other papers that treated this topic required the antenna aperture to be separable from the back-end electronics, which may not be possible in all cases [2-3]. In this paper, we discuss in great detail a slightly modified version of the G/T measurement method presented in [1]. A signal and noise power diagram is presented that can be useful for understanding how system signal-to-noise ratio (SNR) relates to G/T, and a few common misconceptions concerning the topic of G/T are addressed. The CW-Ambient technique for computing G/T of a Unit Under Test (UUT) from measurements in a planar near-field system is described in detail, and a list of assumptions inherent to the CW-Ambient technique is presented. Finally, the validity of the CW-Ambient technique is assessed by analyzing measured data collected from a separable UUT.
View The PaperA robotic near-field antenna measurement system allowing for acquisition over non-canonical measurement surfaces is presented. The robot consists of a six-axis robotic arm and a seventh axis rotary positioner and the created acquisition surface is parametrically reconfigurable. The near-field to far-field transformation required is also described. The success of the technique is demonstrated through measured results, compared to canonical measurement data.
View The PaperThe significant measurement standards in the antenna measurement community all present suggested error analysis strategies and recommendations. However, many of the factors in these analyses are static in nature in that they do not vary with antenna pattern signal level or they deal with specific points in the pattern, such as realized gain, side lobe magnitude error or a derived metric such as on-axis cross polarization. In addition, many of the constituent factors of the error methods are the result of analyses or special purpose data collections that may not be available for periodic measurement. The objective of this paper is to use only a few significant factors to analyze the error bounds in both magnitude and phase for a given antenna pattern, for all levels of the pattern. Most of the standards metrics are errors of amplitude. However, interest is increasing in determining phase errors and, hence, this methodology includes phase error analysis for all factors.
View The PaperThe traditional characterization of the quiet zone for a CATR is to perform field probe scans perpendicular to the range axis at one or more depths of the quiet zone, usually front, middle and back. There is usually no attempt to compare the peak signals across the field probe scans. In recent years, users of CATRs have been using these devices at lower and lower frequencies, sometimes below the lowest frequency that provides the specified performance for the CATR. It is recognized that as a CATR is used at lower and lower frequencies compared to its optics, the quiet zone quality will degrade. The purpose of this study was to create a quiet zone depth variation model to characterize the variation, particularly for low frequencies. The model was to treat the CATR as an antenna aperture and apply a power density versus distance model. It is well known that the extreme near field of an aperture is oscillatory at distances up to approximately 10% of the far-field distance, at which point the power density begins to follow the Fraunhofer approximation. The optics of a CATR place the quiet zone well within the oscillatory zone, indicating that the field will vary through the depth of the quiet zone. This variation will decrease with increasing frequency as the far-field distance for the CATR increases with frequency. The model has been compared to a simulation in GRASP and experimental data collected on a CATR.
View The PaperEffective spherical near-field (SNF) test time reduction approaches are presented for wireless base station antennas that have widely differing beamwidths in the azimuth and elevation planes. The geometry of these antennas allows for the optimization of the SNF angular sampling density by allowing a lower sampling density along one of the acquisition axes. This is validated experimentally and shown to reduce the SNF test time significantly without degrading the measurement accuracy.
Copyright 2019 IEEE. Reprinted from IEEE Antennas and Wireless Propagation Letter, Vol. 18, No. 1, January 2019
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Pyramidal RF absorber, widely used in indoor antenna ranges, is designed to minimize reflectivity by creating an impedance transform from free space to the impedance of the absorber material. The pyramidal shape provides this transition quite well at normal incidence. It has been shown in [1] that pyramidal RF absorber performs very well up to angles of incidence of about 45 degrees off-normal, but at wider angles of incidence, the performance degrades significantly. Unfortunately, it is very difficult to perform RF absorber measurements at large oblique incidence angles. In such measurements, the reflected path and the direct path between the antennas are very close in length, making it difficult to use time-domain gating techniques to eliminate the direct coupling.
In this paper, a novel approach for performing oblique RF absorber measurements is introduced based on spectral domain transformations. Preliminary measurements using this technique have been compared to RF simulations. Results appear to indicate that this approach is a valid way to perform RF absorber reflectivity measurements at highly oblique incidence angles.
View The PaperA new over-the-air (OTA) metric called “spherical coverage” is being discussed in 3GPP. The concept is to test the ability of a device to reliably form beams in any direction, offering connectivity in any orientation and polarization. In this paper, we analyze the effectiveness of various test environments for testing spherical coverage at millimeter-wave frequencies for 5G devices.
Copyright 2019 EurAAP. Reprinted from EuCAP 2019
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The major drawback of Spherical Near-Field (SNF) measurements is the comparatively long measurement time, since the scanning of a whole sphere enclosing an Antenna Under Test (AUT) is required to calculate the Spherical Mode Coefficients (SMCs) required for the computation of the far field. Since the SMCs prove to be sparse under certain conditions, efforts have been made to apply compressed-sensing techniques to reduce the measurement time by acquiring a smaller number of sampling points. These approaches have been successfully tested in simulation using classically acquired measured data. This decouples the measurements from practical problems, such as basis mismatch due to the finite precision of the mechanical positioner and environment effects. In this paper, results from a sparse data acquisition performed with a physical system are reported. To decouple the error introduced by the approach itself from the error introduced by non-idealities in the measurement system, an AUT is measured using both traditional near-field sampling and compressed near-field sampling. The classically acquired data is used both as reference and as source to simulate a synthetic compressed measurement. The effects introduced by real considerations are calculated by comparison between the synthetic compressed measurement and the acquired one, while the error of both is evaluated by comparison to the reference measurement. The results further demonstrate the viability of this method to accelerate SNF measurements and pave the way for further research.
View The PaperAbstract — The last published version of the IEEE Std 1128 is the 1998 edition. It is titled “Recommended Practice for RF Absorber Evaluation in the Range of 30 MHz to 5 GHz”. Over the years, the document has been used widely for absorber evaluations in electromagnetic compatibility (EMC) applications as well as in antenna and microwave measurement applications. Besides the obvious frequency range which needs to be expanded to satisfy today’s applications, several areas are in need of an update. The proposed document will change the upper frequency limit to 40 GHz (with provisions in the document to potentially extend above 40 GHz based on test methods). Measurement uncertainties were not discussed in the IEEE Std. 1128 – 1998. In the new edition, measurement instrumentation and test methods are expected to be updated with guidance on estimating measurement uncertainties. In the proposed document, a section on absorber evaluations for high power applications is planned, and fire properties and test methods will be included.
View The PaperRadar scattering is typically represented as the RCS of the test object. The term RCS evolved from the basic metric for radar scattering: the ratio of the power scattered from an object in units of power per solid angle (steradians) normalized to the plane-wave illumination in units of power per unit area. The RCS is thus given in units of area (or effective cross-sectional area of the target, hence the name). Note that the RCS of the test object is a property of the test object alone; it is neither a function of the radar system nor the distance between the radar and the test object, if the object is in the far field. Because the RCS of a target can have large amplitude variation in frequency and angle, it is expressed in units of decibels with respect to a square meter and is abbreviated as dBsm (sometimes DBSM or dBm2). In terms of this definition, the RCS of a radar target is a scalar ratio of powers. If the effects of polarization and phase are included, the scattering can be expressed as a complex polarimetric scattering (CPS) matrix. The measurement of the RCS of a test object requires the test object to be illuminated by an electromagnetic plane wave and the resultant scattered signal to be observed in the far field. After calibration, this process yields the RCS of the test object in units of area, or the full scattering matrix as a set of complex scattering coefficients.
This paper describes the planned upgrades to the old IEEE Std 1502™-2007 IEEE Recommended Practice for Radar Cross- Section Test Procedures [1]. The new standard will reflect the recent improvements in numerical tools, measurement technology and uncertainty estimates in the past decade.
View The PaperAs 5G systems are developed and deployed, the RF devices comprising these networks require various types of tests at multiple stages of the design and manufacturing processes. The use of millimeter-wave frequencies and massive MIMO, a combination of technologies intended to ensure sufficient bandwidth and SNR to support massive data throughput, is leading to unprecedented levels of integration of antenna arrays and transceivers. Testing these highly integrated devices is becoming increasingly complex and challenging. In this paper, we investigate various test environments for 5G over-the-air (OTA) testing including far-field, compact range, and near-field chambers. We examine the advantages and disadvantages of each for measuring various over-the-air (OTA) test metrics. This paper offers a high-level trade study by broadly analyzing cost, path loss, and applicability of each environment to different types of OTA tests.
Copyright 2019 EurAAP. Reprinted from EuCAP 2019
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Abstract — The IEEE Standards Association Standards Board (IEEE-SASB) approved the IEEE Std 1720™ “Recommended Practice for Near-Field Antenna Measurements” in 2012. More than forty dedicated people from industry, academia and other institutions contributed to the creation of this new document. The main motivation for a new standard dedicated to near-field measurements was to complement the existing IEEE Std 149-1979™ “Test Procedures for Antennas”.
According to the IEEE-SA policies, the existing standard IEEE Std 1720-2012™ is approaching expiration in 2022. A working group of the APS Standard Committee has been formed to review the current document. Most of the current standard is still relevant and useful for individuals designing and evaluating near-field antenna measurement facilities and other people involved in antenna measurements. However, the standard needs update and renewal in areas in which new developments and technologies have matured. This paper gives an overview of the current standards and discusses the suggested potential changes.
View The PaperNSI-MI Technologies has developed a dual polarized compact range feed with a cross-polarization level of -30 dB from 10.7 GHz to 13 GHz.
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We present a methodology to precisely control the gain of an open quad-ridge horn (OQRH) without impacting the input match over the wide band frequency of operation characteristic of this type of horns.
Copyright 2018 IET. Reprinted from EuCAP 2018 Conference.
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The need for thermal stability in a test chamber is a well-established requirement to maintain the accuracy and repeatability sought for high frequency planar near-field (PNF) scanner measurements. When whole chamber thermal control is impractical or unreliable, there are few established methods for maintaining necessary precision over a wide temperature range.
Often the antenna under test (AUT) itself will require a closed-loop thermal control system for maintaining stable performance due to combined effects from transmission heat dissipation and the environment. In this paper, we propose a new approach for nearfield system design that leverages this AUT stability, while relaxing the requirement of strict whole chamber thermal control. Fixed reference monuments strategically placed around the AUT aperture perimeter, when measured periodically with a sensing probe on the scanner, allow for the modeling and correction of the scanner positioning errors. This process takes advantage of the assumed stability of the reference monuments and attributes all apparent monument position changes to distortions in the scanner structure. When this monument measurement process is coupled with a scanner structure that can tolerate wide thermal variations, using expansion joints and kinematic connections, a robust structural error correction model can be generated using a bilinear mapping function. Application of such a structure correction technique can achieve probe positioning performance similar to scanners that require tightly controlled environments. Preliminary results as well as a discussion on potential design variations are presented.
The IEEE Standard 149 has not been revised since 1979. Over the years the Standard was reaffirmed without any changes. Recently the IEEE Standards association stopped the practice of reaffirming standards. This change in policy by the IEEE has been the “medicine” that this standard needed. A working group was organized and a project authorization request (PAR) was approved by IEEE for the document to be updated. In this paper, the expected changes to the document are described. The main change is to convert the standard to a recommended practice document. Additionally, some new techniques to measure antennas such as the use of reverberation chambers and compact ranges is discussed in more detail. Most importantly, a discussion on uncertainty is included. The result will be a very useful document for those designing and evaluating antenna test facilities, and those performing the antenna measurements.
Copyright 2018 IEEE. Reprinted from IEEE Conference on Antenna Measurements & Applications, September 3, 2018.
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Indoor antenna ranges must have walls, floor and ceiling treated with RF absorber. The normal incidence performance of the absorber is usually provided by the manufacturers of the materials; however, the bi-static or off angle performance must also be known. In a recent paper [1], a polynomial approximation was introduced that gave a prediction of the reflected energy from pyramidal absorber. In this paper, the approximations are used to predict the quiet zone (QZ) performance of several anechoic chambers. These predictions are compared with measurements performed per the free space VSWR method of four different chambers. Among the chambers analyzed are a 7.3m by 3.65m by 3.65m range with a 24-inch absorber, operating from 1 to 6 GHz with a 91 cm spherical QZ and a 5.18 m path length. Another chamber is 7 m long by 3.3 m wide with a 2.4 m height. 12-inch absorber is used to treat the internal surfaces and the QZ changes from 63 cm to 20 cm from 2 GHz to 18 GHz. The path length is 5.18m. While performing the comparison, changes are made to the calculations to further improve the predictions of the computations. A chamber previously analyzed is computed again after the changes to see whether there are improvements in the prediction. The results show that the polynomial approximations can be used to give a reasonably accurate and safe prediction of the QZ performance of anechoic chambers and improve some of the previous comparisons especially at lower frequencies where the ray tracing is not that accurate.
Copyright 2018 IET. Reprinted from EuCAP 2018 Conference.
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The fundamental purpose of absorber treatment in an anechoic chamber is to ensure that only the direct-path signal is coupled between the range antenna(s) and the device under test. For many simple and standard geometries, this is readily accomplished with conventional processes and procedures. When the geometry and/or stray-signal requirements deviate from the norm, however, it can be very beneficial to have an easy and reliable way to locate and quantify sources of stray signals.
This paper discusses a straightforward algorithm for creating images of those stray signals in a range when a planar scanner and broad-beamed probe are available in the test zone. Measured data from multiple facilities are evaluated, along with absorber-treatment improvements made based on some of the images produced.
This paper presents the second phase of the development of a new measurement technique to determine antenna system noise temperature using data acquired from a planar near-field measurement. In the first phase, it was shown that the noise temperature can be obtained using the plane-wave spectrum of the planar near-field data and focusing on the portion of the spectrum in the evanescent region or “imaginary space”. Actual evanescent modes are highly attenuated in the latter region and therefore the spectrum in this region must be produced by “errors” in the measured data. Some error sources such as multiple reflections will produce distinct localized lobes in the evanescent region and these are recognized and correctly identified by using a data point spacing of less than λ/2 to avoid aliasing errors in the far-field pattern. It has been observed that the plane wave spectrum beyond these localized lobes becomes random with a uniform average power. This region of the spectrum must be produced by random noise in the near-field data that is produced by all sources of thermal noise in the electronics and radiated noise sources received by the antenna. By analysing and calibrating this portion of the spectrum in the evanescent region the near-field noise power can be deduced and the corresponding noise temperature determined. In the current phase of tests, planar near-field data has been acquired on a measurement system and the analysis applied to determine the system noise parameters. Measurements have been performed with terminations inserted at three different locations in the RF receiving path: the IF input to the receiver, the input to the mixer and the input to the probe that is transmitting to a centre-fed reflector antenna. The terminations consist of either a load that serves as the “cold” noise source or a noise source with a known noise output for the “hot” noise source.
Recently, a paper was presented in which a lithium niobate (LiNbO3) crystal electric field sensor was characterized as a possible probe for near-field antenna measurements. In the present paper, some preliminary measurements are presented. A standard gain horn operating in the X-band was measured in a spherical near-field range using the LiNbO3 probe as the near-field probe. The results are compared to computed data for said horn. An additional flat– plate, slotted array antenna operating in the X-band was also measured. The data was transformed to the far field and compared with previous measurements of said antenna performed using a traditional open-ended-waveguide (OEWG) probe. Additionally, the transform was used to backproject to the aperture of the antenna and the data show the two slots in the array that are covered with metallic tape. The transforms and back-projection suggest that these probes could be used as near-field probes in antenna measurements if some stability issues are corrected.
Copyright 2018 IEEE. Reprinted from EuCAP 2018 Conference.
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It is desirable to have a single facility to serve all the antenna measurement needs of an organization of company. However, there is not a one-size-fits-all antenna measurement facility to meet all antennas and frequency ranges. Trying to design and operate a Jack-of-all-trades facility is unrealistic. Such facilities may be ideal for testing part of the frequency range and types of antennas but may not be the ideal solution for certain applications. In this papers suggestions and recommendations are given for specific types of antennas mainly operating in the 30 to 3000 MHz range.
Copyright 2018 IEEE. Reprinted from IEEE Conference on Antenna Measurements & Applications, September 3, 2018.
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As antenna applications evolve to new technologies, test facilities must adapt to them. Broader application of millimeter-wave frequencies, more frequent use of active electronically steered array technology, and a desire for thorough verification are presenting significant challenges to antenna test ranges. The number of measurement points, frequencies, antenna states, and antenna ports has grown substantially and continues to grow. Some antenna test requirements have resulted in extremely long test times with a vast amount of data to be processes and analyzed. The impact of higher test cost and longer delivery schedules can be substantial, and test facility capacity may be stretched to its limits.
As a result, instrumentation products and automated measurements systems are being pressed to provide enhanced performance to meet the challenges of these rapidly growing test requirements. This presentation describes several ways to reduce test time by optimizing the data acquisition process and by giving due consideration to RF and millimeter-wave performance trade-offs. Specific examples of test ranges that illustrate measurement challenges and solutions will be provided. A view to the future will conclude the talk.
View The PaperThe IEEE Standards Association Standards Board (IEEE-SASB) approved the IEEE Std 1720™ “Recommended Practice for Near Field Antenna Measurements” in 2012 [1]. More than fourty dedicated people from industry, academia and other institutions contributed to the creation of this new document. The main motivation for a new standard dedicated to near-field measurements was to complement the existing IEEE Std 149-1979™ “Test Procedures for Antennas” [2].
Copyright 2018 IEEE. Reprinted from IEEE Conference on Antenna Measurements & Applications, September 3, 2018.
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Spherical near-field (SNF) measurement techniques have become the preferred approach for antenna testing. This paper argues in favour of importing detailed measured pattern data obtained from such testing into the on-platform (and other) modelling of these antennas when in situ performance is required. Two representative examples are given.
Copyright 2018 IEEE. Reprinted from ANTEM Conference, 2018.
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To address dynamic testing requirements of new communications systems and RF processes that use non-static beam forming, NIST proposed the Large Antenna Positioning System (LAPS). The LAPS consists of two kinematically-linked six axis robotic arms, one of which is integrated with a 7 m linear rail system. This repositionable, multi-robot system can perform arbitrary scans around a device under test. The dynamic 13 degree-of-motion capability is designed to perform complex spatial interrogation of systems.
The coordinated-motion capabilities of the system are key to support not only traditional antenna measurement geometries (i.e. spherical, cylindrical, planar, gain-extrapolation), but are also intended to be used to dynamically interact with changing RF conditions. The robots can independently scan or interrogate multiple bearings toward a device under test, perform MIMO illumination, or trace out complex 6D paths during system testing.
Initial RF and mechanical testing results in the factory where it was built show deviations from an ideal linear scan at 0.032 ± 0.02 mm, much better than the l/50 system design specification at 30 GHz. Further improvements to the basic kinematic models of each robot will allow this generation of robotic antenna range to operate open loop without laser tracker feedback.
The topic of non-redundant near-field sampling has received much attention in recent literature. However, a practical implementation has so far been elusive. This paper describes a first step toward such a practical implementation, where the practicality and generality are maximized at the expense of more acquired data points.
Building on the theoretical work of faculty at the University of Salerno and University of Naples [1]-[17], the authors have acquired a set of near-field data using a spiral locus of sample points and, from those data, obtained the far-field patterns. In this paper, we discuss the acquisition system, the calculation and practical implementation of the spiral, the phase transformations, interpolations, and far-field transforms. We also present the resultant far-field patterns and compare them to patterns of the same antenna obtained using conventional near-field scanning. Qualitative results involving aperture backprojection are also given. We summarize our findings with a discussion of error, uncertainty, acquisition time, and processing time in this simplified approach to non-redundant sampling in a practical system.
RF shielded enclosures have been common features in laboratories and manufacturing areas for over 70 years. They provide an environment where work on RF can be performed without interference from outdoor sources. These shielded rooms and areas provide a place where classified frequencies and modulations can be used without leaking out. In general, these shielded rooms have shielded doors to maintain the shielding integrity. These happens until they are opened . To maintain the shielding integrity as personnel moves from the inside to the outside of the room and vice-versa, dual shielded doors with a small vestibule between them are used. However, the presence of multiple doors increases the time to access the enclosure. To solve this, some enclosures are designed featuring access passages to maintain the shielding integrity over a broad frequency without the use of doors. This type of access has been around for over 40 year but its design has never been discussed in the literature. In this paper, a door-less access is analyzed and some design rules are presented. The limitations of these tunnels are also presented. They do not have the shielding performance of a shielded door but they are ideal for certain applications.
Copyright 2017 IEEE. Reprinted from EMC Society of Australia Newsletter, June 2017, Issue 77.
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In this paper, we explore the possibility of using a photonics-based E-field sensor as a near-field probe. Relative to open-ended waveguide (OEWG) probes, a photonics probe could offer substantially larger bandwidths. In addition, since it outputs an optical signal, a photonics probe can offer signal transport through optical fiber with much lower loss than what can be achieved using RF cables.
We begin with a discussion of the theory of the device followed by a summary of results of a photonics sensor that was tested in a spherical near-field (SNF) range. In these tests, data were collected with the photonics probe in the test antenna position to characterize various probe parameters including polarization discrimination, probe gain, effective dynamic range, and probe patterns. Results are presented along with discussions of some of the advantages and disadvantages of using a photonics probe in a practical system based on the lessons learned in the SNF testing.
Indoor antenna ranges must have the walls, floor and ceiling treated with RF absorber. The normal incidence performance of the absorber is usually provided by the manufacturers of the materials; however, the bi-static or off angle performance must also be known. In reference [1], a polynomial approximation was introduced that gave a prediction of the reflected energy from pyramidal absorber. In this paper, the approximations are used to predict the quiet zone (QZ) performance of several anechoic chambers. These predictions are compared with full wave analysis performed in CST Suite®. A 12 m wide by 22 m long with a height of 12 m chamber was analyzed at 700 MHz. The QZ performance was compared to the polynomial predictions showing a difference of less than 2.2 dB. In addition, comparisons are made with measurements of the QZ performance of anechoic chambers. Measurements performed per the free space VSWR method of three different chambers are compared with the prediction that uses the polynomials presented in [1]. The chambers are: a 18 m long by 11.5 m wide and 11.5 m in height operating from 100M MHz to 12 GHz; a 13.41 m by6.1 m by 6.1 m operating from 800 MHz to 6 GHz; and a 14 m long by 4.12 m by 4.27 m operating in the X band. The results show that the polynomial approximations can be used to give a reasonably accurate and safe prediction of the QZ performance of anechoic chambers.
The efficiency of use of the parabolic reflector of a single offset reflector compact antenna test range (CATR) is affected largely by the illumination provided by the range feed and the reflector edge treatment. Thus, when these factors are taken together it is commonly found that the realized quiet zone (QZ) diameter is typically as little as 30% of the diameter of the reflector for the commonly encountered case of a single offset CATR. Furthermore, single offset CATR performance is known to degrade as the wavelength of the illuminating fields becomes more comparable with the physical dimensions of the reflector because the physical optics (PO) assumption needed for collimation of the reflected field becomes less effective. Different reflector edge treatments such as rolled or serrated edges are commonly employed to taper the intensity of the reflected fields at the reflector aperture boundary, seeking to minimize the level of diffracted fields in the quiet-zone (QZ). Such strategies mean that at higher frequencies the transverse dimensions of the QZ are unnecessarily reduced thereby decreasing the spatial efficiency of the CATR and limiting the effective bandwidth of the antenna test system. In this paper we report preliminary results that begin to investigate the alternative strategy for controlling the signal illuminating the CATR reflector by utilising a shaped beam feed antenna. Building on our previously reported work of efficient CATR computational electromagnetic simulation, we report the use of an array feed whose excitation is optimized to achieve maximum QZ size for a given reflector dimension thereby minimising the cost of a new test system or increasing the capacity of an existing one. We illustrate the concept by employing the technique with a sector-shaped, single offset reflector CATR by examining the impact that this has on the amplitude taper and the amplitude and phase peak-to-peak ripple. We demonstrate that a 9-element array feeding an un-serrated rim reflector can attain a useable QZ size approaching 50% the size of the 80λ diameter main reflector.
Copyright 2017 IEEE. Reprinted from EuCAP 2017 Conference.
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Cellular Base Stations require efficient performance validation methods. One performance criterion is the radiation pattern. Our measurements show a radiation pattern change caused by the antenna mount structure, though industry guidance [1] does not yet control or mention this aspect of the test configuration. Consequently, the current guidance leads toward lack of repeatability. We recommend industry guidance be amended to include post-BSA distance as a test configuration parameter.
Measured Data is presented showing radiation pattern dependencies upon the mount in a CATR implementation. Explanations as to the Root Cause are stated.
For a little over a decade now, a measurement and post-processing technique named Mathematical Absorber Reflection Suppression (MARS) has been used successfully to identify and then suppress range multi-path effects in spherical, cylindrical &p; planar near-field antenna measurement systems and far-field and compact antenna test ranges with a detailed theoretical treatment being presented in. Much of this early work concentrated on verification by empirical testing however some corroboration was obtained with the use of computational electromagnetic simulations that considered far-field and subsequently nearfield cases. The recent development of a highly accurate computational electromagnetic simulation tool that permits the simulation of “measured” far-field pattern data as obtained from using a compact antenna test range (CATR) has for the first time permitted the careful verification of the far-field MARS technique for a given AUT and CATR combination. For the first time, this paper presents simulated “measured” far-field pattern data in the presence of a large scatterer and then verifies the successful extraction of the scattering artefacts using standard FF-MARS processing. Results are presented and discussed.
Copyright 2017 IEEE. Reprinted from EuCAP 2017 Conference.
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In this paper, we present a method for measuring antenna group delay (GD) in a planar near-field range. The technique is based on a set of three antenna pairs, measured sequentially, from which the insertion phase of the measurement system and the near-field probe can be resolved. Once these parameters are known, insertion phase for the device under test (that is to say a Tx or Rx antenna) can be measured and GD calculated as the negative frequency derivative of the insertion phase with respect to frequency. An added complexity in the case of a near-field measurement is the near-field probe is in close proximity to the device under test, does not satisfy the far-field condition. We also show that group delay can be extracted from a single near-field measurement point in the antenna’s aperture plane, leading to significant test time savings. Measured results are presented and discussed.
This paper presents the results of a new measurement technique to determine antenna system noise temperature using data acquired from a planar near-field measurement. The ratio of antenna gain to system noise temperature (G/T) is usually determined in a single measurement when the antenna is alternately pointed towards the “cold sky” and a hot radio source such as the sun or a star with a known flux density. The antenna gain is routinely determined from nearfield measurements and with the development of this new technique, the system noise temperature can also be determined using the same measurements. The ratio of G/T can therefore be determined from planar near-field data without moving the antenna to an outdoor range. The noise temperature is obtained by using the plane-wave spectrum of the planar near-field data and focusing on the portion of the spectrum in the evanescent or “imaginary space” portion of the spectrum. Near-field data is obtained using a data point spacing of λ/4 or smaller and the plane-wave spectrum is calculated without applying any probe correction or Cos(θ) factor. The spectrum is calculated over real space corresponding to propagating modes of the far-field pattern and also the evanescent or imaginary space region where kx2+ ky2 ≥ k2. Actual evanescent modes are highly attenuated in the latter region and therefore the spectrum in this region must be produced by “errors” in the measured data. Some error sources such as multiple reflections will produce distinct localized lobes in the evanescent region and these are recognized and correctly identified by using a data point spacing of less than λ/2 to avoid aliasing errors in the far-field pattern. It has been observed that the plane wave spectrum beyond these localized lobes becomes random with a uniform average power. This region of the spectrum must be produced by random noise in the near-field data that is produced by all sources of thermal noise in the electronics and radiated noise sources received by the antenna. By analysing and calibrating this portion of the spectrum in the evanescent region the near-field noise power can be deduced and the corresponding noise temperature determined. Simulated and measured data will be presented to illustrate and validate the measurement and analysis techniques.
RF shielded enclosures have been common features in laboratories and manufacturing areas for over 70 years. They provide a quiet environment where RF measurements can be performed without interference from outdoor sources and are used to keep potentially classified frequencies and modulations from leaking out. In general, these shielded rooms have shielded doors to maintain the shielding integrity of the enclosure until they are opened. In some cases, to maintain the shielding integrity as personnel moves from the inside to the outside of the room and vice-versa, dual shielded doors with a small vestibule between them are used. However, the presence of multiple doors increases the time to access the enclosure. To solve this, some enclosures are designed featuring access passages to maintain the shielding integrity over a broad frequency without the use of doors. Although this type of access has been around for over 40 years, its design has never been discussed in the literature. In this paper, a door-less access is analyzed and some design rules are presented. The limitations of these accesses are also presented. While clearly they do not have the shielding performance of a shielded door, they are ideal for certain applications.
Tilting the receive end wall of a compact range anechoic chamber to improve Radar Cross-Section (RCS) measurements has been a tool of the trade used since the earliest days of anechoic chambers. A preliminary analysis using geometrical optics (GO) validates this technique. The GO approach however ignores the backscattering modes from the reflected waves from a field of absorber. In this paper, a series of numerical experiments are performed comparing a straight wall and a tilted wall to show the effects on both the quiet zone and the energy reflected back towards the source antenna. Two Absorber covered walls are simulated. Both walls are illuminated with a standard gain horn (SGH). The effects of a wall tilted back 20° are computed. The simulations are done for 72-inch long absorber for the frequency range covering from 500 MHz to 1 GHz. The ripple on a 10 ft (3.05 m) quiet zone (QZ) is measured for the vertical wall and the tilted wall. In addition to the QZ analysis a time-domain analysis is performed. The reflected pulse at the excitation antenna is compared for the two back wall configurations Results show that tilting the wall improves measurements at some frequencies but causes a higher return at other frequencies; indicating this method does not provide a broadband advantage.
The evaluation of RF Sensors often requires a test capability where various RF targets are presented to the Unit Under Test (UUT). These targets may need to be dynamic in time, represent multiple targets and/or decoys, emulate dynamic motion, and simulate real world RF environmental conditions. An RF Target Simulator can be employed to perform these functions and is the focus of this paper. The total test system is usually called Hardware in the Loop (HITL) involving the UUT mounted on a Flight Motion Simulator (FMS), the RF Target Simulator presenting the RF Scene, and a Simulation Computer that dynamically controls everything in real-time. The realization of a highly effective target simulator, one that truly meets the user’s needs at an affordable cost, is the result of understanding the complex interrelationship of requirements, architecture and constraints. This paper examines those relationships in seven areas of discussion, employing examples of realized systems;
In order to achieve high accuracy in measuring sidelobes and/or nulls in antenna patterns, it is necessary to use a test system with very high dynamic range. This is particularly important when the antenna has extremely high gain such as those used for certain satellite communications or radio astronomy applications or when transmit power is limited relative to range loss as is often the case in millimeter wave applications. For several years, commercially available antenna measurement receivers have offered a dynamic range as high as 135dB for such applications. This dynamic range has been made possible, in part, by a simple correlator in the receiver’s DSP chain. In a previous paper, noise-free signal models were developed and analyzed to demonstrate the correlator’s ability to reduce carrier frequency offset (CFO) and local oscillator (LO) phase noise to offer the fidelity of test signal necessary to achieve extremely high dynamic ranges of up to 135dB. Building on those models, this paper models the effects of thermal noise and analyzes situations where the correlator works well and where it negatively impacts performance.
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Indoor antenna ranges must have the walls, floor and ceiling treated with RF absorber. The normal incidence performance of the absorber is usually provided by the manufacturers of the materials, however, the bi-static or off angle performance must also be known. In reference [1], a polynomial approximation was introduced that gave a prediction of the reflected energy from pyramidal absorber. In this paper, the approximations are used to predict the quiet zone (QZ) performance an anechoic chambers. These predictions are compared with full wave analysis performed in CST Suite. The results show that the polynomial approximations can be used to give a fairly accurate prediction of the QZ performance of anechoic chambers.
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The Mathematical Absorber Reflection Suppression (MARS) technique is used ordinarily to identify and then suppress effects of spurious scattering within an antenna range measurement. This paper, for the first time, demonstrates by means of computational electromagnetic simulation that MARS can also be used to attenuate feed spillover in an offset parabolic reflector compact antenna test range. Preliminary results are presented and discussed.
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This paper presents the results of a recent study concerning the computational electromagnetic simulation of a spherical near-field (SNF) antenna test system in the presence of a compact antenna test range (CATR). The plane-wave scattering matrix approach [1, 2] allows many of the commonly encountered components within the range uncertainty budget, including range reflections, to be included within the model [3]. This paper presents the results of simulations that verify the utility of the spherical mathematical absorber reflection suppression (S-MARS) technique [3, 4] for the identification and subsequent extraction of artifacts resulting from range reflections. Although past verifications have been obtained using experimental techniques this paper, for the first time, corroborates these findings using purely computational methods. The use of MARS is particularly relevant in applications that inherently include scatterers within the test environment. Such cases include instances where a SNF test system is installed within an existing compact antenna test range (CATR) as is the configuration at the recently upgraded Queen Mary University of London (QMUL) Antenna Laboratory [5, 6]. Thus, this study focuses on this installation with results of CEM simulations being presented. The method enables a quantitative measure of the levels of suppression offered by the MARS system.
P Gain over Temperature (G/T) is an antenna parameter of importance in both satellite communications and radio-astronomy. Methods to measure G/T are discussed in the literature [1-3]. These methodologies usually call for measurements outdoors where the antenna under test (AUT) is pointed to the “empty” sky to get a “cold” noise temperature measurement; as required by the Y-factor measurement approach [4]. In reference [5], Kolesnikoff et al. present a method for measuring G/T in an anechoic chamber. In that approach, the chamber has to be maintained at 290 kelvin to achieve the “cold” reference temperature. In this paper, a new method is presented intended for the characterization of lower gain antennas, such as active elements of arrays. The new method does not require a cold temperature reference; thus alleviating the need for testing outside or maintaining a cold reference temperature in a chamber. The new method uses two separate “hot” sources. The two hot sources are created by using two separate noise diode sources of known excess noise ratios (ENR) or by one source and a known attenuation. The key is that the sources differ by a known amount. This paper builds upon the presented information in [2] providing more measured data using the recommended procedure.
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Indoor antenna ranges must have the walls, floor and ceiling treated with RF absorber. The normal incidence performance of the absorber is usually provided by the manufacturers of the materials, however, the bi-static or off angle performance must also be known. Some manufacturers provide factors at discrete electrical thickness for a discrete range of incident angles. This approximation is based on the curves presented in [1]. In reference [2], a polynomial approximation was introduced.
In this paper, a more accurate approximation of absorber performance is introduced. Pyramidal RF absorber is modeled using CST’s frequency domain solver. The numerical results are compared to results from other numerical methods. The highest reflectivity of the two principal polarizations for a given angle of incidence and thickness of material is calculated. Different physical thickness pyramids are modeled. Once the worst case reflectivity is calculated, a polynomial curve fit is used to derive a set of equations that provide the bi-static performance for absorber as a function of angle of incidence and thickness of material. The equations can be used to predict the necessary RF absorber to treat the walls of an indoor range.
Methods for determining the uncertainty in antenna measurements have been previously developed and presented. The IEEE has published IEEE 1720-2012 that formalizes a methodology for uncertainty analysis of near-field antenna measurements. In contrast, approaches to uncertainty analysis for antenna measurements on a compact range are not covered as well in the literature. A review and discussion of the terms that affect gain and sidelobe uncertainty are presented as a framework for assessing the uncertainty in compact range antenna measurements including effects of the non-ideal properties of the incident plane wave. An example uncertainty analysis is presented.
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Every year, many requests for proposals for an anechoic chamber are generated by companies and institutions that perform antenna measurements. The task of adequately specifying performance for an indoor anechoic chamber without driving unnecessary costs or specifying contradictory requirements requires insight that is not always available to the author of the specification. Although there are some articles and books that address anechoic chamber design [1]–[3], a concise compendium of reference information and rules of thumb on the subject of specifying ranges would be useful.
This article intends to be a helpful tool in that regard. It starts by recommending the proper type of range for different antenna types and frequencies of operation. Rules of thumb are provided to select the best approach for the required test or antenna type. Information is provided on the derivations needed for other ranges, such as compact ranges and near-field test facilities. Simple approximations are used for absorber performance to generate a series of charts that can be used as a guide to specify anechoic chamber performance and size. Company and institution facilities can then define the appropriate square footage necessary to house the required antenna range. This article intends to avoid some of the common contradictory requirements. Some of these contradictory requirements are not enough real estate to accommodate a chamber operating at low frequencies or levels that are not possible given available absorber technology.
Copyright 2016 IEEE. Reprinted from 2016 IEEE Antennas & Propagation Magazine.
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Abstract—“RTCA DO-213 Minimum Operational Performance Standards For Nose-Mounted Radomes” [1] is a document fre-quently referenced in nose-radome testing requirements for commercial aircraft. This document was produced and is maintained by the Radio Technical Commission for Aeronautics (RTCA). The specifications of weather-radar systems have recently changed within RTCA’s DO-220A [2], and as a result DO-213 was updated to DO-213A [3] in March, 2016, to ensure that radome requirements are consistent with those of the weather radar.
In addition to the new requirements for radome evaluation, several existing requirements were clarified. These clarifications addressed such topics as suitability of near-field measurements, proper procedures and processing, and appropriate measure-ment geometries.
RTCA coordinated the document revision, with the bulk of the technical inputs coming from a broad-based working group. This working group had representatives from radar, aircraft, and radome manufacturers, government agencies, and providers and users of radome-testing systems. When requirements were added or when common practice conflicted with existing require-ments, considerable effort and analysis were employed to ensure that each change or clarification was truly required. Never-theless, DO-213A has some significant impacts to many existing radome-testing facilities.
This paper discusses the significant changes from DO-213 to DO-213A and their implications for radome-testing facilities, concen-trating on after-repair radome electrical testing.
This paper extends the authors previous simulation study that predicted the quality of the pseudo plane wave of a single offset compact antenna test range (CATR). In this paper, the quiet-zone performance predictions are extended to rigorously incorporate the effects of probing the CATR quietzone using arbitrary but known field probes. This paper compares and contrasts results obtained using plane-wavespectrum and reaction integral based simulation techniques. This investigation leads to recommendations as to the optimal field probe choice and measurement uncertainties. The results of these new simulations are presented and discussed.
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A high quality antenna feed is an essential element of a compact antenna test range (CATR) in order to ensure the range can achieve the necessary stability in beam width, phase center and the necessary purity of polarization throughout the range’s quiet zone. In order to maintain the requisite quality, such feeds are typically 1) single-port and 2) cover a relatively limited band of frequencies. It is desirable to have a single dual ported, broadband feed that covers multiple waveguide bands to eliminate the need for a polarization positioner and avoid the difficulty associated with changing feeds for a single antenna measurement. Though some such feeds exist in the market, with such feeds, we often see a reduction in polarization purity across the band of interest relative to the more band limited feeds. Previous attempts to utilize dual-port probes and/or extend the bandwidth of the feed have resulted in degraded performance in terms of beam pattern and polarization purity. In an attempt to overcome some of the deficiencies above, the authors have applied polarization processing to dual-pol antennas to correct for the impurity in polarization of the antenna as a function of frequency. We present here a broadband CATR feed solution using a low-cost, dual-port sinuous feed structure combined with polarization processing to achieve low cross-pol coupling throughout the quiet zone. In the following paper, the feed structure, polarization theory, and processing algorithm are described. We also present co- and cross-pol coupling results before and after correcting for the polarization distortion using data collected in two CATRs in Atlanta, GA and Asia.
Antennas utilized as probes, sources, and for gain comparison are typically specified to have excellent cross polarization levels, often on the order of 50 dB below the primary polarization component. In many cases, these antennas are fed with a Waveguide-to-Coaxial adapter, which can be sourced from a multitude of vendors. Depending on the design and construction of the adapter, and the distance from the excitation probe to antenna aperture, the adapter itself can contribute significantly to the degradation of the polarization purity of the antenna. These adapters typically use one of several methods to achieve a good impedance match across their bandwidths, including tuning screws, posts and stubs. These tuning elements may be arranged asymmetrically and can cause the waveguide to be over-moded locally. Additionally, there is wide variance in the distance separating the adapter excitation probe and waveguide electrical flanges, which may not be long enough to suppress the higher order modal content. In this paper, we study the effects of adapter to antenna aperture coupling, including the coupling of fields local to the current probe. The analysis concludes with recommendations to ensure that the antenna polarization purity is optimized.
In order to achieve high accuracy in measuring sidelobes and/or nulls in antenna patterns, it is necessary to use a test system with very high dynamic range. This is particularly important when the antenna has extremely high gain such as those used for certain satellite communications or radio astronomy applications or when transmit power is limited relative to range loss as is often the case in millimeter wave applications. For several years, commercially available antenna measurement receivers have offered a dynamic range as high as 135dB for such applications. This dynamic range has been made possible, in part, by a simple correlator in the receiver’s DSP chain. In this paper, we model the various sources of error in a test signal due to imperfections and uncertainties of the test equipment and the physical environment and analyze these models as they propagate through the receive chain. The results of that analysis demonstrate the correlator’s ability to reduce carrier frequency offset (CFO) and local oscillator (LO) phase noise to offer the fidelity of test signal necessary to achieve extremely high dynamic ranges of up to 135dB.
Near-field antenna test systems are typically designed to optimize measurement results for a specific type of antenna. The measurement system is selected and sized based on the antenna aperture dimensions, directivity, weight and operating frequency, among other parameters. These factors are used to select either a planar, cylindrical, or spherical near-field test system for the given antenna test requirements. Antennas with different characteristics may not be compatible with the selected range and often require costly upgrades to the existing range or a different range altogether. One solution to test a wide variety of antenna types is a combination planar-cylindrical-spherical (PCS) test system. These systems usually require some level of facility re-configuration and present drawbacks when switching between the various modes of operation.
The adaptation of a six-axis robotic test system is an attractive solution in these situations, as the system’s flexibility allows for rapid reconfiguration that is inherent to the system. This allows the user to select the optimal test solution for the antenna under test with little effort. This paper presents the performance of a six-axis robotic near-field measurement system showing nearfield modes of operation and the system’s performance in antenna measurements when compared to a traditional spherical near-field range.
Antenna test facilities are large investments that are expected to be used for decades. Some facilities are wellmaintained with periodic upgrades to the latest equipment, while others attempt to minimize changes to the existing system due to the difficulty of recertifying test procedures or lack of sufficient budget. Meanwhile, antenna designs and test processes continue to evolve and require more extensive antenna performance data packages than ever imagined when the test facility was initially installed. As a result, total test times may grow exponentially, thus limiting range throughput. The size of collected data files can also become extremely large, even exceeding the capacity of some common commercial databases.
This case study illustrates that periodic evaluation and optimization of system test processes and measurement timing can sometimes pay large immediate dividends in range throughput and productivity. In addition, by creating an accurate system measurement timing model, sensitivity studies can easily be conducted to provide guidance in selecting the most effective alternative test plans or incremental investments in new equipment.
This paper describes the upgrade to the indoor antenna range at Stellenbosch University. The previous measurement process relied upon obsolescent control equipment and undocumented software; it was critical that these be replaced. Now, the antenna range supports three measurement types using a commercial integrated measurement control system that provides support for high gain and low gain antennas over a wide frequency range. These are spherical near-field, planar near-field and conventional far-field measurements, with the potential to implement cylindrical near-field. The antenna range potentially supports operations from 1 GHz up to 26.5 GHz, though the currently available probes do not cover the full band. The main physical upgrade was performed during October 2014, though investigations had already begun in 2011, and some supplementary tasks were still ongoing at the time of writing. Several innovative commissioning tests have been undertaken, some of which are only possible with near-field metrology, and these are described in the paper.
Copyright 2016 SAIEE. Reprinted from the Africa Research Journal, March 2016 Issue.
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The evaluation of RF Sensors often requires a test capability where various RF scenes are presented to the Unit Under Test (UUT). These scenes may need to be dynamic, represent multiple targets and/or decoys, emulate dynamic motion, and simulate real world RF environmental conditions. An RF Scene Generator can be employed to perform these functions and is the focus of this paper. The total test system is usually called Hardware in the Loop (HITL) involving the sensor mounted on a Flight Motion Simulator (FMS), the RF Scene Generator presenting the RF Scene, and a Simulation Computer that dynamically controls everything in real time. This paper describes the system concept for an RF Scene Generator that simultaneously represents 4 targets, in highly dynamic motion, with no occlusion, over a wide range of power, frequency, and Field of View (FOV). It presents the test results from a prototype that was built and tested over a limited FOV, while being scalable to the total FOV and full system capability. The RF Scene Generator employs a wall populated with an array of emitters that enables virtually unlimited velocity and acceleration of targets and employs beam steering to provide high angular resolution and accuracy of the presented target positions across the FOV.
The efficiency of use of the parabolic reflector of a single offset reflector compact antenna test range (CATR) is affected largely by the illumination provided by the range feed and the reflector edge treatment. Thus, when these factors are taken together it is commonly found that the realized quiet zone (QZ) diameter is typically as little as 30% of the diameter of the reflector for the commonly encountered case of a single offset CATR. Furthermore, single offset CATR performance is known to degrade as the wavelength of the illuminating fields becomes more comparable with the physical dimensions of the reflector because the physical optics (PO) assumption needed for collimation of the reflected field becomes less effective. Different reflector edge treatments such as rolled or serrated edges are commonly employed to taper the intensity of the reflected fields at the reflector aperture boundary, seeking to minimize the level of diffracted fields in the quiet-zone (QZ). Such strategies mean that at higher frequencies the transverse dimensions of the QZ are unnecessarily reduced thereby decreasing the spatial efficiency of the CATR and limiting the effective bandwidth of the antenna test system. In this paper we report preliminary results that begin to investigate the alternative strategy for controlling the signal illuminating the CATR reflector by utilising a shaped beam feed antenna. Building on our previously reported work of efficient CATR computational electromagnetic simulation, we report the use of an array feed whose excitation is optimized to achieve maximum QZ size. We illustrate the concept by employing the technique with a sector-shaped, reflector single offset CATR having no edge treatment and then using the same reflector with an edge treatment and by examining the impact that this has on the amplitude taper and the amplitude and phase peak-to-peak ripple.
The adaptability of the phased array antenna makes it attractive for a variety of multi-beam applications. Previously, its use was limited mostly to military applications due to the large expenses associated with this type of antenna. In recent years, reduced costs have made phased array antennas viable for a variety of commercial applications. As this technology encroaches on new markets, with it comes the need to learn how to properly calibrate phased array antennas. To date, there have been numerous measurement techniques and methodologies developed for calibrating phased array antennas. This paper discusses those most commonly used in industry, and which could be easily and economically adapted for commercial applications.
Copyright 2016 IEEE. Reprinted from EuCAP 2016 Conference.
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Installing a large compact range reflector and electromagnetically qualifying the quiet zone is a major undertaking, especially for very large panelized reflectors. The approach taken to design the required rolled-edge reflector geometry for achieving a 5 meter quiet zone across a frequency range of 350 MHz to 40 GHz was previously presented [1]. The segmentation scheme, fabrication methodology, and intermediate qualification of panels using an NSI-MI developed microwave holography tool were also presented. This reflector has since been installed and the compact range qualified by direct measurement of the electromagnetic fields in the quiet zone using a large field probe.
This paper presents the comparison and correlation between the holography predictions and the field probe measurements of the quiet zone. Installation and alignment techniques used for the multiple panel reflector are presented. Available metrology tools have inherent accuracy limitations leading to residual misalignment between the panels. NSI-MI has overcome this limitation by using its holography tool along with existing metrology techniques to predict the field quality in the quiet zone based on surface measurements of the panels. The tool was used to establish go/no-go criteria for panel alignment accuracy achieved on site. Correlation of the holography predictions with actual field probe measurements of the installed reflector validates the application of the holography tool for performance prediction of large, multiple-panel, rolled-edge reflectors.
Spherical near-field (SNF) antenna test systems offer unique advantages over other types of measurement configurations and have become increasingly popular as a result. To yield accurate far-field radiation patterns, it is critical that the rotators of the SNF scanner are properly aligned. Many techniques using optical instruments, laser trackers, low cost devices or even electrical measurements have been developed to align these systems. While these alignment procedures have been used in practice with great success, some residual alignment errors always remain. This paper expands on prior work by analyzing the effects of spherical alignment errors for a variety of different measurement grids on a theta-over-phi SNF scanner. Results are presented using a combination of physical alignment perturbations (measured) and errors induced via simulation. A variety of antenna types and directions of radiation within the measurement sphere are considered.
This paper present a specific set-up developed for antenna pattern measurements of probe-fed antenna with a 500mm AUT-probe distance. An example of Near-Field measurement is proposed and shows important errors on phase acquisition at 90GHz. Raw measurements are improved using a spherical correction based on laser-tracker structural data. Phase error and Near-field to Far Field transformation are strongly improved with this technique.
Spherical spiral (Sphiral) scanning involves coordinating the motion of two simultaneous axes to accomplish near-field antenna measurements along a line on a sphere that does not cross itself. The line would ideally start near a pole and trace a path along the sphere to the other pole. An RF probe is moved along this path in order to collect RF measurements at predefined locations. The data collected from these measurements is used along with a near-field to far-field transformation algorithm to determine the radiated far-field antenna pattern.
The method for transforming data collected along a sphiral scan has been previous presented. Later laboratory measurement studies have shown the validity of the technique.
A review of the sphiral scanning technique and its recent advances, resulting from about ten years of research collaboration between the UNISA Antenna Characterization Research Group and MI Technologies is here presented. Such a scan technique relies on the non-redundant sampling representations of EM fields and takes full advantage of moving two axes simultaneously. Accordingly, it allows one to drastically reduce the overall number of required data and the time to collect the data. This scanning technique can be properly applied in testing antennas mounted on automobiles in order to reduce the overall time of the measurement.
Copyright 2016 IEEE. Reprinted from EuCAP 2016 Conference.
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Compact antenna test ranges (CATR) are attractive solutions for far-field measurements in a confined space with the single-offset reflector being the most common variation of deployed CATRs. Reflectors in these ranges emit a collimated plane-wave to simulate the far-field condition. This requires that each CATR must be properly focused and undergo careful alignment and validation, as any misalignment would perturb the plane wave. CATRs are normally designed to operate in environments with tight temperature control, however this is frequently impractical to implement in normal test environments. A carbon-fiber CATR reflector designed to be insensitive to temperature fluctuations can be an effective means to prevent thermally-induced deformation, and thus a corruption of the plane wave. This paper will illustrate the performance of this reflector over across a range of temperatures, and use a computational electromagnetic simulation to predict the impact on antenna measurements when the reflector is subjected to different temperatures.
Copyright 2016 IEEE. Reprinted from EuCAP 2016 Conference.
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MIL STD 461 is the Department of Defense standard that states the requirements for the control of electromagnetic interference (EMI) in subsystems and equipment used by the armed forces. The standard requires users to measure the unintentional radiated emissions from equipment by placing a measuring antenna at one meter distance from the equipment under test (EUT). The performance of the antenna at 1m distance must be known for the antenna to measure objects located at this close proximity. MIL STD 461 requires the antennas to be calibrated at 1 m distance using the Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 958. This SAE ARP 958 document describes a standard calibration method where two identical antennas are used at 1m distance to obtain the gain at 1m for each antenna. In this paper the authors show using simulations that the SAE ARP 958 approach introduces errors as high at 2 dB to the measured gain and AF. To eliminate this problem the authors introduce a new method for calibrating EMC antennas for MIL STD 461. The Method is based on the well-known extrapolation range technique. The process is to obtain the polynomial curve that is used to get the far field gain in the extrapolation gain procedure, and to perform an interpolation to get the gain at 1 m. The results show that some data in the far field must be collected during the extrapolation scan. When the polynomial is calculated the antenna performance values at shorter distances will be free of near field coupling. Measured results for a typical antenna required for emissions testing per the MIL STD 461 match well with the numerical results for the computed gain at 1 m distance. Future work is required to study the use of this technique for other short test distances used in other electromagnetic compatibility standards, such as the 3 m test distance used by the CISPR 16 standard.
MI has recently developed and installed two separate real-time laser-correction mechanisms for large planar scan-ners. One mechanism employs a spinning laser, while the other uses a tracking laser with multiple SMR constellations. The spinning-laser system is limited to planarity correction, and is appropriate for any planar scanner up to a diagonal of about 15 meters. The tracking-laser system compensates X, Y, and Z, and is intended for a horizontal planar scanner of larger size or when X and Y positions also require dynamic correction. This paper will provide an overview of the two correction mechanisms, contrast the two approaches, and include measured performance data on scanners employing each mechanism.
P Gain over Temperature (G/T) is an antenna parameter of importance in both satellite communications and radio-astronomy. Methods to measure G/T are discussed in the literature [1-3]. These methodologies usually call for measurements outdoors where the antenna under test (AUT) is pointed to the “empty” sky to get a “cold” noise temperature measurement; as required by the Y-factor measurement approach [4]. In reference [5], Kolesnikoff et al. present a method for measuring G/T in an anechoic chamber. In this approach the chamber has to be maintained at 290 kelvin to achieve the “cold” reference temperature. In this paper, a new method is presented intended for the characterization of lower gain antennas, such as active elements of arrays. The new method does not require a cold temperature reference, thus alleviating the need for testing outside or maintaining a cold reference temperature in a chamber. The new method uses two separate “hot” sources. The two hot sources are created by using two separate noise diode sources of known excess noise ratios (ENR) or by one source and a known attenuation. The key is that the sources differ by a known amount. In a conventional Y-factor measurement [4], when the noise source is turned off, the noise power is simply the output attenuator acting as a 50 ohm termination for the rest of the receive system. But by using two known noise sources, the lower noise temperature source takes the place of T-cold in the Y-factor equations. The added noise becomes the difference in ENR values. An advantage of this approach is that it allows all the ambient absorber thermal noise temperature change effects to be small factors, thus reducing one of the sources of uncertainty in the measurement. This paper provides simulation data to get an approximation of the signal loss from the probe to the antenna under test (AUT). Another critical part of the method is to correctly define the reference plane for the measurement. Preliminary measurements are presented to validate the approach for a known amplifier attached to a standard gain horn SGH) which is used as the AUT.
Highly accurate antenna measurements can require precise alignment and positioning of the probe antenna to the antenna under test. The positioning of the antenna during acquisition can involve the movement of several simultaneous axes of motion. This places a global positioning accuracy requirement on the positioning system. To achieve precision in global positioning and alignment, an understanding of dominant error factors such as load induced deflection/resonance, thermal deflection, positioning error sources and mechanical alignment tolerances is essential. This paper focuses on how global accuracy and stability were achieved, addressing these factors, on a recently delivered large far field antenna measurement system. The system involved eight axes of positioning with the ability to position 950 lbs. antenna under test 5.94 meters above the chamber floor achieving 0.84 mm and 0.027 degrees positioning accuracy relative to the global range coordinate system. Stability of the probe antenna after motion was within 0.076 mm.
NSI has developed a high precision articulated swing arm system for millimeter wave spherical near-field measurements. This paper presents this system along with structural analysis and characterization, and explains what error corrections are performed in order to produce high accuracy results. Both simulated and measurements are shown to demonstrate the effectiveness of these correction measures.
Copyright 2015 European Space Agency. Reprinted from 2015 ESA ESTEC Workshop on Antenna Measurements.
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Is there a gap between Computational and Measurement Electromagnetics? The author believes that there is. That those involved in engineering electromagnetics using numerical methods and those performing measurements of electromagnetic devices have drifted apart in the recent years. The improvement of numerical tools available commercially seems to have some part in the widening of this gap. Additionally the improvement of measurement tools and instrumentation seem to have given the community of EM metrologist, the belief of better measurements and results. As the author studied this apparent gap he believes that the ease of use of these tools has reduced the amount of training necessary. Also the author believes that the confidence on the tools has eliminated the use of a priori knowledge of the problem’s solution as well as a dose of skepticism. While computational electromagneticists seem to have a blind faith on their tools, the metrology group seem to believe that the computational results are models with little place in the real world where they perform the measurements. As a way to try to bridge this gap the author looks at a series of case studies where numerical results benefit form measurements and measurements benefit from numerical results. In conclusion, the author believes that the most important ingredient in closing the apparent gap between measurements and computed results is to question the results of the simulation or measurement and to understand if they are physical results or errors of some kind. The lack of that skepticism may be tied to the easy-to-use tools available that minimize the need for training on the underlining theory of the phenomena being measured or computed.
Copyright 2015 IEEE. Reprinted from EuCAP 2015 Conference.
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The successful design and implementation of a compact antenna test range (CATR) is predicated upon the availability of highly accurate and precise computational electromagnetic (CEM) modelling tools. As the accuracy of these models is paramount to both the design of the CATR and the subsequent determination of the facility level uncertainty budget, this paper presents an accuracy evaluation of five different CEM simulations. We report results using methods of CATR modelling including: geometrical-optics with geometrical theory of diffraction, plane-wave spectrum, Kirchhoff-Huygens and current element, before presenting results of their use in the farfield antenna pattern measurement prediction for given CATRAUT combinations.
If a planar near-field (PNF) scanner is large and there is insufficient temperature regulation in the chamber to keep ordinary thermal expansion/contraction from causing unacceptable position errors, then consideration must be given to compensation techniques that can adjust for the changes. Thermal expansion/contraction will affect almost everything in the chamber including the floor, the scanner structure, the encoder or position tapes, the AUT support, and the mount for any extra instrument(s) used to measure and correct for position error. Since the temperature will generally cycle several times during a lengthy acquisition, error-correction solutions must account for the dynamic nature of the temperature effects.
This paper describes a new automated tracking-laser compensation subsystem that has been designed and developed for very large horizontal PNF systems. The subsystem is active during the acquisition to account for both static and dynamic errors and compensates for those errors in all three dimensions. The compensation involves both open-loop corrections for repeatable errors with high spatial frequency and closed-loop corrections for dynamic errors with low spatial frequency. To close the loop, laser data are measured at a user-defined interval between scans and each scan that follows the laser measurements is fully compensated. The laser measurements are fully automated with no user interaction required during the acquisition.
The challenges, goals, and assumptions for this development are listed, the high-level implementation concept is described, and resulting measured data are presented.
Copyright 2015 IEEE. Reprinted from EuCAP 2015 Conference.
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This paper presents the results of a recent computational electromagnetic (CEM) simulation campaign for a single offset reflector CATR, where a number of models employing different field propagation methods were compared and contrasted both qualitatively and quantitatively using objective, non-local statistical image classification techniques.
The challenges, goals, and assumptions for this development are listed, the high-level implementation concept is described, and resulting measured data are presented.
Copyright 2015 IEEE. Reprinted from The Loughborough Antennas and Propagation Conference, 2015.
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Within the standard scheme for probe-corrected spherical data-processing, it has been found that for an efficient computational implementation it is necessary to restrict the characteristics of the probe pattern such that it contains only azimuthal modes for which μ = ±1 [1, 2, 3]. This first-order pattern restriction does not however extend to placing a limit on the polar index mode content and therefore leaves the directivity of the probe unconstrained. Clearly, when using this widely utilized approach, errors will be present within the calculated probe-corrected test antenna spherical mode coefficients for cases where the probe is considered to have purely modes for which μ = ±1 and where the probe actually exhibits higher order mode structure. A number of analysis [4, 5, 6, 7, 8] and simulations [9, 10, 11, 12] can be found documented within the open literature that estimate the effect of using a probe with higher order modes. The following study is a further attempt to develop guidelines for the azimuthal and polar properties of the probe pattern and the measurement configuration that can be utilized to reduce the effect of higher order spherical modes to acceptable levels. Included in this study are the cases when an Open Ended Waveguide (OEWG) is simulated at a series of measurement distances, a Quad Ridged Horn Probe (QRHP) with very large higher order modes is also simulated, the AUT is offset from the origin of the measurement sphere and the AUT is simulated with its main beam along the equator rather than along the pole. These new simulation cases provide additional guidelines when selecting a probe for spherical near-field measurements and answer some questions that have been raised about generalizing past results.
Methods for determining the uncertainty in antenna measurements have been previously developed and presented. The IEEE has published IEEE 1720-2012 that formalizes a methodology for uncertainty analysis of near-field antenna measurements. In contrast, approaches to uncertainty analysis for antenna measurements on a compact range are not covered as well in the literature. A review and discussion of the terms that affect gain and sidelobe uncertainty are presented as a framework for assessing the uncertainty in compact range antenna measurements including effects of the non-ideal properties of the incident plane wave. An example uncertainty analysis is presented.
Antennas operating at mm-wave frequencies have led to the development of spherical near-field test systems that have to function at higher frequencies than before. This paper addresses some of the factors limiting the upper frequency bound of spherical near-field test systems in terms of what is practical with current technology. This includes mechanical positioning systems, RF sub-systems and the spherical near-field sampling requirements. Correction techniques that have been developed to enhance the performance of such measurement systems are also presented.
Copyright 2015 IEEE. Reprinted from 2015 EuCAP Conference.
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This paper summarizes the design, fabrication and characterization of a coplanar waveguide fed modified aperture bowtie antenna operating in the 60 to 90 GHz range. Modifications to the bowtie edges extend the bandwidth up to 40% without increasing radiator area. The antenna was initially designed and measured in the 3-8 GHz frequency band and then frequency scaled to 60-90 GHz. The millimeter wave antenna is implemented on FR408 (r=3.65) and a multilayer laminate. Both substrates can be used in millimeter-wave system design where efficient antennas are needed. Return loss measurements of the antennas are made on a Cascade probe station. The results agree well with simulations in ANSYS HFSS. Until recently, only simulated radiation patterns were available illustrating broadside gain of 5 to 7 dB for these antennas. With the acquisition of a spherical scanner, near-field measurements have been taken of the three antennas from 67 to 110 GHz. The broadside radiation pattern results are compared with simulation. The NSI 700S-360 spherical near-field measurement system used in conjunction with an Agilent network analyzer, GGB Picoprobes and Cascade manipulator allow for on-wafer measurements of the antenna under test.
Performance requirements for compact ranges are typically specified as metrics describing the quiet zone's electromagnetic-field quality. The typical metrics are amplitude taper and ripple, phase variation, and cross polarization. Acceptance testing of compact ranges involves field probing of the quiet zone to confirm that these metrics are within their specified limits. It is expected that if the metrics are met, then measurements of an antenna placed within that quiet zone will have acceptably low uncertainty.
Various methods for determining the uncertainty in antenna measurements have been previously developed and presented for far-field and near-field antenna measurements. An uncertainty analysis for a compact range would include, as one of its terms, the quality of the field illuminating on the antenna of interest. In a compact range, the illumination is non-ideal in amplitude, phase and polarization. Error sources such as reflector surface inaccuracies, chamber-induced stray signals, reflector and edge treatment geometry, and instrumentation RF leakage, perturb the illumination from ideal.
This paper will review, in a summary fashion, the equations that estimate the effect of a non-ideal incident electromagnetic field on an antenna. It will calculate the resulting antenna pattern for a candidate antenna and compare it to the ideal antenna pattern thus showing the induced errors. Parametric studies will be presented studying the error effects of varying illumination metrics on the antenna measurement. In addition, measured field probe data from a compact range will also be used with the candidate antennas to investigate induced errors.
The intent is to provide the reader with insight as to how the typical compact range metrics affect the accuracy of an antenna measurement.
Copyright 2015 IEEE. Reprinted from EuCAP 2015 Conference.
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Achieving a very large quiet zone across a wide frequency band in a compact range system requires a physically large reflector with a suitable surface accuracy. The size of the required reflector dictates attention to several important processes such as how to manufacture the desired surface across a large area and the practicality of transportation and installation. This inevitably leads to the segmentation of the reflector into multiple panels; which must be fabricated, installed, and aligned to each other to conform to the required geometry. Performance predictions must take into account not only the surface accuracy of the individual panels, but also their alignment errors.
This paper presents the design approach taken on a recent project for a compact range system utilizing a blended rolled-edge reflector that produces a 5 meter quiet zone across a frequency range of 350 MHz to 40 GHz. It discusses the physical segmentation strategy, the fabrication methodology, the intermediate qualification of panels, the panel alignment technique, and the laser-based metrology methodology employed. Performance analysis approach and results will be presented for the geometry as conceived, and then for the realized panelized reflector as machined and aligned.
Spherical spiral scanning involves coordinating the motion of two simultaneous axes to accomplish near-field antenna measurements along a line on a sphere that does not cross itself. The line would ideally start near a pole and trace a path along the sphere to the other pole. An RF probe is moved along this path in order to collect RF measurements at predefined locations. The data collected from these measurements is used along with a near-field to far-field transformation algorithm to determine the radiated far-field antenna pattern.
The method for transforming data collected along a spherical spiral scan has been previous presented [12, 13]. Later laboratory measurement studies have shown the validity of the technique
A review of the spherical spiral scanning technique and its recent advances, resulting from about ten years of research collaboration between the UNISA Antenna Characterization Research Group and MI Technologies is here presented. Such a scan technique relies on the non-redundant sampling representations of EM fields and takes full advantage of moving two axes simultaneously. Accordingly, it allows one to drastically reduce the overall number of required data and the time to collect the data. This scanning technique can be properly applied in testing antennas mounted on automobiles in order to reduce the overall time of the measurement.
A specific set-up for probe-fed antenna pattern measurements with an articulated arm has been developed with a 500mm AUT-probe distance. This paper will give an example of far-field measurement and highlight its advantages and limitations. A near-field approach to filter the probe effect is investigated. First measurement results, including amplitude and phase patterns, will be presented. Phase data will be leveraged to develop post-processing techniques to filter probe and environmental effects.
Spherical near-field error analysis is extremely useful in allowing engineers to attain high confidence in antenna measurement results. NSI has authored numerous papers on automated error analysis and spherical geometry choice related to near field measurement results. Prior work primarily relied on comparison of processed results from two different spherical geometries: Theta-Phi (0 ≤θ≤ 180, -180 ≤ φ ≤ 180) and Azimuth-Phi (-180 ≤θ≤ 180, 0 ≤ φ ≤ 180). Both datasets place the probe at appropriate points about the antenna to measure two different full spheres of data; however probe-to-antenna orientation differs in the two cases. In particular, geometry relative to chamber walls is different and can be used to provide insight into scattering and its reduction.
When a single measurement is made which allows both axes to rotate by 360 degrees both spheres are acquired in the same measurement (redundant). They can then be extracted separately in post-processing. In actual fact, once a redundant measurement is made, there are not just two different full spheres that can be extracted, but a continuum of different (though overlapping) spherical datasets that can be derived from the single measurement. For example, if the spherical sample density in Phi is 5 degrees, one can select 72 different full sphere datasets by shifting the start of the dataset in increments of 5 degrees and extracting the corresponding single-sphere subset. These spherical subsets can then be processed and compared to help evaluate system errors by observing the variation in gain, sidelobe, cross pol, etc. with the different subset selections.
This paper will show the usefulness of this technique along with a number of real world examples in spherical near field chambers. Inspection of the results can be instructive in some cases to allow selection of the appropriate spherical subset that gives the best antenna pattern accuracy while avoiding the corrupting influence of certain chamber artifacts like lights, doors, positioner supports, etc.
A spherical near-field test system allowing for the antenna under test to remain stationary during testing is described. The system is suitable for use at mm-wave ( > 50 GHz) frequencies. Fidelity of the structure for testing at these frequencies is critical and since the structure experiences a gravitational force as a function of probe position, a complex deformation map results. There is also a radial distance variation of the probe and a technique to correct for this variation (presented before) is expounded upon. We describe the structural perturbation observed on such a scanner and assess to what extent this limits high frequency application for spherical nearfield testing.
Antenna calibrations for EMC emissions and immunity measurement require gain characterization at reduced distances. The current standards for EMC antenna calibrations do not address the near-field antenna-to–antenna interactions that are present during calibration at these reduced distances. These interactions are not present when using these antennas to measure a device and can result in large measurement errors. Extrapolation measurements have been used for many years to measure the far field gain of antennas at reduced distances. This paper uses both computations and measurements to show how the use of interpolation results in a more accurate assessment of antenna gain at distances required for EMC antenna calibrations.
Copyright 2015 IEEE. Reprinted from EuCAP 2015 Conference.
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Planar near-field antenna measurements have largely been performed within fully absorber lined anechoic chambers. However, when measuring medium to high gain antennas, one can often obtain excellent results when testing within only a partially absorber lined chamber [1], or in some cases even when using absorber placed principally behind the acquisition plane.
As absorber can be bulky and costly, its usage often becomes a significant factor when planning a new facility. This situation becomes more difficult when the designated test environment is not exclusively devoted to antenna pattern testing with non-ideal absorber coverage being, in some cases, mandated, c.f. EMC testing. Planar test systems lend themselves to deployment within multipurpose installations as they are routinely constructed so as to be portable [2] thereby allowing partial or perhaps complete removal of the test system between measurement campaigns. Many of NSI’s large planar near-field system installations are implemented with only a partially lined chamber [3]
This paper will present measured data taken using a number of different planar antenna test systems with and without anechoic chambers to summarize what is achievable and to provide design guidelines for testing within non-ideal anechoic environments. NSI’s Planar Mathematical Absorber Reflection Suppression (MARS) technique [4, 5, 6] will be utilized to show additional improvements in performance that can be achieved through the use of modern sophisticated post processing.
Antenna, Radome, and RCS measurement systems rely on high-fidelity positioner systems to provide high-precision positioning of measurement articles. The industry currently relies on linear PID control techniques in current, velocity, and position control loops on individual axes to drive the positioners. Recent control advancements have been made in the use of position feedback devices, brushless DC motors, VFD AC motors, and multi-drive torque-biased actuation along with high-speed computing in all digital controllers. Current advanced control techniques including open-loop error correction and multi-axis global error compensation have been implemented to improve positioner accuracy. Here, an assessment is conducted on the viability of advanced control techniques in similar positioner industries to provide insight into the potential future control and capabilities of positioning systems in the RF measurement industry. Candidate advanced techniques include closed-loop error compensation using laser feedback devices to provide superior positioning accuracy. Input-shaping and feedforward model-based techniques could help suppress dynamic vibrations and nonlinear behavior to improve dynamic tracking for improved continuous-measurement scanning accuracy. Gain scheduling and sliding-mode control could provide improved motion over a wider range of conditions to maintain scanning motion fidelity.
This paper will discuss a highly customizable and integrated waveform generator (WFG) subsystem used to coordinate timing and command sequences between a phased array antenna and the measurement system during antenna testing. The WFG subsystem is an automated digital pattern generator that orchestrates the command and triggering interface between an NSI measurement system and a phased array beam steering computer. The WFG subsystem is controlled directly by the NSI 2000 software and allows the test designer to select and generate a sequence of up to sixteen unique synchronized timing waveforms. Test scenarios, results and data for the WFG subsystem will be presented along with plots showing the key timing characteristics of the system.
Copyright 2014 IEEE. Reprinted from EuCAP 2014 Conference.
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Applications using millimeter-wave antennas have seen a strong growth in recent years. Examples are wireless HDTV, automotive radar, imaging and space communications. NSI has delivered dozens of antenna measurement systems operating at mm-wave frequencies. These are all based on standard mm-wave modules from vendors such as OML, Rohde & Schwarz and Virginia Diodes. This paper will present considerations for implementation of these systems, including providing the correct power levels, and interoperability with coaxial solutions. Many new mm-wave applications employ low to medium gain antennas that are more suited to spherical near-field and far-field measurement geometries. NSI has developed solutions to meet these needs.
When performing far-field or near-field measurements on large antennas, it is often necessary to consider the advantages and disadvantages of various mechanical positioning configurations for achieving the required hemispherical scan coverage. Measurement systems employing a single-arm gantry, a dual-arm gantry, a fixed arch moving probe, and a fixed arch multi-probe have been paired with either an azimuth positioner or a vehicle turntable in order to provide hemispherical scanning of the article being tested.
This paper will highlight the key characteristics of various scanning methods and provide comparisons among the different techniques. Positioning and system accuracy, speed, stowing capability, calibration, frequency range, upgradability, relative cost and other key aspects of the various techniques will be discussed in detail to help the end user during the system design and selection process. In addition, the paper will highlight novel hemispherical and truncated spherical scanning approaches.
In many applications, the success of meeting the measurement requirements often centers on the judicious selection of the positioning subsystem. This paper will provide guidance toward making the proper selection of the scanning concept as well as of the positioning system.
Several methods have been proposed to identify potential sources of Spherical Near-Field measurement uncertainties, including simulated, empirical, and analytical studies. One of the goals of such work is to understand the degree to which the measured results of a given system should be trusted. Another important benefit can be obtained by applying uncertainty analysis during system design to achieve better performance in the realized system.
This paper shows how a Roll over Azimuth positioning system was analyzed to determine the major mechanical contributors to uncertainty in a W-band Spherical Near-Field system. The results of the analysis were used to target specific improvements to components of the positioning system. These changes resulted in better measurements at minimal incremental cost, yet without resorting to the expense of high accuracy position feedback. The relationship between the primary mechanical sources of uncertainty and the quality of the Spherical Near-Field measurement is described. This example illustrates the detailed work behind uncertainty analysis and shows its value in making appropriate design decisions.
Copyright 2014 IEEE. Reprinted from EuCAP 2014 Conference.
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Band limiting radiating fields from a finite sized field distribution has been shown to be a highly effective way to eliminate spurious scattered fields from antenna measurements [1, 2, 3]. These techniques have been used with impressive results in many antenna measurement geometries including spherical, cylindrical and planar near-field and far-field [3, 4, 5, 6]. Generally, the objective verification of these suppression techniques on scattered fields can be readily demonstrated, while their impact on the overall facility level uncertainty budget has perhaps been less clear.
The use of the NIST 18 term range assessment [7, 8] for error analysis of planar near-field antenna test facilities has become a widely accepted technique for antenna measurement error evaluation. This technique identifies the overall effect on the measurement as well as each of the 18 terms individually. Thus, range assessment evaluation provides an effective way to evaluate the impact of planar MARS processing on a given antenna measurement or planar near-field facility. This paper presents results from a recent range assessment campaign that illustrates and quantifies the impact of MARS processing on the facility level error budget on a large planar near-field antenna test system.
Copyright 2014 IEEE. Reprinted from EuCAP 2014 Conference.
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Mathematical Absorber Reflection Suppression (MARS) is a well-established, widely used measurement and postprocessing mode orthogonalization and filtering technique [1, 2] that has been extensively used to locate and then supress measurement errors arising from scattered fields when antenna testing is performed in echoic environments. Furthermore, it has been shown that this form of processing has reduced those uncertainties associated with bias leakage error, second order truncation effects, and mutual coupling (i.e. multiple reflections between the test antenna and the probe) leading to a worthwhile reduction in the overall range uncertainty budget. The success of MARS, and other mode orthogonalization and filtering strategies [3], is dependent upon the behaviour of the orthogonal vector wave functions (that are used to expand the electromagnetic fields) under an isometric translation of co-ordinate systems. This translation of origins is applied as part of the digital post-processing with the resulting mode orthogonalization being observed irrespective of whether plane, cylindrical, or spherical elemental vector mode bases are used. Within this paper and presentation, simulated and measured data will be used to demonstrate the power and flexibility of this technique when correcting measured data highlighting the specific behaviour of the various commonly used vector wave functions.
Copyright 2014 IEEE. Reprinted from CEM 2013 Conference.
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If a planar near-field (PNF) scanner is large and there is insufficient temperature regulation in the chamber to keep ordinary thermal expansion/contraction from causing unacceptable position errors, then consideration must be given to compensation techniques that can adjust for the changes. Thermal expansion/contraction will affect almost everything in the chamber including the floor, the scanner structure, the encoder or position tapes, the AUT support, and the mount for any extra instrument(s) used to measure and correct for position error. Since the temperature will generally cycle several times during a lengthy acquisition, error-correction solutions must account for the dynamic nature of the temperature effects.
This paper describes a new automated tracking-laser compensation subsystem that has been designed and developed for very large horizontal PNF systems. The subsystem is active during the acquisition to account for both static and dynamic errors and compensates for those errors in all three dimensions. The compensation involves both open-loop corrections for repeatable errors with high spatial frequency and closed-loop corrections for dynamic errors with low spatial frequency. To close the loop, laser data are measured at a user-defined interval between scans and each scan that follows the laser measurements is fully compensated. The laser measurements are fully automated with
The challenges, goals, and assumptions for this development are listed, high-level implementation considerations are provided, and resulting measured data are presented.
Broad band, dual polarized probes are becoming increasingly popular options for use in near-field antenna measurements. These probes allow one to reduce cost and setup time by replacing several narrowband probes like open-ended waveguides (OEWG) with a single device covering multiple waveguide bands. These probes are also ideal for production environments, where chamber throughput should be maximized. Unfortunately, these broadband probes have some disadvantages that must be quantified and corrected for in order to make them viable for high accuracy near-field measurements. Most of these broadband probes do not have low cross polarization levels across their full operating bandwidths and may also have undesirable artifacts in the main component of their patterns at some frequencies. Both of these factors will result in measurement errors when used as probes. Furthermore, the use of a dual port RF switch adds an additional level of uncertainty in the form of port-to-port channel balance errors that must be accounted for. This paper will describe procedures to calibrate the pattern and polarization properties of broad band, dual polarized probes with an emphasis on a newly developed polarization correction algorithm. A simple procedure to measure and correct for amplitude and phase imbalance entering the two ports of the near-field probe will also be presented. Measured results of the three calibration procedures (pattern, polarization, channel balance) will be presented for a dual polarized, broad band quad-ridged horn antenna. Once calibrated, this probe was used to measure a standard gain horn (SGH) and will be compared to baseline measurements acquired using a good polarization standard OEWG. Results with and without the various calibration algorithms will illustrate the advantage to using all three routines to yield high accuracy far-field pattern data.
In recent years a number of analyses and simulations have been published that estimate the effect of using a probe with higher order azimuthal modes with standard probe corrected spherical transformation software. In the event the probe has higher order modes, errors will be present within the calculated antenna under test (AUT) spherical mode coefficients and the resulting asymptotic far-field parameters [1, 2, 3, 4]. Within those studies, a computational electromagnetic (CEM) simulation tool was developed to calculate the output response for an arbitrary AUT/probe combination where the probe could be placed at arbitrary locations on the measurement sphere ultimately allowing complete near-field acquisitions to be simulated. The planar transmission equation was used to calculate the probe response using the plane wave spectra for actual AUTs and probes derived from either planar or spherical measurements. The planar transmission formula was utilised as, unlike the spherical analogue, there is no limitation on the characteristics of the AUT or probe thereby enabling a powerful, entirely general, model to be constructed. This paper further extends this model to enable other measurement configurations and errors to be considered including probe positioning errors which can result in ideal first order probes exhibiting higher order azimuthal mode structures. The results of these additional simulations are presented and discussed.
Performance requirements for compact ranges are typically specified as metrics describing the quiet zone's electromagnetic-field quality. The typical metrics are amplitude taper and ripple, phase variation, and cross polarization. Acceptance testing of compact ranges involves field probing of the quiet zone to confirm that these metrics are within their specified limits. It is expected that if the metrics are met, then measurements of an antenna placed within that quiet zone will have acceptably low uncertainty. However, a literature search on the relationship of these parameters to resultant errors in antenna measurement yields limited published documentation on the subject.
Various methods for determining the uncertainty in antenna measurements have been previously developed and presented for far-field and near-field antenna measurements. An uncertainty analysis for a compact range would include, as one of its terms, the quality of the field illuminating on the antenna of interest. In a compact range, the illumination is non-ideal in amplitude, phase and polarization. Error sources such as reflector surface inaccuracies, chamber-induced stray signals, reflector and edge treatment geometry, and instrumentation RF leakage, perturb the illumination from ideal.
This paper will review, in a summary fashion, the equations that estimate the effect of a non-ideal incident electromagnetic field on an antenna. It will calculate the resulting antenna pattern for a candidate antenna and compare it to the ideal antenna pattern thus showing the induced errors. Parametric studies will be presented studying the error effects of varying illumination metrics on the antenna measurement. In addition, measured field probe data from a compact range will also be used with the candidate antennas to investigate induced errors.
The intent is to provide the reader with insight as to how the typical compact range metrics affect the accuracy of an antenna measurement. This work is intended to be the foundation for future work to develop a comprehensive uncertainty analysis for compact range measurements.
EIRP (Equivalent Isotropically Radiated Power) and SFD (Saturating Flux Density) are two system level parameters often sought during test campaigns. Measurement of these under far-field conditions is well defined and common. Although valid techniques for measuring these parameters on near-field ranges exist, these techniques are not commonly used. This paper presents measurement techniques for both parameters in detail in an attempt to illustrate the process. The techniques presented are also valid if the source (in the EIRP case) or receiver (in the SFD case) ports of the device under test are inaccessible.
The standard numerical analysis used for efficient processing of spherical near-field data requires that the far-field pattern of the probe can be expressed using only azimuthal spherical modes with indices of μ = ±1 [1, 2, 3]. In this commonly used approach, the probe is assumed to have only modes for μ = ±1, and if the probe has higher order modes, errors will be present within the calculated AUT spherical mode coefficients and the resulting asymptotic far-field parameters. In the event that the probe satisfies this symmetry requirement, then nearfield data is only required for two angles of probe rotation about its axis of χ = 0 and 90 degrees and numerical integration in χ is not required. This reduces both measurement and computation time as only two orthogonal tangential near electric field components are sampled and processed. Thus, it is highly desirable to use probes that satisfy the μ = ±1 criteria. Circularly symmetric probes can be constructed that reduce the higher order modes to very low levels. Examples of these devices include cylindrical waveguide probes that are excited by the TE11 fundamental mode. However for probes using open ended rectangular waveguides (OEWG) the effect of the higher order modes can also be limited by using a measurement radius that reduces the subtended angle of the AUT at the probe.
Some analysis and simulation have been published that estimate the effect of using a probe with higher order modes [4, 5, 6, 7, 8] and the following study is another effort to develop further guidelines for the properties of the probe and the measurement radius that will reduce the effect of higher order azimuthal modes to acceptable levels. Previous simulation studies [9, 10, 11] have focused primarily on the effect of higher order azimuthal probe modes in rectangular OEWG probes. These showed that for radii of twice the maximum radial extent (MRE) of the test antenna the differences in the near-field, and far-field, are on the order of -50 dB below the peak amplitudes. For larger measurement radii, the differences were found to be below -60 dB. In contrast to the previously published work, this paper presents the results of a similar study in which a broadband dual ridged horn antenna was used as a near-field probe for spherical testing. Such probes are often utilized for spherical near-field testing as they have wide frequency bandwidths that cover several, typically three or more, rectangular or circular waveguide bands. Thus, the use of such devices is attractive as they can greatly simplify measurement setup, and minimize test times. Thus, although it does not satisfy the μ = ±1 criteria, these probes are widely used within the antenna measurement community. However, comparatively little information is available within the published open literature regarding specific guidelines for the properties of the probe and the measurement radius needed that will reduce the effect of higher order azimuthal modes to acceptable levels and is the motivation for this work. The results of these additional simulations are presented and guidelines developed to aid in the choice of spherical near-field probes and measurement radii for typical antennas are presented and discussed.
Copyright 2014 IEEE. Reprinted from EuCAP 2014 Conference.
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Two efficient techniques to compensate known positioning errors in a nonredundant spherical near-field – far-field (NF–FF) transformation, using a cylinder ended in two halfspheres for modelling long antennas, are developed and experimentally assessed in this communication. In order to evaluate the NF data at the points fixed by the nonredundant sampling representation from the acquired irregularly spaced ones, the former approach uses the singular value decomposition method and can be applied when the nonuniformly distributed samples lie on nonuniform parallels. The latter, based on an iterative technique, can be adopted even if such a hypothesis is not satisfied, but requires the existence of a biunique correspondence associating at each uniform sampling point the nearest nonuniform one. Once the uniform samples have been retrieved, those needed by the classical spherical NF–FF transformation are efficiently determined via an optimal sampling interpolation algorithm.
Copyright 2014 University of Salerno. Reprinted from AMTA 2014 Conference.
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For the purposes of antenna or radome measurement, a gimbal may be thought of as a compact, two or three axis antenna positioner with mutually orthogonal, intersecting axes. The unrelenting demand for higher accuracy in positioners of this type is driving innovation in mechanical architecture and design. Refined position feedback techniques, reflecting enhanced understanding of position errors, and delivering unprecedented native encoder accuracy, have been developed and tested. New mechanical architecture has been created that allows for fullyfeatured two-axis gimbals to exist in the restricted confines behind an aircraft radome. The principal result of these developments is increasingly accurate and capable systems, particularly in the field of radome measurements. These new applications, techniques, architectures, and their results are explored in the following pages.
Radar antennas are typically required to operate in using various transmit and receive conditions, depending on the particular radar mode. In active antenna applications, these modes may require different antenna operating parameters, which currently dictate testing the antenna independently in transmit and receive using different test system configurations. In testing highly-integrated active arrays, electrical and thermal considerations make it desirable to test the antenna in its nominal Tx/Rx (Transmit/Receive) operating mode as opposed to transmit-only or receive-only. An extension to the NSI Panther 9100 RF measurement system has been developed to support multiplexed transmit and receive, pulse-mode measurements with different measurement parameters during the course of a single data acquisition. This capability allows pulsed transmit and receive tests to be interleaved using a single measurement setup, reducing overall test time and improving the real-world accuracy of the test results.
Copyright 2014 IEEE. Reprinted from EuCAP 2014 Conference.
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This paper investigates practical aspects of measuring antennas operating with pulsed RF signals of very narrow width, down to 0.1 μs or less. The current generation of network analyzers provides very high IFBW, theoretically allowing measurement of very narrow pulses. But in practice there are several factors that limit the minimum pulse width, including IF path filtering, receiver dynamic range, trigger synchronization issues and test/reference path delays. The paper will compare the performance of both narrow-band and wideband pulse measurements on a Keysight PNA.
This paper describes an articulated arm spherical near-field scanner design which transports a probe over a hyperhemispherical surface that surrounds a stationary test antenna. Surface profile data collected with a laser tracker is presented and a parametric study performed to investigate the viability of testing at mm-wave frequencies is described. Parameters such as probe radial distance, and angular positioning are investigated to assess to what extent spherical near-field testing can be performed using this structure.
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Introduction, Vertical Half-Loop Antenna, Inverted-L Antenna, Scaled Model Design, Measurements, Summary
Near-field antenna measurement accuracy is reduced by various factors associated with the antenna test range environment and instrumentation hardware used. One such factor is associated with energy leakage in the RF hardware used in the measurement. In spherical near-field measurements, the sidelobe accuracy can be greatly impacted by RF leakage.
In this brief note on the use of spherical modal filtering and the IsoFilterTM technique, it will be shown how the IsoFilterTM technique can be used to reduce the impact of leakage and stray signals in spherical near-field measurements.
Copyright 2014 IEEE. Reprinted from EuCAP 2014 Conference.
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A truncation study is presented to assess the impact of theta truncation in a spherical near-field antenna test system. The study is relevant for antennas with broad beams in the plane of truncation and where antenna weight prevents alternate mounting options, while facility space limitations necessitate truncation.
Copyright 2014 IEEE. Reprinted from ANTEM 2014 Conference.
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During recent years, far-field mathematical absorber reflection suppression (FF-MARS) has become a widely adopted and accepted technique for suppressing spurious range reflections within far-field and compact antenna test ranges (CATR) [1, 2, 3, 4]. Far-field measurements are particularly prone to the effects of range-reflections [5] which, when combined with the increase in signal to noise ratio that MARS often provides, makes this form of post-processing particularly beneficial to these applications [6]. FF-MARS processing is rigorous with its theoretical basis being resolutely founded within standard cylindrical near-field theory [6, 7]. FF-MARS offers the unique, and particularly pertinent for far-field applications, attributes of being able to process one-dimensional, singularly polarised, mono-chromatic, frequency-domain, far-field data without approximation or loss of generality. Hitherto, for cases where two-dimensional far-field data needed processing, recourse to standard spherical-MARS (S-MARS) was unavoidable [8, 9]. However, there are occasions when complex spherical mode based post-processing is unavailable, undesirable, or even inappropriate (such as when only a small portion of the far-field sphere is acquired, or when only a single polarisation component is available) and under these circumstances the ability to process two-dimensional data with this new FF-MARS technique can become highly desirable. For the first time, this paper shows how the existing one-dimensional FF-MARS technique can be extended to enable two-dimensional data to be processed with the success of the measurement and post-processing technique being illustrated with results obtained from actual range measurements.
Copyright 2014 IET. Reprinted from The Loughborough Antennas and Propagation Conference, 2014.
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Most antenna measurement systems are designed to acquire and process amplitude and phase data from a transmitting or receiving antenna in order to determine the antenna pattern, gain, sidelobes and other parameters of interest. The antenna under test (AUT) is typically assumed to have fixed operating parameters, except in the case of phased arrays with beam-steering capabilities. These phased array antennas are capable of changing the antenna transmit and receive characteristics under program control and present a unique challenge for the designer of antenna measurement systems. A key requirement for phased array antenna testing is that the RF stimulus and receiver measurement interval must be coordinated with the AUT timing and control commands. Optimization of this measurement process requires the measurement system to be capable of interfacing directly with the AUT beam-steering computer (BSC) in order to coordinate the various timing and control signals.
This paper will discuss a highly customizable and integrated waveform generator (WFG) subsystem used to coordinate the phased array test process. The WFG subsystem is an automated digital pattern generator that orchestrates the command and triggering interface between the NSI measurement system and a phased array beam steering computer. The WFG subsystem is controlled directly by the NSI 2000 software and allows the test designer to select and generate a sequence of up to sixteen unique synchronized timing waveforms. Test scenarios, results and data for the WFG subsystem will be presented along with plots showing the key timing characteristics of the system.
What is achievable in compact range performance is usually constrained by several factors. The desire for lower frequency performance must be weighed against the economics of reflector and chamber size. The desire for higher frequency performance puts demands on the reflector’s surface accuracy. Consistency of performance across a waveguide band levies demands on compact range feeds.
This paper addresses a recent compact range development by MI Technologies that achieves desired extended low frequency and millimeter wave performance (1 to 110GHz) while maintaining a cost effective reflector size and a small range footprint. The paper will explore the conventional rule-of-thumb relationships between feed, reflector, edge treatments and range geometries while contrasting them to the resultant design. The paper will highlight an impressive new family of compact range feeds and advancements in cost effectively achieving a superior reflector surface.
Over the years, spherical near-field (SNF) antenna measurements have become increasingly popular for characterizing a wide variety of antenna types. The SNF configuration allows one to measure data over a sphere surrounding the antenna, which provides it a unique advantage over planar and cylindrical near-field systems where measurement truncation is inherent. Like all antenna measurement configurations, SNF systems are susceptible to a number of measurement errors that, if not properly understood, can corrupt the antenna’s far-field parameters of interest (directivity, beamwidth, beam pointing, etc.). The NIST 18-term error assessment originally developed for planar near-field measurements [1] has been adapted for SNF systems [2] and provides an accurate measure of the uncertainty in a particular SNF measurement. Once particular measurement errors are known, steps can be taken to reduce their impact on far-field radiation patterns. When manually assessing all 18 terms of the NIST uncertainty budget this procedure becomes tedious and time consuming.
This paper will describe an acquisition algorithm that allows one to analyze all 18 error terms or a subset of those in automated fashion with minimal user intervention. Building upon previous research toward developing an automated SNF error assessment algorithm [3, 4], this new procedure will automatically generate tabulated and plotted uncertainty data for directivity, beamwidth and beam pointing of a particular farfield radiation pattern. Once measurement uncertainties are known, various post-processing techniques can be applied to improve far-field radiation patterns. Results will be shown for three antennas measured on large phi-over-theta SNF scanners.
Copyright 2013 IEEE. Reprinted from The Fifth European Conference on Antennas and Propagation (EuCAP 2013) 08-12 April 2013.
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As part of a target simulator, a linearly polarized signal was required with a variable tilt angle that could be controlled electronically and changed at a 1 kHz rate. The signal simulates the effect of rapid polarization changes that a missile might encounter in real time during flight.
Tilt angles can be varied by adjusting the amplitude of the vertical and horizontal inputs to an orthomode transducer. To produce good cross-polarization, independent phase adjustments are also required. However, microwave components available in the 33.4 – 36 GHz operating range were inadequate to achieve the desired performance.
A novel approach was developed to downconvert the input signal to a lower frequency range and use vector modulators available in the lower band to produce the appropriate phase and amplitude changes in each path, then upconvert back to the desired operating frequency to drive the orthomode transducer. A device was built and tested using this approach.
A calibration and measurement procedure was developed to determine the vector modulator input settings that produced the most accurate tilt angles and best cross-polarization performance. By iteratively measuring cross-polarization and tilt angle, then adjusting the vector modulator controls, a tilt angle accuracy of +/-1 degree was achieved with a cross-polarization of -25 dB, exceeding the required performance.
By implementing the architecture described, both the phase and amplitude of the horizontal and vertical signals to the orthomode transducer can be controlled. In addition to linearly polarized signals, other types of polarization signals can also be generated, including left-hand and right-hand circular, as well as the general case of elliptical polarization.
Copyright 2013 IEEE. Reprinted from EuCAP 2013 Conference.
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Antenna test engineers are faced with testing increasingly complex antenna systems, one of these being the AESA (Active Electronically Steered Array) antennas used for cell communications, jammers, and radars. Often these antennas have integrated electronics and RF components that are an intricate part of the antenna, and as a result must be tested with the waveforms generated by the antenna itself. One cannot simply inject an unmodulated continuous wave signal. These antennas require new measurement techniques which are compatible with their broadband waveforms.
The reference channel of a measurement receiver can be used to collapse the spectrum of the modulated signal into a single CW measurement. Done properly all the energy in the signal is captured with noise and interference being dispersed, resulting in no loss of DR (dynamic range) over a CW measurement. A receiver employing this technique can capture all the energy in modulated and pulsed signals wielding wide dynamic range measurements. Phased locked loops (PLL) are not used as they can preclude such measurements.
A measurement receiver that uses a digital correlator to collapse the spectrum of modulated and pulsed signals will be presented. This paper will describe the technique used to do this and show measured results on example broadband signals.
The measurement and post-processing mode orthogonalisation and filtering technique, named Mathematical Absorber Reflection Suppression (MARS) [1, 2], has been used extensively to identify and subsequently extract measurement artefacts arising from spurious scattered fields that are admitted when antenna testing is performed in non-ideal anechoic environments. Underpinning the success of the MARS postprocessing, and other mode orthogonalisation and filtering strategies [3], is the behaviour of the orthogonal vector wave (mode) expansions that are employed to describe the radiated fields and in particular their behaviour under the isometric coordinate translations that are central to the post processing. Within this paper, simulated and measured data will be used to illustrate the applicability of this measurement and post processing technique paying particular attention to the behaviour of the various modal expansions examining and confirming specific, commonly encountered, measurement conventions.
Copyright 2013 IEEE. Reprinted from The Loughborough Antennas and Propagation Conference, 2013.
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There are several applications in which knowledge of the location of the phase center of an antenna, and its twodimensional variation, is an important feature of its use. A simple example occurs when a broad-beam antenna is used as a feed for a reflector, where the center of the spherical phase fronts should always lie at the focal point of the paraboloidal surface. Here, the ability to determine the phase center of the feed from knowledge of its far-field phase/amplitude pattern is critical to the reflector’s design.
Previously published methods process a single cut of data at a time, yielding 2D lateral and longitudinal phase-center offsets. Eand H-plane cuts are thus processed separately, and will, in general, yield different answers for the longitudinal offset. The technique presented here can process either one line cut at a time or a full Theta-Phi raster. In addition, multiple frequencies can be processed to determine the average 3D phase-center offset. The technique can merely report the phase-center location, or it can also adjust the measured phases to relocate the origin to the computed phase center. Example results from measured data on multiple antenna types are presented.
Nearfield Systems Inc. (NSI) has been contracted by the Department of Electrical and Computer Engineering of the University of Waterloo to install a unique antenna test system with multiple configurations allowing it to characterize a wide variety of antenna types over a very wide bandwidth. The system employs a total of 10 positional axes to allow near-field and far-field testing in various modes of operation with great flexibility. A 4 m x 4 m planar near-field (PNF) scanner is used for testing directive antennas operating at frequencies up to 110 GHz with laser interferometer position feedback providing dynamic probe position correction. The PNF’s Y-axis can also be used for cylindrical near-field (CNF) testing applications when paired with a floor mounted azimuth rotation stage. A single phi-over-theta positioner permits both spherical near-field (SNF) testing from L-band to W-band and far-field testing down to 0.2 GHz. This positioner is installed on a translation stage allowing 1.8 m of Z-axis travel to adjust the probe-to-AUT separation. In addition, a theta-over-phi swing arm SNF system is available for testing large, gravitationally sensitive antennas that may be easily installed on a floor mounted rotation stage. In order to ensure system and personnel safety, a complex interlock system was designed to reduce the risk of mechanical interference and ease the transition from one configuration to another. The system installation and validation was completed in March 2013. We believe that this facility is unique in that it encompasses all commonly used near-field configurations within one chamber. It therefore provides a perfect environment for the training of young engineers and could potentially form the baseline of future academic test facilities. This paper will outline the technical specifications of the scanner and discuss the recommended applications for each configuration. It will also describe the details of the safety interlock system.
This paper extends a previous simulation study (1, 2) of the effect of higher order probe modes when the spherical numerical software uses the orthogonality approach to solve for the spherical modes of the AUT. In this commonly used approach, the probe is assumed to have only modes for μ = ±1, and if the probe has higher order modes, errors will be present in the calculated AUT spherical coefficients and the resulting far-field parameters. In the previous studies, a computer simulation was developed to calculate the output response for an arbitrary AUT/probe combination when the probe is placed at arbitrary locations on the measurement sphere. The planar transmission equation was used to calculate the probe response using the plane wave spectra for actual AUTs and probes derived from either planar or spherical near-field measurements. The positions and orientations of the AUT and probe were specified by a combination of rotations of the antenna’s spectra and the x, y, z position of the probe used in the transmission equation. The simulation was carried out for rectangular Open Ended Waveguide (OEWG) probes using all of the higher order modes and also for the same probe where only the μ = ±1 modes were used to calculate the probe patterns. The parameter that was used to estimate the error in the measured near-field data was the RMS combination of the complex differences between near-field polarization curves over a χ rotation span of 100º. This RMS combination represented the estimated error signal level relative to the peak near-field amplitude. Using two different AUTs, different measurement radii and a sequence of θ-positions on the measurement sphere, the error signal levels were between -35 and -80 dB and the initial conclusion was that the effect of the higher order modes on typical measurements using OEWG probes would be smaller than other typical measurement errors and therefore have little practical effect on far-field results. In this phase of the study the goal was to develop general guidelines to predict the error signal level for a given AUT/probe/measurement radius combination. The same simulation software was used in this study with the following changes and additions. Rather than use all of the points in the polarization curves to derive an RMS error signal level, only the χ-rotation angles of 0º and 90º were used since these are the only two probe rotation angles used in a typical spherical near-field measurement. In addition to deriving the error estimates for specific spherical angles and measurement radii, complete sets of near-field data were derived for some cases, the far-fields calculated and compared to derive estimates of far-field error levels.
The results of these simulations are presented and guidelines developed to aid in the choice of spherical near-field probes and measurement radii for typical antennas.
The use of pre-conditioning interpolation schemes, as a possible means of enhancing the performance of previously introduced adaptive acquisition algorithms for spherical near-field (SNF) test time reduction, is evaluated. Investigations have been carried out to establish whether the adaptive SNF approach is suited to test engineering practice are reported. The pre-conditioning method involving the acquisition of two orthogonal polar cuts on the near-field sphere and the separate linear interpolation of two complex spherical components of the NF data is shown to be the preferred scheme. This method is evaluated for three different antennas using specific acquisition rules, and decision functions related to directivity, amplitude error, and side lobe level.
A diagnostic technique was published over 20 years ago on imaging compact-range reflectors by focusing plane-polar field-probe data. At that time, only synthesized data had been evaluated. Since then, a few reflectors have exhibited performance lower than expected, and this technique has been successfully employed to improve that performance based on their measured data. This paper reviews the technique and discusses the results of processing those measured data sets.
The technique produces an image of the estimated field amplitudes at the reflector surface that do not contribute to the desired quiet-zone plane wave. Point sources, line sources, and deformations over an area have all been successfully identified, often outside the projected circular boundary of the field-probe data. All measurements to date have used very coarse angular spacing with acceptable degradation in image quality.
In this communication, the experimental verification of a probe compensated near-field – far-field (NF–FF) transformation with spherical spiral scanning particularly suitable for elongated antennas is provided. It is based on a nonredundant sampling representation of the voltage measured by the probe, obtained by using the unified theory of spiral scans for nonspherical antennas and adopting a cylinder ended in two half-spheres for modelling long antennas. Its main characteristic is to allow a remarkable reduction of the measurement time due to the use of continuous and synchronized movements of the positioning systems and to the reduced number of required NF measurements. In fact, the NF data needed by the classical NF–FF transformation with spherical scanning are efficiently and accurately reconstructed from those acquired along the spiral, by employing an optimal sampling interpolation formula. Some experimental results, obtained at the Antenna Characterization Lab of the University of Salerno and assessing the effectiveness of such a NF–FF transformation technique, are presented.
Copyright 2013 The University of Salerno. Reprinted from AMTA 2013 Conference
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There are a number of near-field measurement scenarios where the use of broad band probes is desirable. These probes allow one to make the most efficient use of chamber occupancy time by covering a wide bandwidth using a single probe in place of a collection of narrow band probes. Dualpolarized probes allow one to further reduce test time by a factor of two by eliminating the need to rotate the probe by 90 degrees to perform two-polarization antenna measurements. However, the reduction in test times yielded using these probes can also lead to a decrease in performance if the probe is not properly calibrated. This paper will describe a procedure to calibrate both the pattern and polarization properties of broad band, dualpolarized probes for use in near-field antenna measurements. Results of the calibration procedure for two of these probes will be presented here. Once calibrated, these antennas were used to measure the performance of standard gain horns (SGH) and compared to baseline measurements acquired using a good polarization standard open-ended waveguide (OEWG). Examples of these results from 300 MHz to 12 GHz will be presented.
Copyright 2013 IEEE. Reprinted from EuCAP 2013 Conference.
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The planar near-field technique is one of the most widely used, methods for measuring electrically large, medium to high gain antennas [1, 2]. Plane rectilinear systems consist of two intersecting, orthogonal linear translation stages which produces data in a convenient regularly spaced rectilinear co-ordinate system. Although the plane rectilinear geometry is by far the most commonly encountered implementation, plane-polar [2, 3] and plane-bipolar [4] geometries can also be constructed using mechanically convenient, commercially available, positioners. Plane-polar acquisition systems typically comprise the intersection of a rotation stage mounted behind the AUT, with a linear probe translation stage acting as a radial arm. This geometry yields data tabulated in a plane-polar co-ordinate system. Although comparatively rare in industry, plane-polar systems are important because they present certain distinct advantages [3, 5]. They do however pose some unique challenges within their implementation. As with all near-field methodologies, accurate and precise probe positioning is of paramount importance to the success of the technique and comprises an important term within the facility level uncertainty budget [6, 7]. Clearly, this is equally valid for the planar-polar technique with the hybrid angular/linear positioning system presenting unique challenges to the mechanical alignment.
This paper will describe newly developed mechanical and electrical alignment techniques for use with plane-polar near-field test systems. A simulation of common plane-polar alignment errors will illustrate, and quantify, the alignment accuracy tolerances required to yield high quality far-field data, as well as bounding the impact of highly repeatable systematic alignment errors. The new plane-polar electrical alignment technique comprises an adaptation of the existing, widely used, spherical near-field electrical alignment procedure [8] and can be used on small, and large, plane-polar near-field antenna test systems.
Panasonic Avionics Corporation has developed an innovative antenna design that provides Ku-band connectivity for its In Flight Entertainment and Communications (IFEC) systems. Traditional approaches suffer outages when crossing the equator due to Adjacent Satellite Interference (ASI). Through the addition of a second antenna panel, Panasonic has both increased the overall performance and eliminated the ASI problem by cutting the elevation beam width by 50% when operating in the tropics and when crossing the equator. Due to the great success of its IFEC systems, Panasonic had a need to dramatically increase the production and test capacity for its antenna all while transitioning the manufacturing to a new facility in Lake Forest California.
MI Technologies has delivered two new state of the art compact range measurement systems and has worked with Panasonic to develop automated test systems that have reduced the test time by more than a factor of 4. The range design includes significant automation, integration with the antenna’s built in up and down converters, and the ranges are reversible.
More complex antennas with higher transmit power levels are being tested in compact range environments. AESA’s and other phased array antennas can transmit significant power levels from a relatively small volume. Without consideration of the impact of the transmitted power levels for a given test article, human and facility safety could be at risk. This paper addresses designing a test chamber in light of these power handling considerations for high power antennas on two fronts: 1) A methodology is presented to determine the power levels seen by surfaces in the chamber that are covered with absorber material and 2) Calculating the power levels seen at the compact range feed due to the focusing effect of the compact range itself. A test case is presented to show the application of the methods.
Radar Cross Section (RCS) test systems typically employ 2-axis compact positioners mounted atop low-observable support structures. The positioners are most often configured as azimuth over elevation, and are referred to as rotators. The support structures, called pylons, are built with very specific geometry that exhibits extremely low RCS. The rotator/pylon system mounts a model, often a full size aircraft, and presents it to the RCS measurement system in various spatial orientations.
The need to maintain very low observability, along with the need to manipulate the model through a large range of motion, result in a challenging set of problems. These have been effectively addressed over decades of RCS equipment design. In recent years however, RCS applications have become much more demanding. Models are ever larger and heavier, with length exceeding 150 feet, and with weight up to 50,000 lbs. Required accuracy with some applications has increased to ±0.01°, an increase of 67% as compared to legacy values.
MI Technologies has developed products that significantly expand the structural and operational envelopes of rotator/pylon systems to meet the demand for higher performance. This paper presents the various challenges encountered in RCS Rotator and Pylon design, and the innovative solutions that have arisen from recent engineering efforts.
Several gain measurement techniques exist for near-field antenna ranges. These include Comparison-gain, Direct-gain and Three-antenna gain methods. Each technique has its own unique advantages and disadvantages in terms of accuracy, cost and measurement time. Range operators must understand the differences between these techniques in order to properly configure their test system to best suit their requirements. This paper surveys each of the gain techniques and identifies the relative advantages of each. As part of the survey, all three techniques were performed on three types of near-field antenna measurement systems: Planar, Cylindrical and Spherical. The results of this paper provide the reader with a practical understanding of each technique, the formulas required, and real-world examples for the trade-offs needed to outfit a range for fast and accurate gain measurements while balancing cost and schedule.
Attempts to produce robust, objective, and quantitative measures of similarity between antenna pattern data sets using statistical methods have been widely reported in the open literature [1, 2]. Hitherto, such techniques have primarily been restricted to the purposes of comparing two or more images as a means in itself. However, no measurement can be considered to be completely free from error, and as such each data set inevitably contains an associated uncertainty. Therefore, in contrast to previous work, this paper discusses and extends some commonly used comparison techniques to take account of the finite, non-zero, measurement uncertainties that complicate the comparison process. Results are presented that illustrate the effectiveness of the comparison method and conclusions drawn.
Northrop Grumman Aerospace Systems (NGAS), working with Nearfield Systems Inc. (NSI) and others, has installed a state-of-the-art near-field antenna measurement system to test various payload antenna systems. This horizontal planar near-field system was designed to measure antennas with up to 30’ diameter apertures. In addition, a second bridge was included in the design so that the range can operate either as one very large scanner or as two autonomous ranges and double the testing throughput of the range. This near-field system features a large scan plane of nearly 40 ft. x 47 ft. with two smaller scan planes of 17’ x 47’ each. This horizontal nearfield measurement system has the capability to operate from 500 MHz to 75 GHz using NSI’s high speed Panther receiver and high speed microwave synthesizers. The system is capable of performing conventional raster scans, as well as directed plane-polar scans tilted to the plane of a specific Antenna Under Test (AUT). The range was completed in December 2011. This paper will describe this near-field range’s design and installation, present test data and plots from its acceptance test including results of a NIST 18- term error assessment.
Highly accurate spherical near-field measurement systems require precise alignment of the probe antenna to the measurement surface. MI Technologies has designed and constructed a new spherical near-field arch positioner with a 1.5 meter radius to support measurements requiring accurate knowledge of the probe phase center to within .0064 cm throughout its range of travel.
To achieve this level of accuracy, several key design elements were considered. First, a highly robust mechanical design was considered and implemented. Second, a tracking laser interferometer system was included in the system for characterization of residual errors in the position of the probe. Third, a position control system was implemented that would automatically correct for the residual errors.
This paper defines the spherical near-field system and relation of each axis to the global coordinate system, discusses their associated error sources and the effect on global positioning and presents achieved highly accurate results.
Copyright 2012 IEEE. Reprinted from EuCAP 2012 Conference.
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The use of adaptive acquisition techniques to reduce the overall test time in planar near-field antenna measurements was presented in [1] & [2]. In those publications the concept of a decision function to track the uncertainty of a measurement as the data acquisition proceeds and also to adapt the acquisition region dynamically, was introduced. In this publication we build upon that work and present the concept of near-field array initialization. This is tested on different antennas and simulation results are presented. We also present actual measurement results to validate simulations that have to date been used to demonstrate advantages of the adaptive techniques.
Since the early days of antenna pattern recorders, advances in instrumentation and computers have enabled measurement systems to become highly automated and much more capable providing higher productivity, more efficient use of test facilities, and reduced data acquisition time.
Recently, measurement speeds of microwave receivers and vector network analyzers have advanced considerably. To take full advantage of these speed improvements, the measurement system architecture must be carefully considered. A comparison of several system architectures is given, along with discussion of key concepts and parameters that affect system timing, and a general method of calculating overall test time.
A summary table illustrates that small timing differences due to instrumentation and system architecture can have a significant impact on overall test time.
Further advances in system throughput are being explored using techniques such as simultaneous multi-frequency measurements in conjunction with a narrowband or wide band Receiver. A brief description of these techniques and initial proof of concept results are included.
Copyright 2012 IEEE. Reprinted from EuCAP 2012 Conference.
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This work concerns the experimental validation of a probe compensated near-field – far-field transformation technique using a spherical spiral scanning, which allows one to significantly reduce the measurement time by means of continuous and synchronized movements of the positioning systems of the probe and antenna under test. Such a technique relies on the nonredundant sampling representations of the electromagnetic fields and makes use of a two-dimensional optimal sampling interpolation formula to recover the near-field data needed to perform the classical spherical near-field – far-field transformation. The good agreement between the so reconstructed far-field patterns and those obtained via the classical spherical near-field – far-field transformation assesses the effectiveness of the approach.
Copyright 2012 The University of Salerno. Reprinted from AMTA 2012 Conference
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Antenna measurement systems employ mechanical positioners to spatially orient antennas, vehicles, and a variety of other test articles. These mechanical devices exhibit native positioning accuracy in varying degrees based on their design and position feedback technology. Even the most precise positioning systems have insufficient native accuracy for some specific applications.
As the limits of economical positioning accuracy are approached, a new error correction technique developed by MI Technologies satisfies these higher accuracy requirements without resorting to extreme measures in positioner design. The new technique allows real-time correction of repeatable positioning errors. This is accomplished by (1) performing a finely grained measurement of positioner accuracy, (2) creating a map of the errors in both spatial and spatial frequency domains, (3) separating the errors into their various components, and (4) applying correction filters to algorithmically perform error correction within the positioner control system.
The technique may be used to achieve extreme positioning accuracy with positioners of high native accuracy. It may also be applied to conventional (synchro feedback) positioners to achieve impressive results with no modifications at all to the positioner. The following paper discusses the new error correction technique in detail.
Phased-array antennas have always presented challenges in their interface to an acquisition system. Active arrays, especially those with a Digital Beam- Former (DBF), further complicate this interface. Whereas a passive phased array might be readily controlled with a simple digital code from the acquisition system, an active array tends to require more sophisticated communication to exercise the capabilities that must be tested. Furthermore, a DBF has receivers built into the array, and the simultaneous readings on these multiple receivers represent the data to be stored by the acquisition system vs. position and frequency.
The increased complexity of an active array's transmit beams by itself elevates the need for an interface between the array and the acquisition system. With the embedded receivers of a DBF, however, standard antenna testing of a DBF becomes nearly impossible without such an interface.
MI Technologies has developed a reasonably general interface between its acquisition system and active arrays with digital beamformers. MI has produced minor variations of this interface for multiple customers, and these customers will each use the interface to test multiple types of DBF active arrays. This paper discusses the challenges, capabilities, and architecture of this interface.
In this paper, we apply a recently developed 3D source reconstruction algorithm to perform antenna diagnostics on a planar array configuration. The test case is a planar X-band slot array measured in a spherical near-field facility and two slots were intentionally covered during the measurement campaign to test the performance of the algorithm. These measured data have previously been analyzed in [1] using two different methods for planar back-projection. For the purpose of comparison, results obtained with a planar reconstruction method based on conversion of spherical waves are also presented.
Copyright 2012 IEEE. Reprinted from EuCAP 2012 Conference.
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The Mathematical Absorber Reflection Suppression (MARS) technique which is used to identify and supress effects of spurious scattering within antenna measurement systems is demonstrated and its effectiveness examined through computational electromagnetic simulation and actual range measurements which were taken using a dual cylindrical reflector compact antenna test range.
Copyright 2012 IEEE. Reprinted from EuCAP 2012 Conference.
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The numerical analysis used for efficient processing of spherical near-field data requires that the far-field pattern of the probe can be expressed using only azimuthal modes with indices of μ = ±1. (1) If the probe satisfies this symmetry requirement, near-field data is only required for the two angles of probe rotation about its axis of χ = 0 and 90 degrees and numerical integration in χ is not required. This reduces both measurement and computation time and so it is desirable to use probes that will satisfy the μ = ±1 criteria. Circularly symmetric probes can be constructed that reduce the higher order modes to very low levels and for probes like open ended rectangular waveguides (OEWG) the effect of the higher order modes can be reduced by using a measurement radius that reduces the subtended angle of the AUT. Some analysis and simulation have been done to estimate the effect of using a probe with the higher order modes (2) – (6) and the following study is another effort to develop guidelines for the properties of the probe and the measurement radius that will reduce the effect of higher order modes to minimal levels. This study is based on the observation that since the higher order probe azimuthal modes are directly related to the probe properties for rotation about its axis, the near-field data that should be most sensitive to these modes is a near-field polarization measurement. This measurement is taken with the probe at a fixed (x,y,z) or (θ,φ,r) position and the probe is rotated about its axis by the angle . The amplitude and phase received by the probe is measured as a function of the χ rotation angle. A direct measurement using different probes would be desirable, but since the effect of the higher order modes is very small, other measurement errors would likely obscure the desired information. This study uses the plane-wave transmission equation (7) to calculate the received signal for an AUT/probe combination where the probe is at any specified position and orientation in the near-field. The plane wave spectrum for both the AUT and the probe are derived from measured planar or spherical near-field data. The plane wave spectrum for the AUT is the same for all calculations and the receiving spectrum for the probe at each χ orientation is determined from the far-field pattern of the probe after it has been rotated by the angle χ. The far-field pattern of the probe as derived from spherical near-field measurements can be filtered to include or exclude the higher order spherical modes, and the near-field polarization data can therefore be calculated to show the sensitivity to these higher order modes. This approach focuses on the effect of the higher order spherical modes and completely excludes the effect of measurement errors. The results of these calculations for different AUT/probe/measurement radius combinations will be shown.
The mathematical absorber reflection suppression (MARS) technique has been used to identify and then suppress the effects of spurious scattering within spherical, cylindrical, and planar near-field antenna measurement systems, compact antenna test ranges (CATRs), and far-field measurement facilities for some time now. The recent development of a general-purpose threedimensional computational electromagnetic model of a spherical antenna test system has enabled the MARS measurement and postprocessing technique to be further investigated. This paper provides an overview of the far-field MARS technique and presents an introduction to the computational electromagnetic range model. Preliminary results of computational electromagnetic range simulations that replicate typical MARS measurement configurations are presented and discussed which, for the first time, confirm through simulation many of the observations that have previously been noted using purely empirical techniques.
Copyright 2012 International Journal of Antennas and Propagation. Reprinted from 2012 International Journal of Antennas and Propagation.
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The feasibility of using adaptive acquisition techniques to reduce the overall testing time in spherical near-field (SNF) antenna measurements is investigated. The adaptive approach is based on the premise that near-field to far-field (NF-FF) transformation time is small compared to data acquisition time, so that such computations can be done repeatedly while data is being acquired. This allows us to use the transformed FF data to continuously compute and monitor pre-defined decision functions (formed from the antenna specifications most important to the particular AUT) while data is being acquired. We do not proceed with a complete scan of the measurement sphere but effectively allow the probe to follow a directed path under control of an acquisition rule, so that the sampled NF datapoints constitute an acquisition map on the sphere (the geographical allusion being purposeful). SNF data acquisition can be terminated based on decision function values, allowing the smallest amount of data needed to ensure accurate determination of the AUT performance measures. We demonstrate the approach using actual NF data for several decision functions and acquisition rules.
Recent work in developing a mode orthogonalisation and filtering post processing algorithm for multipath suppression in far-field and planar near-field antenna measurement systems has enabled worthwhile improvements to be obtained from the analogous, but mathematically and computationally distinct, cylindrical analogue. This paper presents an overview of the measurement and novel post-processing algorithm as embodied within the cylindrical mathematical absorber reflection suppression technique as well as comparing and contrasting results obtained from the new post-processing algorithms with previously published data.
Copyright 2012 IEEE. Reprinted from The Loughborough Antennas and Propagation Conference, 2012.
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We review two conventional algorithms for aperture back-projection from spherical near-field data, with the goal of quantifying array-element excitations. The first algorithm produces that portion of the near field that radiates to the far field. The second algorithm divides out the element pattern prior to the transformation, and produces an estimate of the element excitations. We introduce a variation of this element-excitation algorithm that, for some arrays, can improve the fidelity of this conventional estimate. For the array geometries measured and simulated, this new algorithm shows dramatic improvement.
Copyright 2012 IEEE. Reprinted from EuCAP 2012 Conference.
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Some antenna measurement applications require the precise positioning of an antenna along a prescribed path which may be realized by a combination of several, independent physical axes. Coordinated motion allows for emulation of a more complex and/or precise positioning system by utilizing axes which are mechanically less complex or precise and are correspondingly more easily realizable.
An ideal coordinated motion system should 1) Allow for the description of coordinated paths as parametric mathematical functions and/or interpolated look-up tables 2) Support control variable parameters which affect the trajectory 3) Compute a feasible trajectory within given kinematical constraints 4) Generate measurement trigger signals along the trajectory 5) Minimize control-induced vibration 6) Compensate for multivariate positioning errors.
This paper will describe a novel approach to virtual-axis coordinated motion which offers significant improvements over existing motion control systems. This advancement can be applied to many antenna measurement problems such as Helicoidal Near-Field Scanning and Radome Characterization.
In addition to steady state performance, antennas also have transient responses that need to be characterized. As antennas become more complex, such as active phased arrays, the transient responses of the antennas also become more complex. Transient responses are a function of internal antenna interactions such as coupling and VSWR, active circuitry, and components such as phase shifters and attenuators. This paper will show techniques for measuring antenna transient responses.
The first measurements utilize standard instrumentation capable of sampling at up to 4 MHz, giving 250 nS time resolution of the transient effect. Recognizing that some transient measurements require finer time resolution, a higher sampling rate prototype receiver was developed with 1 nS time resolution. After verification of its performance, the prototype receiver was used to measure the transient effects of a 50 nS pulse through a broadband antenna. The spectrum of the pulse yields information on the time and frequency domain responses of the antenna.
Phased arrays may exhibit transient signals when switching between beam directions as well as switching between frequencies. The methods presented in this paper are applicable to both.
There are a number of near-field measurement situations where it is desirable to use a broad band probe to avoid the need to change the probe a number of times during a measurement. But most of the broad band probes do not have low cross polarization patterns over their full operating frequency range and this can cause large uncertainties in the AUT results. Calibration of the probe and the use of probe pattern data to perform probe correction can in principle reduce the uncertainties. This paper reports on a series of measurements that have been performed to demonstrate and quantify the cross polarization levels and associated uncertainties that can be measured with typical log periodic (LP) probes. Two different log periodic antennas were calibrated on a spherical near-field range using open ended waveguides (OEWG) as probes. Since the OEWG has an on-axis cross polarization that is typically at least 50 dB below the main component, and efforts were made to reduce measurement errors, the LP calibration should be very accurate. After the calibration, a series of standard gain horns (SGH) that covered the operating band of the LP probe were then installed on the spherical near-field range in the AUT position and measurements were made using both the LP probes and the OEWG in the probe position. The cross polarization results from measurements using the OEWG probes where then used as the standard to evaluate the results using the LP probes. Principal plane patterns, axial ratio and tilt angles across the full frequency range were compared to establish estimates of uncertainties. Examples of these results over frequency ranges from 300 MHz to 12 GHz will be presented.
The development of a wideband, high-power capable 18-45 GHz quad-ridge horn antenna for a small towed decoy platform is discussed. Similarity between the system-driven antenna specifications and typical requirements for gain and probe standards in antenna measurements (that is, mechanical rigidity, null-free forward-hemisphere patterns, wide bandwidth, impedance match, polarization purity) is used to assess the quad-ridge horn as an alternative probe antenna to the typical open-ended rectangular waveguide probe for measurements of broadband, broad-beam antennas. Suitability for the spherical near-field measurements is evaluated through the finite elementbased full-wave simulations and measurements using the in-house NSI 700S-30 system. Comparison with the near-field measurements using standard rectangular waveguide probes operating in 18-26.5 GHz, 26.5-40 GHz, and 33-50 GHz ranges is used to evaluate the quality of the data obtained (both amplitude and phase) as well as the overall time and labor needed to complete the measurements. It is found that, for AUTs subtending a sufficiently small solid angle of the probe’s field of view, the discussed antenna represents an alternative to typical OEWG probes for 18-45 GHz measurements.
This paper describes a parametric study performed to investigate the viability of testing at frequencies above 100 GHz, using positioners implementing a theta/phi scanner. Parameters like probe radial distance, axis intersection and angular positioning are investigated to assess to what extent spherical near-field testing can be performed using commercially available positioners.
Copyright 2012 URSI. Reprinted from The International Union of Radio Science (URSI) 2012 Conference, Chicago, July 2012.
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Polarization properties (e.g. axial ratio, sense, and tilt) of an antenna under test (AUT) are often calculated from measurements with a linear (or dual-linear) polarized range antenna. At first, these calculations appear to be simple and straightforward. However, there are several different conventions used in the literature and some important practical aspects of the measurements are often omitted. Neglect of these small details can easily lead to incorrect results, with the most common error being the reversal of the right-hand-circular and left-hand-circular polarization components. We note the differences in the published polarization conventions and provide practical tips for good polarization measurement practices. We also describe stepby- step procedures for determining AUT polarization properties from two styles of polarization measurements using a linear (or dual-linear) polarized range antenna.
View The PaperNumerous applications for antenna, radome and RCS measurements require a very accurate positioning capability to properly characterize the product being tested. Testing of weapons (missiles), guidance systems, and satellites, among other applications, require multi-axis position accuracies of a few thousandths of an inch or degree. For global positioning, spherical error volumes can be extremely small having diameters of .002 inches to .005 inches. This paper addresses the issues that must be resolved when highly accurate mechanical positioning is required. Many factors such as thermal stability, axis configuration, bearing runout and mechanical alignment can adversely affect the overall system accuracy. Additionally, when examined from a global positioning system perspective, the accuracy of the entire system is further degraded as the number of axes increases. Successful system implementation requires carefully examining and addressing the most dominant error factors. The paper will cover current tools and techniques available to characterize and correct the contributing errors in order to achieve the highest possible system level accuracy. A recently delivered 4 ft radius SNF arch scanner, which achieved ± .0043° global positioning accuracy, will provide insight into these methods and show how the dominant factors were addressed.
This paper details a recent advance that, for the first time, enables the Mathematical Absorber Reflection Suppression (MARS) technique to be successfully deployed to correct measurements taken using plane-polar near-field antenna test systems with reduced AUT-to-probe separation. This paper provides an overview of the measurement, transformation, and post-processing. Preliminary results of range measurements are presented and discussed that illustrate the success of the new planepolar MARS technique by utilising redundancy within the near-field measured data that enables comparisons to be obtained and verified by using two existing, alternative, scattering suppression methodologies.
Streaming SDR (software defined radio) communication receivers are now high performance, common place and cost effective. Yet these receivers are not easily used for antenna measurements for a variety of reasons including their inability to accept phase reference and measurement trigger information. A new technique called Time Space Coherence Interferometry (TSCI) solves this problem in a simple and elegant manner. TSCI combines the concepts of temporal phase coherence with spatial division multiple access (SDMA) to directly encode phase and spatial information into a single continuous receiver data stream. The stream can be recorded for later analysis or efficiently decoded in real time producing conventional spatially sampled S21 amplitude and phase measurements. Additional data including antenna pulse timing, dynamics and signal quality metrics can be extracted from the TSCI data stream. Several representative TSCI systems are described.
Highly accurate spherical near-field measurement systems require precise alignment of the probe antenna to the measurement surface. MI Technologies has designed and constructed a new spherical near field arch positioner with a 1.5 meter radius to support measurements requiring accurate knowledge of the probe phase center to within .0064 cm throughout its range of travel.
To achieve this level of accuracy, several key design elements were considered. First, a highly robust mechanical design was considered and implemented. Second, a tracking laser interferometer system was included in the system for characterization of residual errors in the position of the probe. Third, a position control system was implemented that would automatically correct for the residual errors.
The scanner includes a two position automated probe changer for automated measurements of multi-band antennas and a high accuracy azimuth axis. The azimuth axis includes an algorithm for correcting residual, repeatable positioning errors.
This paper defines the spherical near-field system and relation of each axis to the global coordinate system, discusses their associated error sources and the effect on global positioning and presents achieved highly accurate results.
The early work with the IsoFilterTM
technique demonstrated that the radiation emanating from the aperture of a horn,
located several wavelengths above a ground plane, could be separated from the radiation due to the sidelobe and backlobe
illumination of the ground plane itself. The success of this demonstration encouraged us to pursue further the question of
how well the IsoFilterTM
technique worked to suppress other types of secondary signals – such as signals coming from
other elements of an array antenna or another individual first-order primary radiator nearby. [1] In the process of
evaluating the goodness of the secondary signal suppression we devised a method for identifying the locations and
strengths of an antenna's radiation sources that is an alternative to conventional back-projection. The alternative method
utilizes the antenna's far-field measured radiation pattern and successive spherical modal analyses to ascertain the relative
strength of the antenna's sources that give rise to its far field. We believe that this alternative technique has applicability to
the general problem of antenna diagnostics. Please see the Figure below for an example of an IsoFilterTM rejection curve. [2]
Copyright 2011 IEEE. Reprinted from EuCAP 2011 Conference.
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The desire to acquire Radar Cross Section (RCS) data on full scale models poses a number of challenges to the users of pylon / rotator systems. Typically, these full scale models have significant mass but have a relatively small foot print on which it is acceptable to mount the model to the rotational flange. The challenges to be addressed in this paper include designing a rotator that will have sufficient strength to support the weight of the model and the stress generated by the overturning moment. This rotator must have a sufficiently low profile and small volume so that it will conveniently fit within the model volume but still achieve a sufficient elevation travel to meet test objectives. This rotator must still properly close out the pylon at all elevation angles to prevent unwanted reflections. Additional design considerations include the test conditions and the test environment. A rigorous test requirement can demand special engineering features to mitigate the demands of relatively high scan speeds and extended run times. Environmental concerns including wind loads, temperature, humidity, and contaminants, must be factored into the design of modern RCS rotators.
This paper presents the system design approach to address the requirements of a full scale model rotator. The paper examines consequences of selected potential design solutions and demonstrates the importance of performing trade studies.
The use of adaptive acquisition techniques to reduce the overall test time in planar near-field antenna measurements is described. A decision function is used to track the accuracy of a measurement as the data acquisition proceeds, and to halt such acquisition when this is considered sufficient for the measured quantity of importance. Possible decision functions are defined and compared. Several test cases are presented to show that significant test time reduction is possible when compared to traditional acquisition schemes.
Since the early days of antenna pattern recorders, advances in instrumentation and computers have enabled measurement systems to become highly automated and much more capable. Automated systems have provided higher productivity, more efficient use of test facilities, and the ability to acquire more data in less time. In recent years, measurement speeds of microwave receivers and vector network analyzers have advanced considerably. However, to take full advantage of these speed improvements, the measurement system architecture must be carefully considered. Small differences in instrument timing that are repeated many times can make large differences in system measurement time. This paper describes a general method of calculating system measurement time based on the primary factors that affect system timing, including position trigger detection, frequency switching time, multiplexer switching time, receiver measurement time, and timing overhead associated with triggers, sweeps, and measurements. It also shows how key features of instruments available today can be used along with improved antenna measurement system architectures to optimize system throughput.
When making antenna measurements, great care must be taken in order to obtain high quality data. This is especially true for near-field antenna measurements as a significant amount of mathematical post-processing is required in order that useful far-field data can be determined. However, it is often found that the integrity of these measurements can be compromised in a large part through range reflections, i.e. multipath. For some time a technique named Mathematical Absorber Reflection Suppression (MARS) has been used to reduce range multi-path effects within spherical, cylindrical and most recently planar nearfield antenna measurement systems. This paper presents the results of a recent test campaign which yields further verification of the effectiveness of the technique together with a reformulation of the post-processing algorithm which, for the first time, utilises a rigorous spherical wave expansion based orthogonalisation and filtering technique.
This paper discusses a technique that can be used when comparing two antenna patterns that produces a measure of the difference between the patterns and an associated confidence level for the results that is derived from a statistical analysis of the pattern differences. The first step in the process is to verify that the same coordinate system is used, and that the AUT is precisely aligned to those coordinates in all measurements. Small differences in beam pointing and polarization that correspond to rotations about the three rectangular axes can arise due to AUT alignment differences or due to some measurement errors. These changes in beam pointing can produce apparent pattern differences in the pattern comparison process that can distort the results, and adjustments in the AUT alignment or pattern data should be done before the pattern comparisons are carried out. It is easy to interpolate the pattern or change the pattern centering using standard processing software to try and correct for the angular misalignments, however the interpolation or centering may not completely correct for rotations about all three axes and some effects may remain after software adjustments...
Copyright 2011 IEEE. Reprinted from EuCAP 2011 Conference.
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Mathematical Absorber Reflection Suppression (MARS) has been used successfully to identify and extract range multi-path effects in a great many spherical [1, 2], cylindrical [3, 4], and planar [5, 6] near-field antenna measurement systems. This paper details a recent advance that enables the MARS measurement and postprocessing technique to be used to correct antenna pattern data from far-field or compact antenna test ranges (CATRs) where only a single great circle pattern cut is taken. This paper provides an overview of the measurement and novel data transformation and postprocessing chain that is utilised to efficiently correct farfield, frequency domain antenna pattern data. Preliminary results of range measurements that illustrate the success of the technique are presented and discussed.
For some time now, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used successfully to identify range multi-path effects in spherical, cylindrical, and planar near-field antenna measurement systems. This paper detail a recent advance that, for the first time, allows the MARS measurement and post-processing technique to be successfully deployed to correct antenna pattern data taken using direct far-field or compact antenna test ranges (CATRs) where only a single great circle cut is acquired. This paper provides an overview of the measurement and novel data transformation and post-processing chain that is utilized within the far-field MARS (F-MARS) technique to efficiently correct far-field, frequency domain data. Preliminary results of range measurements that illustrate the success of the technique are presented and discussed.
Copyright 2011 IEEE. Reprinted from The Loughborough Antennas and Propagation Conference, 2011.
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Nearly all antenna measurements are contaminated to some degree with fields scattered by objects within the environment of the test system. In many instances these reflections (i.e. multi-path) are found to constitute one of the most significant contributors to the facility-level error budget [1]. For some time, a frequency domain measurement and postprocessing technique named Mathematical Absorber Reflection Suppression (MARS) has been successfully used to reduce range multi-path effects within spherical [2, 3, 4] and cylindrical nearfield antenna measurement systems [5, 6]. More recently, a related technique has been developed for use with planar nearfield antenna measurement systems [7]. This paper provides an introduction to the measurement technique and novel probe pattern corrected near-field to far-field transform algorithm. It then presents the most recent results of an on-going validation campaign which have been found to yield improvements comparable to those attained with the corresponding spherical and cylindrical MARS implementations. These results are discussed and conclusions presented.
Copyright 2011 IEEE. Reprinted from EuCAP 2011 Conference.
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When making measurements on an outdoor range, susceptibility to electromagnetic interference is a common problem. In a dense spectral environment, television and radio broadcast signals, radars, and various communications systems emit signals that can be received by antenna measurement instrumentation. When these combine with the signal from the range transmit signal source at the receive antenna, measurement errors result.
To overcome interfering signals, one may consider increasing range transmit power sufficiently to limit the effects of interference on the measurement. However, the test range may then cause interference to other nearby systems. A better solution is to keep transmit power at a lower level and filter out the interferer.
The filtering techniques used in measurement instrumentation may or may not be adequate to prevent interference from affecting the measurement results. This paper illustrates that by choosing a sensitive receiver with high selectivity filtering, measurements can be made in the same band as the interferer.
It has become of current interest to understand how best to draw conclusions as to the effects of sources of error upon the accuracy of measured antenna patterns.[1] In particular, when for a particular antenna, two pattern measurements made under slightly or somewhat different conditions are compared, how might one arrive at a conclusion as to which is the more nearly correct or the least uncertain pattern result ? Before attempting to address this question, it is important to realize that the method used to compare patterns must be examined for any anomalous artifacts. Here in this presentation I provide some background on how the equivalent stray signal method of pattern comparison came to be used in assessment of pattern accuracy and goodness of pattern result...
Copyright 2011 IEEE. Reprinted from EuCAP 2011 Conference.
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We review two conventional algorithms for aperture back-projection from spherical near-field data, with the goal of quantifying array-element excitations. The first algorithm produces that portion of the near field that radiates to the far field. The second algorithm divides out the element pattern prior to the transformation, and produces an estimate of the element excitations. We introduce a variation of this element-excitation algorithm that, for some arrays, can improve the fidelity of this conventional estimate. We apply the three algorithms to measured data, where the algorithms’ assumptions are tested, and to synthesized data, where the expected results are known exactly. For the array geometries measured and simulated, this new algorithm shows dramatic improvement.
Two of the three algorithms require an estimate of the element pattern, which they assume to be common to all the elements. We describe our measurement of our array’s element pattern, as well as the use of the IsoFilterTM to center the element pattern and limit the edge effects.
The commercial aviation industry faces several issues in regard to servicing and maintaining the radomes that abound in the aircraft fleet flying today. The first issue is the historically high cost of radome test systems. As a result of this, there are limited numbers of test systems in operation today and some geographic regions have insufficient radome test capacity. Advances in weather radar and increased reliance on them for turbulence avoidance and more efficient route planning around storm systems will increase the importance of ensuring that weather radar systems are performing well and consequently that weather radar radomes are in good condition and have been adequately tested. Because of the potential consequence of flying with a bad radome and the demands of new radar systems, its more important than ever to ensure test systems in use adhere to requirements and to spread awareness of these challenges within the aviation community.
Recently, a design effort was conducted specifically geared towards developing a system concept for radome testing that would both provide a robust test capability that fully meets the RTCA-DO-213 after repair test requirements and one that is much lower in cost than traditional systems that are fielded today. This paper describes the issues cited above and provides a description of the low cost - compliant solution
A distributed RF subsystem is a common solution for antenna test ranges where dynamic range is to be optimized. This is especially relevant for electrically large1 systems since cable losses become prohibitive. For such solutions the locations of the components comprising the RF subsystem are critical. By using remote mixers, amplifiers and multipliers, the use of lower frequency cables with lower loss becomes feasible and this approach dramatically increases the available power level at the transmitting antenna and the sensitivity of the receiver.
Radar antennas are typically required to operate in using various transmit and receive conditions, depending on the particular radar mode. In active antenna applications, these modes may require different antenna operating parameters, which currently dictate testing the antenna independently in transmit and receive using different test system configurations. In testing highly-integrated active arrays, electrical and thermal considerations make it desirable to test the antenna in its nominal Tx/Rx (Transmit/Receive) operating mode as opposed to transmit-only or receive-only. An extension to the NSI Panther 9100 RF measurement system has been developed to support multiplexed transmit and receive, pulse-mode measurements with different measurement parameters during the course of a single data acquisition. This capability allows pulsed transmit and receive tests to be interleaved using a single measurement setup, reducing overall test time
Collecting accurate gain measurements on antennas is one of the primary tasks for our community. One of the primary concerns in making gain measurements is choosing one of the well known gain measurement techniques to make the measurement. Each gain measurement technique has an inherent accuracy limit based on the measurements made, the measurement environment and the equipment required. The frequency band of interest may also have an impact on the gain measurement scheme employed. In addition, each technique can affect the throughput of the range in question. Balancing the cost of obtaining the gain versus the required accuracy of the gain measurement is a difficult task. This paper will discuss the basic accuracy limitations for several of the standard gain measurement techniques and will catalog the accuracy limitations of the various gain measurement techniques versus the cost associated with obtaining that quality of measurement.
RF guided missile developers require flight simulation of their target engagements to develop their RF seeker. This usually involves the seeker mounted on a Flight Motion Simulator (FMS) as well as an RF target simulator that simulates the signature and motion of the target. Missile defense developers, whose job it is to defend against guided missiles, require a similar test environment adding the ability to insert decoy RF targets that can spoof the seeker. Both seeker development and counter-measure development can benefit from an RF test facility that can provide RF targets and decoys controlled by a real-time simulation.
This paper addresses an RF Target and Decoy Simulator developed by MI Technologies that provides this test capability. The direction of the target and decoy emitters is independently controlled such that the centerlines of their radiated main antenna lobes are always directed at the RF seeker. Each emitter can be independently and simultaneously commanded along a spherical surface. High rates of acceleration and velocity are achieved all the way out to the ends of the test area to simulate the high line of sight rate that occurs at missile closure. The simulator is capable of safely stopping a decoy racing to the ends of the target area with minimal over-travel. Collision avoidance provisions prevent target and decoy from damaging each other during the simulation.
The paper presents a description of the simulator, pertinent tradeoffs considered in the design and accuracy data of the simulator’s performance.
This paper describes a novel method termed the IsofilterTM Technique, of isolating the radiation pattern of an individual radiator from among a composite set of radiators that form a complex radiation distribution. This technique proceeds via three successive steps: A spherical transform on an over-sampled data set, followed by a change of coordinate system followed in turn by filtering in the domain of the spherical modes to isolate a radiating source. The result is an approximate pattern of the individual radiator largely uncontaminated by the other competing sources of radiation.
Copyright 2011 IEEE. Reprinted from AP-S/URSI 2011 Conference.
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Attempts to produce robust, objective, quantitative measures of similarity between antenna pattern data sets using statistical methods have been widely reported in the open literature [1, 2, 3, 4, 5]. This paper presents an introduction to the physical validity and use of generalised statistical analysis along with specialist statistical image classification concepts as applied to the assessment of antenna pattern functions. Before presenting results the paper describes some of the more recent developments in the field relating to, nominal, ordinal, interval and the more usual ratio type levels of measurement data available within near-field measurement data sets. Finally the paper describes how these assessment techniques could be implemented in a formal gage repeatability and reproducibility (R&R) [6] numerical quality analysis of a measurement system.
Copyright 2011 IEEE. Reprinted from EuCAP 2011 Conference.
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Conventional antenna measurement systems use a multiplexer or polarization positioner to sequence polarization and or antenna elements as a function of time, requiring two or more measurement intervals. However, a simpler, more cost effective, and faster technique can be implemented by using frequency diversity to distinguish between polarizations or antenna elements. This paper describes how two slightly different frequencies can be used to make two measurements simultaneously instead of sequentially, cutting the measurement time in half or even more. Additional considerations must be taken into account to achieve good measurements. This paper addresses these issues. Actual measurements are presented.
An arch-based spherical near-field measurement system has been commissioned at Lockheed Martin’s facility in Syracuse, New York. This system is designed for highfidelity testing of large, low-frequency, phased-array radars. The near-field scanner system consists of a 9.5- meter-radius arch with an active probe-position-error correction, and a large azimuth axis capable of carrying large arrays. The shielded anechoic chamber designed to house the measurement system includes full treatment with curvilinear absorber to achieve low levels of stray signal at UHF band frequencies, FM-200 / VESDA fire protection, and a glycol based system for removing heat loads generated by the radars.
The overall measurement system details are presented, along with mechanical accuracies achieved for the scanner system. Details of the chamber and host facility are described. Finally, the paper concludes with measurements of a UHF-band Standard Gain Horn using the system. The challenges and benefits of such a system will be highlighted.
In a 2008 AMTA paper the concept of the IsoFilterTM rejection curve was described. The steps to generate this rejection curve consist simply of (1) translating the coordinate origin of the measured pattern to a new location (2) performing a spherical modal analysis of the pattern, and (3) taking the total power in the lowest order mode as a measure of the strength of the radiation source at that location. Stepwise repetition of this process then generates the IsoFilterTM rejection curve. The basis for the process of generation was an empirical recipe for which no theoretical basis was presented. In this paper we relate the rejection curve to conventional electromagnetic theory. We begin with the general free space Green's function assuming a general distribution of current sources, and show how one may plausibly describe the IsoFilterTM rejection curve, and how it operates to reveal an arbitrary source distribution.
In many instances, reflections within antenna test ranges constitute the largest single component within the facilitylevel error budget. For some time, a frequency domain measurement and post-processing technique named Mathematical Absorber Reflection Suppression (MARS) has been successfully used to reduce range multi-path effects within spherical near-field and far-field antenna measurement systems. More recently, a related technique has been developed for use with cylindrical near-field antenna measurement systems. This paper provides an introduction to the measurement technique and novel probe pattern corrected near-field to farfield transform algorithm. It then presents the most recent results of an ongoing validation campaign detailing a number of the most recent advances which are found to yield improvements comparable to those attained with the corresponding spherical MARS technique. The results are discussed and conclusions presented.
Copyright 2010 IEEE. Reprinted from EuCAP 2010 Conference.
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Millimeter-wave measurements on spherical nearfield scanning systems present a number of technical challenges to be overcome to guarantee accurate measurements are achieved. This paper will focus on the affect of mechanical alignment errors of the spherical rotator system on the antenna’s measured performance. Methods of precision alignment will be reviewed. Sensitivity to induced mechanical alignment errors and their affect on various antenna parameters will be shown and discussed. Correction methods for residual alignment errors will also be described. The study includes 38 and 48 GHz data on the Alphasat EM model offset reflector antenna measured by TeS in Tito, Italy on a NSI-700S-60 Spherical Nearfield system, as well as a 40 GHz waveguide array antenna measured by NSI on a similar NSI-700S-60 Spherical Nearfield System at its factory in Torrance, CA, USA.
We have devised a method for identifying the locations and strengths of an antenna's radiation sources that is an alternative to conventional back-projection. This alternative method utilizes the antenna's measured far-field radiation pattern and successive spherical modal analyses to ascertain the relative strength of the antenna's sources, as a function of position, that give rise to the far-field.
Copyright 2010 IEEE. Reprinted from EuCAP 2010 Conference.
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We describe two theoretical bases for an algorithm for back-projection. The first is (1) Fourier inversion of the mathematical expression for the far electric field components in terms of the aperture electric field. The second is (2) Fourier inversion of the complete vectorial transmitting characteristic of Kerns' scattering matrix. It is this characteristic that results from the standard process of planar near-field (PNF) scanning and the ensuing reduction of the PNF transmission equation. We demonstrate that the theoretical approaches (1) and (2) yield identical back-projection algorithms. We report on backprojection measurements of an 18 inch X-band flat plate phased array using the far-field obtained from spherical near-field scanning. The spherical measurements were made on a large arch range.
Copyright 2010 IEEE. Reprinted from EuCAP 2010 Conference.
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Reflections in antenna test ranges can often constitute the largest single term within the error budget of a given facility. For some time, a frequency domain measurement and postprocessing technique named Mathematical Absorber Reflection Suppression (MARS) has been successfully used to reduce range multi-path effects within spherical near-field and far-field antenna measurement systems. More recently, a related technique has been developed for use with cylindrical near-field antenna measurement systems. This paper provides an introduction to the measurement techniques and a description of the novel near-field to far-field transform algorithms before presenting preliminary results of actual range measurements of a low gain antenna taken using a combination spherical / cylindrical system that was installed within a partially anechoic chamber. These results illustrate the success of the techniques which are found to provide comparable improvements yielding far-field patterns that are in encouraging agreement despite every step within the data acquisition, transformation and postprocessing chain being different thereby providing further compelling evidence of the success of the MARS technique.
Copyright 2010 IEEE. Reprinted from Loughborough Conference on Antennas and Propagation, 2010.
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This article describes a repeatable reference environment for over-the-air (OTA) testing of multiple input multiple output (MIMO) devices and why the reverberation chamber is uniquely suited to provide insight into MIMO device performance. The importance of reference environments and the characteristics of the reverberation chamber in the context of 4G wireless systems are reviewed. Statistically based measurements are described and compared with the line-of-sight solutions implemented in an anechoic chamber environment.
Microwave Journal Copyright ©2010. Reprinted from August 2010 issue.
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Operating microwave receivers with remote mixers in a system requires the LO power to be flat over broadband frequencies. In large systems, this is difficult to attain due to long RF cables. Most systems require significant engineering to ensure the LO power level to the mixer is adequate. To help understand the problem, commonly used techniques have been evaluated while recommending a particular approach.
Operating over a small fundamental frequency range with harmonic mixing has the advantage of lower RF cable insertion loss but results in high mixer conversion loss. Using negative slope equalizers and amplifiers, RF cable slope and attenuation can be sufficiently combated. However, this requires extensive system engineering and customization to match cable losses, thereby making it expensive. The approach is also designed to only work with a certain set of RF cables.
A more viable approach includes independently controlling the attenuators and amplifiers for the signal and reference channels which can be configured to provide optimal LO power to the respective mixer. A simple setup file configures components in each channel to adapt to any set of RF cables. Positive experimental results of implementing this technique in different configurations are presented.
Obtaining a quantitative accuracy qualification is one of the primary concerns for any measurement technique. This is especially true for the case of near-field antenna measurements as these techniques consist of a significant degree of mathematical analysis. When undertaking this sort of examination, room scattering is typically found to be one of the most significant contributors to the overall error budget. Previously, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used with considerable success in quantifying and subsequently suppressing range multi-path effects in first spherical and then, cylindrical near-field antenna measurement systems. This paper details a recent advance that, for the first time, enables the MARS technique to be successfully deployed to correct data taken using planar near-field antenna measurement systems. This paper provides an overview of the measurement and novel data transformation and post-processing chain. Preliminary results of computational electromagnetic simulation and actual range measurements are presented and discussed that illustrate the success of the technique.
Most conventional spherical near-field scanning systems require the antenna under test to rotate in one or two axes. This paper will describe a novel rolling arch near-field scanner that transports a microwave probe over a hyper-hemispherical surface in front of the antenna. This unique scanning system allows the antenna to remain stationary and is very useful for cases where motion of the antenna is undesirable, due to its sensitivity to gravitational forces, need for convenient access, or special control lines or cooling equipment. This allows testing of stationary antennas over wide angles with accuracies and speeds that historically were only available from planar near-field systems.
The probe is precisely positioned in space by a high precision structure augmented by dynamic motion compensation. The scanner can complete a hyperhemispherical multi-beam, multi-frequency antenna measurement set of up to eight feet in diameter in less than one hour.
The design challenges and chosen techniques for addressing these challenges will be reviewed and summarized in the paper.
The science of spiral scanning techniques have been advancing for a number of years at the University of Salerno.
Experimental validation of the technique has been presented in the past couple of years validating the techniques.
This latest work shows the ability to compensate for known residual errors in the probe position.
Copyright 2010 The University of Salerno. Reprinted from AMTA 2010 Conference
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This paper provides an overview of antenna test systems that operate in the millimeter and sub-millimeter wave bands. Techniques that have been developed to overcome technical restrictions that usually limit performance at very high RF frequencies are presented. Aspects such as thermal structural change, RF cable phase instability, scanner planarity, and probe translation during polarization rotation are addressed. These methods have been implemented and validated on test systems operating from 50 GHz up to 950 GHz.
Making measurements on an outdoor range can be challenging for many reasons, including test article size, weather, and undesired electromagnetic effects. The challenges this paper addresses are those associated with the dense spectral environment in which measurements must often be made. Signals from external emitters must be prevented from causing interference with the measurement, and the outdoor range must not cause interference with other nearby systems. These criteria oppose each other in that if range transmit power is increased sufficiently to limit the effects of interference on the measurement, the range may cause interference to other systems. If low power is used in the range to avoid causing interference to others, the external emitter may make measurements on the range difficult to impossible. This paper demonstrates how, by using a sensitive receiver with high selectivity, one can make measurements right in the band of the interferer. By changing how the signal is processed, measurement capability is enhanced.
The Global Precipitation Measurement (GPM) mission is a satellite based Earth science mission that will study the global precipitation from rain, ice and snow. A critical part of this satellite is the multifrequency radiometer system that covers frequencies up to 183 GHz. Beam pointing and beam efficiency must be measured very accurately to calibrate the radiometer response. This paper will focus on the measurements of the offset reflector antenna operating up to 183 GHz using a Nearfield Systems Inc. (NSI) planar near-field measurement system and the special challenges that this presents. Results will be presented and the uncertainty in beam pointing will be discussed.
This paper elaborates on certain aspects of a new measurement process that permits an assessment of spherical near-field (SNF) measurement errors based on a set of practical tests that can be done as part of any SNF measurement. It provides error bars for a measured radiation pattern in an automated fashion.
Specially designed pyramidal horn antennas known as standard-gain horns are accepted as gain standards throughout the antenna community. The unknown gain of an AUT is determined by comparing its gain to that of a standard gain horn. Slayton of the US Naval Research Laboratory in 1954 developed a design method and gain curves for standard gain horns. This paper examines the ability of modern numerical electromagnetic modeling to predict the gain of these horns and possibly achieve greater accuracy than with the NRL approach.
Copyright 2010 IEEE. Reprinted from 2010 Antenna Measurement Conference, ANTEM 2010, July 5-9, 2010, Ottawa, Canada.
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Monopulse antennas typically have a sum and two difference channels allowing for the accurate tracking of radar targets. Measuring the radiation patterns of these three channels often include establishing the electrical boresight of the antenna. Planar near-field test systems allow for the accurate determination of difference pattern nulls and locations. In this paper we present an iterative process requiring the use of all three channels to achieve an accurate null depth and location result. The impact of near-field truncation on the boresight pointing angle is also addressed and achievable accuracy numbers are presented.
Copyright 2010 IEEE. Reprinted from 2010 Antenna Measurement Conference, ANTEM 2010, July 5-9, 2010, Ottawa, Canada.
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This paper describes a measurement process that permits an assessment of spherical near-field (SNF) measurement errors based on a set of tests that can be done as part of any SNF measurement. A test system has been implemented that, in an automated fashion, derives error bars for the measured radiation patterns.
Copyright 2010 IEEE. Reprinted from EuCAP 2010 Conference.
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Time domain gating has been widely used in antenna measurements for many years. The technique has proven useful in gating out extraneous signals from the range that can be uniquely separated from the primary desired signal. The well known process involves collecting data in the frequency domain and then transforming the data to the time domain for processing. In far-field antenna measurements the technique is limited in its applicability by factors such as antenna bandwidth and pattern shape vs. frequency, and internal time delays within the device under test. Spherical near-field presents additional challenges to the effective use of time domain gating.
These challenges of performing time domain gating in spherical near-field measurements are presented along with measured results from a small spherical near-field range. These results show the significant reduction in stray signals and the resulting increase in accuracy that can be achieved via time domain gating.
Finally, the paper concludes with measurements made on a large spherical near-field arch over a ground plane system. Additional challenges of such a system will be highlighted.
Copyright 2010 IEEE. Reprinted from 2010 Antenna Measurement Conference, ANTEM 2010, July 5-9, 2010, Ottawa, Canada.
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In this work, a probe compensated near-field – farfield transformation technique with spherical spiral scanning suitable to deal with elongated antennas is developed by properly applying the unified theory of spiral scans for nonspherical antennas. A very flexible source modelling, formed by a cylinder ended in two half-spheres, is considered as surface enclosing the antenna under test. It is so possible to obtain a remarkable reduction of the number of data to be acquired, thus significantly reducing the required measurement time. Some numerical tests, assessing the accuracy of the technique and its stability with respect to random errors affecting the data, are reported.
Copyright 2009 University of Salerno. Reprinted from AMTA 2009 Conference.
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We describe two theoretical bases for an algorithm for back-projection. The first is (1) Fourier inversion of the mathematical expression for the far electric field components in terms of the aperture electric field. The second is (2) Fourier inversion of the complete vectorial transmitting characteristic of Kerns' scattering matrix. It is this characteristic that results from the standard process of planar near-field (PNF) scanning and the ensuing reduction of the PNF transmission equation. We demonstrate that the theoretical approaches (1) and (2) yield identical back-projection algorithms. We report on backprojection measurements of an 18 inch X-band flat plate phased array using the far-field obtained from spherical near-field scanning. The spherical measurements were made on a large arch range.
Copyright 2009 IEEE. Reprinted from The Loughborough Antennas and Propagation Conference, 2009.
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This work concerns the experimental validation of a new near-field – far-field transformation technique with helicoidal scanning tailored for elongated antennas. Such a transformation is based on the theoretical results relevant to the nonredundant sampling representations of the electromagnetic fields and uses a proper source modelling particularly suitable to deal with this kind of antennas. By employing such an effective modelling, instead of the spherical one, it is possible to remarkably reduce the error related to the truncation of the scanning zone, since measurement cylinders with a diameter smaller than the source height can be used. The validity of this innovative scanning technique is assessed by comparing the reconstructions obtained from the data directly measured on the classical cylindrical grid with those recovered from nonredundant measurements on the helix.
Copyright 2009 IEEE. Reprinted from EuCAP 2009 Conference.
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We describe two theoretical bases for an algorithm for back-projection. The first is (1) Fourier inversion of the mathematical expression for the far electric field components in terms of the aperture electric field. The second is (2) Fourier inversion of the complete vectorial transmitting characteristic of Kerns' scattering matrix. It is this characteristic that results from the standard process of planar near-field (PNF) scanning and the ensuing reduction of the PNF transmission equation. We demonstrate that the theoretical approaches (1) and (2) yield identical back-projection algorithms.
We report on back-projection measurements of an 18 inch X-band flat plate phased array using the far-field obtained from both planar and spherical near-field scanning. The spherical measurements were made on a large arch range.
It is well known that modern, high-performance antenna range instrumentation requires fast sources and receivers. What is often overlooked is that the locations of the components making up the RF subsystem need to be considered as well. RF sources and receivers that are controlled over a LAN interface can easily be located remotely, where they are closer to the transmitting and receiving antennas. In addition to using remote mixers, other components such as amplifiers and multipliers can be mounted remotely, on a positioner or probe carriage. This allows using lower-frequency cables with lower loss, and dramatically increases the available power level at the transmitting antenna. Use of fiber optics is also becoming an option for transmission of RF signals in distributed RF systems. Automated configuration control can be achieved using remotely controlled switches.
This paper will present comparisons of distributed and more traditional geometries, including performance and cost benefits.
Measurement of radome beam deflection and/or Boresight shift in a compact range generally requires a complicated set of positioner axes. One set of axes usually moves the radome about its system antenna while the system antenna remains aligned close to the range axis. Another set of axes is normally required to scan the system antenna through its main beam (or track the monopulse null) in each plane so the beam pointing angle can be determined. The fidelity required for the beam pointing angle, combined with the limited space inside the radome, usually make this antenna positioner difficult and expensive to build...
The NIST 18 Term Error Analysis Technique uses a combination of mathematical analysis, computer simulation and near-field measurements to estimate the uncertainty for near-field range results on a given antenna and frequency range. A subset of these error terms is considered for alignment accuracy of an antenna’s RF main beam. Of the 18 terms, several have no applicable influence on determining the beam pointing or the terms have a minor effect and when an RSS estimate is performed they are rendered inconsequential. The remainder become the dominant terms for identifying the alignment accuracy. There are six terms that can be evaluated to determine the main beam pointing uncertainty of an antenna with respect to dual band performance. Analysis of the near-field measurements is performed to identify the alignment uncertainty of the main beam with respect to a specified mechanical position as well as to the main beam of the second band.
The Mathematical Absorber Reflection Suppression (MARS) technique is a method to reduce scattering errors in near-field and far-field antenna measurement systems. Previous tests by the authors had indicated that NSI’s MARS technique was not as effective for directive antennas. A recent development of a scattering reduction technique for cylindrical near-field measurements has demonstrated that it can also work well for directive antennas. These measurements showed that the AUT should be offset from the origin by a distance at least equal to the largest dimension of the AUT rather than only 1-3 wavelengths which had been used for smaller antennas in the earlier MARS measurements. Spherical near-field measurements have recently been concluded which confirm that with the larger offsets, the MARS technique can be applied to directive antennas with excellent results...
A near-field measurement technique for the prediction of wide-angle asymptotic far-field antenna patterns from data obtained from a modified small combination planar/cylindrical near-field measurement system is presented. This novel technique utilises a simple change in the alignment of the robotic positioners to enable near-field data to be taken over the surface of a conceptual right conic frustum. This configuration allows existing facilities to characterise wide-angle antenna performance in situations where hitherto they could have been limited by the effects of truncation. This paper aims to introduce the measurement technique, present a measurement campaign, describes the novel probe-corrected near-field to farfield transform algorithm before presenting preliminary results. As this paper recounts the progress of ongoing research, it concludes with a discussion of the remaining outstanding issues and presents an overview of the planned future work.
Copyright 2009 IEEE. Reprinted from EuCAP 2009 Conference.
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Antenna characterization requires making large quantities of RF measurements across a known spatial position. In practice, this involves coordinated positioning equipment, test instruments, and software processing. The overhead of networking, handshaking, sequencing, triggering, and data management between these devices is often the limiting factor in total system performance. Antenna test engineers are often frustrated by slow system performance, despite using the fastest equipment available.
The gain-transfer technique is the most commonly used antenna gain measurement method and involves the comparison of the AUT gain to that of another antenna with known gain. At microwave frequencies and above, special pyramidal horn antennas known as standard-gain horns are universally accepted as the gain standard of choice. A design method and gain curves for these horns were developed by the US Naval Research Laboratory in 1954. This paper examines the ability of modern numerical electromagnetic modeling to predict the gain of these horns and possibly achieve greater accuracy than with the NRL approach.
Similar computational electromagnetic modeling is applied to predict the gain and pattern of open-ended waveguide probes which are used in near-field antenna measurements. This approach provides data for probes that are not available in the literature.
RF guided missile developers require flight simulation of their target engagements to develop their RF seeker. This usually involves the seeker mounted on a Flight Motion Simulator (FMS) as well as an RF target simulator that simulates the signature and motion of the target. Missile intercept engagements are unique in that they involve highly dynamic relative motion in a short period of time. This puts demanding requirements on the RF target simulator to adequately present the desired phase slope, amplitude, and polarization to the seeker antenna and electronics under test.
This paper describes a newly installed RF Target Simulator that addresses these requirements in a unique fashion. The design utilizes a compact range reflector, dynamically rotated in two axes as commanded by the flight simulation computer, to produce the desired changing phase slope and an RF feed network dynamically controlled to produce the desired changing polarization and amplitude. Physical optics analysis establishes an accurate correlation between reflector physical rotation and resulting angle-of-arrival of the wave front in the quiet zone. The RF Target Simulator is self contained in a two-man portable anechoic chamber that can be disengaged from the FMS and rolled to and from the FMS as needed. Measurements are presented showing the performance of the RF Target Simulator.
Measurements of a conical micro-strip WraparoundTM antenna array mounted on a portion of the entry vehicle for NASA’s Mars Science Laboratory mission were completed at Nearfield Systems, Inc.’s new spherical near-field range facility. The WraparoundTM antenna, designed and manufactured by Haigh-Farr, Inc., provides nearly full spherical coverage and operates in the UHF frequency band for telecommunications to orbiting assets at Mars. A summary of the measurements techniques and results are presented, along with a comparison of the measured and calculated patterns.
Reflections in antenna test ranges can often be the largest source of measurement error within the error budget of a given facility. Previously, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used with considerable success in reducing range multi-path effects in spherical near-field antenna measurements. Whilst the technique presented herein is also a general purpose measurement and postprocessing technique; uniquely, this technique is applicable to cylindrical near-field antenna test ranges. Here, the postprocessing involves the analysis of the cylindrical mode spectrum of the measured field data which is then combined with a filtering process to suppress undesirable scattered signals.
This paper provides an introduction to the measurement technique and a description of the novel near-field to farfield transform algorithm before presenting preliminary results of actual range measurements. These results illustrate the success of the technique by showing a circa 10 to 20 dB reduction in spectral reflections (i.e. a reduction in the scattering from a known scatterer within the measurement environment) which is comparable to the degree of improvement attained with the pre-existing, comparable, spherical MARS technique.
This paper provides an overview of planar near-field antenna test systems developed for sub-millimeter wave testing. Special techniques that have been developed to overcome technical restrictions that usually limit performance at very high RF frequencies are presented. Aspects like thermal structural change, RF cable phase instability, scanner planarity and probe translation during polarization rotation are addressed. These methods have been implemented and validated on systems up to 660 GHz and 950 GHz. These cases have lead to the development of low cost commercial test systems, making antenna testing in the V and W-bands (40 – 110 GHz) cost effective.
Copyright 2009 IEEE. Reprinted from EuCAP 2009 Conference.
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This paper describes measurements performed at the National Physical Laboratory (NPL) and Near Field Systems Inc (NSI) on Open Ended Waveguide (OEWG) probes that are typically used for near-field measurements. The effect of the size and location of the absorber collar placed behind the probe was studied. It was found that for some configurations, the absorber collar could cause noticeable ripples in the far-field patterns of the probe and this in turn could affect the probe correction process when the probe was used in near-field measurements. General guidelines were developed to select an absorber configuration that would have minimal effect on the patterns, polarization and gain of the probes.
Lockheed Martin MS2 has a long history of utilizing antenna ranges for calibration, test and characterization of the phased array antennas. Each range contains an integrated RF receiver subsystem for performing antenna measurements, typically on the full array. For solid-state phased array testing, what is often needed, however, is a test station capable of performing complex S-parameter measurements on a subarray or subset of the full antenna system without incurring the expense of a test chamber. To address this requirement, Lockheed Martin, working with Nearfield Systems, has developed a portable standalone RF measurement system...
Vector Network Analyzers (VNA’s) are finding increasing utilization in antenna measurement ranges. At the same time, complex measurement scenarios involving many data channels in the antenna under test along with integration to beam steering computers for phased array antennas require management of the data collection beyond the VNA. Traditional methods have added control cards in the measurement control computer, increasing software complexity and reducing measurement throughput. The MI-788 Networked Acquisition Controller is designed to manage the hardware handshakes between position controllers, external sources and VNA’s, control up to 16 channels of multiplexed data from the antenna under test and/or interface with a beam steering computer. The MI-788 tremendously increases system throughput, particularly in these more complex measurement scenarios by removing real time data collection responsibilities from the measurement control computer. In addition, this unit makes all instrument communication Ethernet based, eliminating the spacing and operational limitations of GPIB based measurement systems. This paper will describe the operation of the MI- 788 and demonstrate the increased measurement capabilities while using VNA’s in antenna measurements.
This paper provides an overview of planar nearfield antenna test systems developed for sub-millimeter wave applications used for earth observation and radio astronomy. Examples are shown of some of these test systems and methods described to overcome technical restrictions, limiting performance at very high RF frequencies. Aspects like thermal structural change, RF cable phase instability and scanner planarity are addressed. These methods have been implemented and validated on practical real-world applications up to 660 GHz and 950 GHz. These extreme cases have lead to the development of low cost commercial test systems, making antenna testing in the V and W-bands cost effective and viable today.
Copyright 2008 IEEE. Reprinted from the COMITE Conference, Czech Republic, 23-24 April 2008.
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In this paper a technique is described that allows for the determination and correction of probe translation during polarization rotation in planar near-field measurements. The technique, which relies on the independent translation of coordinate systems for the two orthogonally polarized data sets, has significance for mm-wave testing, where bulky RF components makes probe alignment difficult. Measured data is presented to demonstrate the success of the technique.
A near-field measurement technique for the prediction of asymptotic far-field antenna patterns from data obtained from a modified cylindrical, or plane-polar, near-field measurement system is presented. This technique utilises a simple change in facility alignment to enable near-field data to be taken over the surface of a conceptual right cone [1, 2], or right conic frustum [3, 4] thereby allowing existing facilities to characterise wide-angle antenna performance in situations where hitherto they would perhaps have been limited by truncation.
This paper aims to introduce the measurement technique, describe the novel probe-corrected near-field to far-field transform algorithm which is based upon a cylindrical mode expansion of the measured fields before presenting preliminary results of both computational electromagnetic simulations and actual range measurements. As this paper recounts the progress of ongoing research, it concludes with a discussion of the remaining outstanding issues and presents an overview of the planned future work.
The probe correction of near-field measured data can be considered as being composed of two parts. The first part is a pattern correction that corrects for the effects of the aperture size and shape of the probe and can be analyzed in terms of the far-field main component pattern of the probe. The second part is due to the non-ideal polarization properties of the probe. If the probe responded to only one vector component of the incident field in all directions, this correction would be unnecessary. But since all probes have some response to each of two orthogonal components, the polarization correction must be included. The polarization correction will be the focus of the following discussion. Previous studies have derived and tested general equations to analyze polarization uncertainty12. This paper simplifies these equations for easier application. The results of analysis and measurements for Planar, Cylindrical and Spherical near-field measurements will be summarized in a form that is general, easily applied and useful. Equations and graphs will be presented that can be used to estimate the uncertainty in the polarization correction for different AUT/Probe polarization combinations and measurement geometries. The planar case will be considered first where the concepts are derived from the probe correction theory and computer simulation and then extended to the other measurement geometries.
The IsoFilterTM technique was originally demonstrated to operate by rejecting secondary signals that derive from reflections off of a nearby metallic object – namely, the ground plane surface supporting a small pyramidal horn. The aperture of the horn was located several wavelengths above the ground plane and the sidelobes and backlobes of the horn illuminated the ground plane itself. The success of this demonstration has been sufficient to encourage us to pursue further the question of how well the IsoFilterTM technique will work to suppress other types of secondary signals– such as signals coming from other elements of an array antenna or another individual first-order primary radiator nearby.
Here we report on some of the results of that investigation. We have calculated the far-field patterns of a sparsely populated array and applied the IsoFilterTM technique. The goodness of the suppression is judged by how well the “IsoFiltered” result agrees with the calculated pattern of the individual radiator.
In this work an experimental validation of the nearfield – far-field transformation technique with helicoidal scanning tailored for elongated antennas is provided. Such a transformation relies on the theoretical results relevant to the nonredundant sampling representations of the electromagnetic fields and makes use of an optimal sampling interpolation algorithm, which allows the reconstruction of the near-field data needed by the near-field – far-field transformation with cylindrical scan. In such a case, a prolate ellipsoid is employed to model an elongated antenna, instead of the sphere adopted in the previous approach. It is so possible to consider measurement cylinders with a diameter smaller than the source height, thus reducing the error related to the truncation of the scanning surface. The comparison of the reconstructions obtained from the data directly measured on the classical cylindrical grid with those recovered from the nonredundant measurements on the helix assesses the validity of this innovative scanning technique.
Measurement of radome beam deflection and/or Boresight shift in a compact range generally requires a complicated set of positioner axes. One set of axes usually moves the radome about its system antenna while the system antenna remains aligned close to the range axis. Another set of axes is normally required to scan the system antenna through its main beam (or track the monopulse null) in each plane so the beam pointing angle can be determined. The fidelity required for the beam pointing angle, combined with the limited space inside the radome, usually make this antenna positioner difficult and expensive to build.
With a far-field range, a common approach to the measurement of beam deflection or Boresight shift uses a down-range X-Y scanner under the range antenna. By translating the range antenna, the incident field's angle of arrival is changed slightly. Because the X-Y position errors are approximately divided by the range length to yield errors in angle of arrival, the fidelity required of the X-Y scanner is not nearly as difficult to achieve as that of a gimbal positioner for the system antenna.
This paper discusses a compact-range positioner geometry that approximates the simplicity of the downrange- scanner approach commonly used on far-field radome ranges. The compact-range feed is mounted on a small X-Y scanner so that the feed aperture moves in a plane containing the reflector's focal point. Translation in this 'focal plane' has an effect very similar to the X-Y translation on a far-field range, altering the direction of arrival of the incident plane wave. Measured and modeled data are both presented.
This paper gives a detailed account of free space Voltage Standing Wave Ratio (VSWR) method. We first review the formulations and terms commonly used in this method. We then discuss errors involved in its direction determination of extraneous signals, contrasting them among plane wave, spherical wave and specular reflection. We highlight issues relating to its application in anechoic chamber electromagnetic performance. Also discussed is the practice of data processing through analyzing a measured VSWR pattern.
In the 1980’s new requirements were issued to make airframes meet criteria for low radar cross-section characteristics. These requirements simultaneously created a need for improvement of radar cross-section measurement techniques to validate the performance of the designs. Thus was the advent of the modern technologies of short pulse instrumentation radars, specially designed outdoor RCS ranges, advanced indoor compact ranges, and low RCS pylons for mounting and positioning the test articles. In this talk I review the current state of these technologies and the measurement systems they support.
Copyright 2008 IEEE. Reprinted from The Loughborough Antennas and Propagation Conference, 2008.
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NSI’s MARS technique (Mathematical Absorber Reflection Suppression) has been used to improve performance in anechoic chambers and has been demonstrated over a wide range of frequencies on numerous antenna types. MARS is a post-processing technique that involves analysis of the measured data and a special filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical or far-field range or Compact Antenna Test Range (CATR). It has also been applied to extend the useful frequency range of microwave absorber to both lower and higher frequencies than its normal operating band. This paper will demonstrate the use of the MARS capability in evaluating the performance of anechoic chambers used for spherical near-field measurements, as well as in improving chamber performance.
Here we address some of the issues faced by antenna engineers when testing active antennas. We review some features of phased arrays and report on how MI Technologies has dealt with the issue of measuring the effective isotropic radiated power (EIRP) and antenna patterns by use of the spherical near-field scanning technique. We take as an example the test methodology of the SAMPSON radar antenna.
Copyright 2008 IEEE. Reprinted from IET 2008.
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Previous AMTA papers have discussed pulsed antenna measurements and the importance of parameters such as pulse width, pulse repetition frequency (PRF) and receiver dynamic range in determining the appropriate technique for performing pulsed measurements. Typically, the pulse width and PRF determine the IF bandwidth required of the instrumentation receiver to achieve a specific level of receiver performance. Less emphasis has been given to the receiver timing and synchronization required to achieve optimum performance for a full pulsed antenna measurement scenario.
This paper will discuss receiver timing considerations and show examples of scan time performance during highspeed pulsed measurements. Inter-pulse and intra-pulse measurements will be compared with respect to their impact on measurement time. Pulse profile measurements will be examined to show the importance of a fast synchronous receiver for sub-microsecond pulse characterization. Pulsed antenna pattern results will also be presented and compared with CW measurements.
Reflections in antenna test ranges can often be the largest source of measurement errors. This paper will show the results of a new technique developed by NSI to reduce scattering from the walls or other objects in the measurement chamber. The technique, named Mathematical Absorber Reflection Suppression (MARS), is a post-processing technique that involves analysis of the measured data and a filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical near-field test range. It has also been applied to extend the useful frequency range of microwave absorber in an anechoic chamber. The paper will show typical improvements in pattern performance and directivity measurements, and will show validation of the MARS technique by comparing results between a high quality anechoic chamber and a range with limited or no absorber.
Copyright 2008 IEEE. Reprinted from 2008 IEEE AP-S/URSI Symposium.
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The number of users involved in spherical near-field (SNF) antenna measurement continues to grow rapidly due to the popularity of this technology. Pressured from competition in today’s commercial market, some must perform SNF antenna measurements and produce results before they have an opportunity to gain adequate understanding of the technology and the theory behind it. This note attempts to link some SNF measurement practices to theoretical requirements from a user’s perspective and to promote a disciplined approach in measurement practice. Topics covered include range setup, probe alignment, SNF data output and AUT mounting.
To achieve the above goal, we first lay out the coordinate system in which SNF transformation is formulated, and then illustrate in detail how this coordinate system is realized by practical SNF range and probe. We then discuss the interpretation of SNF output data and AUT mounting. We also address possible misunderstandings and misconceptions. To ensure consistency, this note follows a single set of conventions and definitions for symbols and formulations detailed in [1].
This note highlights the connection of antenna gain to the measurement of insertion loss based on established SNF formulations, relating directly among antenna transmission coefficients, antenna gain, acquired SNF raw scan data and the parameters acquired during a range insertion loss measurement. It shows how the measured insertion loss parameters are applied in normalizing raw SNF scan data in determining antenna gain.
Most traditional antenna measurement techniques presume that the antenna under test (AUT) is accurately aligned to the mechanical axes of the test range. Sometimes, however, it is not possible to achieve such a careful antenna alignment [1]. In these cases, standard post processing techniques can be used to accurately correct antenna-to-range misalignment. Alternatively, similar results may be obtained by approximation in the form of piecewise polynomial interpolation. When carefully employed, this method will result in only a small increase in uncertainty, but with a significant reduction in computational effort.
This paper describes this far-field alignment correction method, which is closely related to standard active alignment correction methods [2]. This paper then proceeds to use numerical simulation as well as actual range measurements to demonstrate the effectiveness of this method. Finally, the utility of this technique in the presentation of far-field antenna pattern functions is illustrated.
The large variety of today’s wireless applications is also matched by the demands placed on the transmitting and receiving antennas they require. Therefore, antennas are probably the most highly varied of components in wireless communications systems, with virtually no restrictions in size, shape and structure. Yet, all of these antennas serve basically the same purpose: As transmitting antennas, they must convert conducted electromagnetic waves to free-space waves, and as receiving antennas they must convert these free-space waves back to conducted waves. To determine if the antenna properties are optimally suited for the application at hand, they are precisely analyzed using antenna measurement systems.
Copyright 2008 Rohde & Schwarz. Reprinted from Rohde & Schwarz News 198/08 Magazine.
This material is posted here with permission of Rohde & Schwarz. Such permission of the URSI does not in any way imply Rohde & Schwarz endorsement of any of NSI-MI Technologies' products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the Rohde & Schwarz.
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This paper describes an existing antenna range that uses a unique dual receiver configuration to solve the problem of measuring both conventional microwave antennas and the new digital beam-forming antennas in a single facility. The paper will include a comparison of antenna measurements from tests performed on actual antennas using the two different receivers.
An efficient algorithm for calculating the position of the phase center of an antenna from a measurement is derived and implemented in software. Application of the algorithm to actual measurements shows that the success of the algorithm depends on characteristics of the antenna and a weighing parameter derived from the amplitude pattern.
Reduce Near-Field Data Acquisition Time > Using an Existing Conventional PNF Facility & Operation > Data Acquisition Time (Physical Movement of the Probe) > Work Represents Our Moving in the Direction of Building Feedback/Adaptivity into Near-Field Measurements Dominates Testing Time
Copyright 2007 IEEE. Reprinted from 2007 IEEE AP-S Symposium.
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Antenna measurement data is collected over a surface as a function of position relative to the antenna. The data collection coordinate system directly affects how data is mapped to the surface: planar, cylindrical, spherical or other types. Far-field measurements are usually mapped or converted to spherical surfaces from which directivity, polarization and patterns are calculated and projected. Often the collected coordinate system is not the same as the final-mapped system, requiring special formulas for proper conversion. In addition, projecting this data in two and three-dimensional polar or rectangular plots presents other problems in interpreting data. This paper presents many of the most commonly encountered coordinate system formulas and shows how their mapping directly affects the interpretation of pattern and polarization data in an easily recognizable way.
This paper discusses design aspects related to a tiltable lightweight near-field scanning system for use at submillimeter frequencies. It addresses design issues as they relate to accuracy and scanner distortions from multiple causes. Calibration methods to measure and correct for anticipated and unanticipated errors are briefly addressed. Actual test results are presented.
The tiltable scanner being discussed was designed for the Atacama Large Millimeter/submillimeter Array (ALMA) [1] and is being used by the National Radio Astronomy Observatory (NRAO) [2]. It has many other applications by virtue of its light weight (approx. 120 lbs) and ability to be oriented at different angles. These include flightline testing and other in-situ antenna test applications.
Realized gain measurements of a 44 beam 44 element linear array over a 43.5 to 45.5 GHz design frequency range are presented. The prototype array1 is designed as a single column of a 50 column multibeam 2200 element planar active receive array for geostationary satellite communications payload. The 2200 element planar array is designed to form 1760 simultaneous narrow 0.4 degree beams, 1463 of which intercept the earth. The multibeam single prototype column realized gain was tested at the Nearfield Systems Inc.'s (NSI) facility using a 12’ x 12’ Planar Near-Field Range. Two different linear array configurations were tested. Each configuration utilized the same WR-19 waveguide fed 44 beam, 44 element Rotman lens and integrated RF distribution network (RFD). An active receive array utilizing only the center 8 array elements of the Rotman lens feed was tested first. This was followed by a 44 array element passive array test demonstrating the narrow 0.4 degree half power beamwidth. Summary and specific examples of the NFR test results will be presented. These will be compared with that predicted using the previously measured lens array factor gain (AFG) and embedded element realized gain. The AFG was measured using a HP8510C automatic network analyzer.
The success of most traditional implementations of antenna measurement techniques whether near field, far field or compact, assume that a fiducial mechanical datum associated with the antenna under test (AUT) can be accurately and precisely aligned to the mechanical axes of the test range. Unfortunately, an alternative approach is sometimes necessary, as achieving such careful alignment is not always convenient or possible. Instead, if the relationship between the frame of reference associated with the antenna and the frame of reference associate with the range can be acquired, i.e. assuming that it is known, then in principal any misalignment can be corrected for within the data processing chain. Techniques for rigorously implementing the necessary vector isometric rotation are well documented and usually utilise the concepts of a modal expansion. In general this is not always convenient as these methods can be difficult to implement and often require the transformation of one modal expansion to another, e.g. planar or cylindrical to spherical, etc.. This paper describes the additional post processing that is required to yield alignment corrected far field data from an acquisition of an imperfectly aligned antenna. A general-purpose vector isometric rotation strategy is utilised that is reliant upon interpolation, rather than a particular modal expansion. The interpolation is performed using a polar, i.e. amplitude and phase, implementation of a two dimensional bi-cubic convolution interpolation algorithm. The effectiveness of this technique is then demonstrated through the use of range measurements.
Copyright 2007 IEEE. Reprinted from Loughborough Antennas and Propagation Confererence, 2007.
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Attempts to produce robust, objective, quantitative measures of comparison between data sets using statistical methods have been widely reported in the literature. Recently, techniques have been developed that require the antenna pattern functions to be converted into histograms before the comparison, i.e. the measure of adjacency, is made.
The success of such a tactic can be crucially dependent upon the choice of categorising “bins”. As the number, level, and size of these bins can be chosen both a priori and freely, it is possible that the resulting histograms will be sparsely populated with the majority of the samples falling within only a few of the categories. This difficulty can be avoided if the bins are defined in such a way that roughly equal numbers of samples fall within each of the categorising bin.
This paper describes an efficient method for “equalising” any histogram and illustrates the effectiveness of this strategy with example, measured data.
Copyright 2007 IEEE. Reprinted from EuCAP 2007 Conference.
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The calibration of electronically steered arrays require the determination of the aperture distribution of the antenna for a given set of excitation coefficients. A common approach taken in this process is to measure the radiating near-field of the antenna and to then mathematically back project to the actual aperture surface of the array to form an estimate of the aperture distribution. It is well known that the result obtained in this way is of limited resolution and accuracy and these limitations may at times render the result of little use for adjusting the array feed distribution.
In this paper theoretical results will be presented for a steered array antenna, which will demonstrate the limitations of this back projection technique. These results will also show the accuracy with which the original excitation vector can be recovered. Parametric studies showing the impact of spherical angular truncation and over-sampling in the nearfield will be presented.
This paper will also investigate the impact of spherical near-field scanner alignment anomalies (i.e. axis non-intersection, axis orthogonality and axis zero reference) on the achieved back projected resolution. Comparison to equivalent results obtained from a planar near-field back projection technique will also be presented.
Copyright 2007 URSI. Reprinted from The International Union of Radio Science (URSI) 2007 Conference, Ottawa Canada, July 2007.
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Reflections in anechoic chambers can limit the performance and can often dominate all other error sources. NSI’s MARS technique (Mathematical Absorber Reflection Suppression) has been demonstrated to be a useful tool in the fight against unwanted reflections. MARS is a post-processing technique that involves analysis of the measured data and a special mode filtering process to suppress the undesirable scattered signals.
The technique is a general technique that can be applied to any spherical near field or far-field range. It has also been applied to extend the useful frequency range of microwave absorber down to lower frequencies. This paper will show typical improvements in pattern performance, and will show results of the MARS technique using data measured on numerous antennas.
The next generation of antennas will benefit from advanced instrumentation receivers capable of providing simultaneous analog and digital IF inputs, better TR pulse synchronization and high resolution pulse profiling. One such receiver uses a synergistic combination of a tightly coupled FPGA based beam controller, high performance analog digitizers, multiple FPGA based digital signal processors and a new mathematical programming environment. The FPGA signal processor provides direct digital downconversion, high resolution pulse processing and dynamically reconfigurable time and frequency gated matched filter signal integration. The signal processing functions are fully scriptable, providing spectral analysis, various other types of transform analysis, instantaneous demodulation, pulse characterization, noise estimation and more. Advanced mathematical tools combined with novel user interface technologies provide multiple intuitive views into the test setup, error analysis and measurement environment.
Comparisons of the far-field results from two different ranges are a useful complement to the detailed 18 term uncertainty analysis procedure. Such comparisons can verify that the individual estimates of uncertainty for each range are reliable or indicate whether they are either too conservative or too optimistic. Such a comparison has recently been completed using planar and spherical nearfield ranges at Nearfield Systems Inc. The test antenna was a mechanically and electrically stable slotted waveguide array with relatively low side lobes and cross polarization and a gain of approximately 35 dBi.
The accuracies of both ranges were improved by testing for, and where appropriate, applying small corrections to the measured data for some of the individual 18 terms. The corrections reduce, but do not eliminate the errors for the selected terms and do not change the basic near-to-far field transformations or probe correction processes. The corrections considered were for bias error leakage, multiple reflections, rotary joint variations and spherical range alignment. Room scattering for the spherical measurements was evaluated using the MARS processing developed by NSI.
The final results showed a peak equivalent error signal level in the side lobe region of approximately -60 dB for both main and cross component patterns for angles of up to 80 degrees off-axis.
Following an introductory review of spherical near-field scanning measurements, with emphasis on the general applicability of the technique, we present a survey of the various methods to improve measurement accuracy by correcting the acquired data before performing the transform and by special processing of the resulting data following the transform. A post-processing technique recently receiving additional attention is the IsoFilter™ technique that assists in suppressing extraneous stray signals due to scattering from antenna range apparatus.
Copyright 2007 IET. Reprinted from EuCAP 2007 Conference.
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The NIST 18 term error analysis has been used for some time to estimate the uncertainty in the far-field antenna parameters determined from near-field measurements. Each of the error terms is evaluated separately to estimate the uncertainty it produces in parameters such as gain, directivity, side lobe level, cross polarization level and beam pointing angle. This identification and evaluation of uncertainties has led to the development of procedures that can be used to reduce the effect of individual error sources and therefore improve the reliability of the results.
Automated, real time systems have been added to the measurement hardware and electronics that can reduce the effect of such things as probe position errors and cable flexing. Measurement and special computer processing techniques have also been developed to self-calibrate and correct for transmission path differences of dual mode probes.
More recently, a number of techniques have been developed that provide a means to reduce the effect of measurement errors without the need of special hardware or additional measurements. These procedures often involve additional data processing steps to identify and reduce the presence of the error in the measured data, but the processing time is small and the improvement in some parameters can be very significant. In some cases, the error signal level can be reduced by 10 to 20 dB. Such techniques have been developed for errors due to bias error leakage in the receivers, non-ideal rotary joints, spherical rotator misalignment, and room scattering. Further improvements can be realized by making additional measurements to reduce multiple reflection effects, position errors and room scattering in spherical systems.
Examples of these techniques will be presented to illustrate the methods and demonstrate typical improvement.
Copyright 2007 IEEE. Reprinted from The Second European Conference on Antennas and Propagation (EuCAP 2007) 11-16 November 2007.
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The IsoFilterTM Technique is a novel method of isolating the radiation pattern of an individual radiator from among a composite set of radiators that form a complex radiation distribution. This paper demonstrates that the technique is viable and applicable to cases where the individual radiator of interest is near the boundary of the minimum sphere that encloses the entire collection of sources.
Copyright 2007 IEEE. Reprinted from EuCAP 2007 Conference.
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The utility of a variety of objective, quantitative and robust methods of assessing similarities between antenna measurement data have already been highlighted in the literature. These techniques essentially involved the extraction of interval, ordinal, and categorical features from antenna pattern data sets that can then be effectively compared to establish a measure of their adjacency, i.e. similarity. Hitherto, such techniques have primarily been limited to the purposes of comparing two or more images as a means in itself, e.g. far-field three-dimensional radiation pattern of a given antenna having been characterised using two different facilities. In contrast, this paper discusses the utility of such techniques for the purposes of establishing convergence within an iterative optimisation process, namely the phase retrieval (PR) plane-to-plane algorithm. Within this paper, in addition to the conventional holistic error metrics, an alternative image classification comparison technique is employed as an error metric. The convergence properties, as reported by these various metrics are compared and contrasted using empirical mm-wave measured data taken using a planar near-field scanner and processed using a commonly encountered plane-to-plane PR algorithm.
In this paper we show how a modification in the choice of mode normalization changes the pair-wise transmitting-receiving conversion to a one-to-one equality for a reciprocal antenna... This change affords us greater simplicity and the opportunity to avoid confusion when manipulating the scattering coefficients. For this relation to hold, there is a useful convention defining two fiducial coordinate systems for the antenna – one a transmitting and the other a receiving coordinate system.
A sophisticated strategy for extrapolating the samples external to the measurement region in the helicoidal scanning is proposed in this paper. It relies on the nonredundant sampling representations of the electromagnetic field an on the optimal sampling interpolation expansions of central type. Such a technique uses the singular value decomposition method for evaluating the outside samples. The estimation of such data allow one to reduce the truncation error affecting the field interpolation in the zone close to the ends of the scanning cylinder, thus giving rise to a more accurate far-field reconstruction. Some numerical tests, assessing the accuracy fo the technique and its stability with respect to random errors affecting the data, are reported.
An effective approach for the estimation of the data external to the near-field measurement region in the helicoidal scan is developed in this work. It is based on the nonredundant sampling representations of the electromagnetic field and uses the singular value decomposition method for extrapolating the outside samples. It is so possible to reduce the inevitable truncation error affecting the near-field reconstruction, due to the finite dimension of the helix. Numerical examples assess the effectiveness of the proposed technique.
Copyright 2006 IEEE. Reprinted from Loughborough Antennas and Propagation Confererence, 2006.
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Active antenna measurements are familiar to traditional antenna test range operators. Pulsed, multi-beam, multi-frequency, phased-array measurements have become quite popular for military and space-based applications. These combine typical antenna patterns with active RF excitation in order to create a system-like capability. A new type of measurements called Mobile Station Over-the-Air (OTA) Measurements is emerging, which attempts to include even more of the communication system (antenna, amplifier, receiver and electronics) in the measurement. Promoted by CTIA (Cellular Telecommunications & Internet Association), OTA measurements attempt to test system components closer to the environment in which they will be used. RF excitation is no longer just an RF source in pulsed or CW modes, but requires a Base Station Simulator (BSS) with protocols such as GSM, CDMA, Bluetooth, 802.11g, etc. Traditional antenna patterns are less important than the newly required measurements of TRP (Total radiated power) and TIS (Total isotropic sensitivity). Range operators must become familiar with these concepts in order to keep up with the ever-changing requirements of the future. This paper provides the reader with an overview of Mobile Station OTA measurements, techniques and sample data.
Characterization of radome performance involves measuring the radome-induced changes in the microwave signals that are transmitted and received by the antenna through the radome. The standard quantities that characterize a radome’s performance are Beam Deflection, Boresight Shift, Transmission Efficiency, Change in Antenna Reflection Characteristic, and Pattern Distortion. Typical test system configurations include an RF subsystem and a mechanical positioning subsystem interfaced to a host computer. In this paper we review three types of radome test systems that have been implemented recently − far-field range, compact range, and near-field scanning ranges. We point out the salient features of each. We note the importance of digital automation to making these measurements practical.
Copyright 2006 IEEE. Reprinted from Loughborough Antennas and Propagation Confererence, 2006.
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This paper presents an overview of work carried out in developing the probe-corrected, poly-planar near-field antenna measurement technique. The poly-planar method essentially entails a very general technique for deriving asymptotic far-field antenna patterns from near-field measurements taken over faceted surfaces.
The probe-corrected, poly-planar near-field to far-field transformation, consisting of an innovative hybrid physical optics (PO) plane wave spectrum (PWS) formulation, is summarised, and the importance of correctly reconstructing the normal electric field component for each of the discrete partial scans to the success of this process is highlighted. As an illustration, in this paper the poly-planar technique is deployed to provide coverage over the entire far-field sphere by utilising a small planar facility to acquire two orthogonal tangential near electric field components over the surface of a conceptual cube centred about the antenna under test (AUT). The success of the poly-planar technique is demonstrated through numerical simulation and experimental measurement. A discussion into the limitations of the partial scan technique is also presented.
NSI has developed a novel technique for automating antenna range configurations. Although automation has shown to dramatically improve range productivity, most of today’s antenna ranges are reconfigured manually. Today’s automated ranges use electromechanical RF switches to control the RF signal path, which is contained primarily in a central rack, thereby limiting automation to ranges that are relatively small in size. Larger ranges, however, tend to locate many of the RF components such as mixers, couplers, amplifiers and multipliers remotely near the probe or AUT, sometimes 100 ft (30 m) or more from the rack, making the remote RF components more difficult to access and control. To address this problem, NSI has developed the Range Transition Manager (RTM) for automating large antenna ranges. The RTM uses modular packaging with a LAN interface and embedded processor to provide commonality and flexibility in automating various range sizes and types. The RTM family of modules provide a full range of automation capability for 0.5 to 18 GHz and higher frequencies.
This paper will describe the capabilities of the Range Transition Manager developed for a large near-field scanner and describe how the RTM improves overall range productivity.
In this paper a circular planar near-field scan region is considered as an alternative to the commonly used rectangular boundary. It is shown how the selection of this alternative boundary can reduce test time and also to what extent the alternative truncation boundary will affect far-field accuracy. It is also shown how well known single dimensional filter functions can be applied over a two-dimensional region of test and how these attenuate the truncation effect. The boundary and filter functions are applied to measured data sets, acquisition time reduction is demonstrated and the impact on far-field radiation pattern integrity in assessed.
This paper describes some improvements in the measurement process of the NIST 18 term error analysis that reduces the required measurement time and also improves the sensitivity of some of the tests to the individual sources of uncertainty. As a result, the measurement time is reduced by about 40 % and some of the estimated uncertainties are also improved without a reduction in the confidence levels. The reduction in measurements is accomplished by using one measurement for two or more error terms or using centerline rather than full 2D data scans for some of the terms.
An elaborate and effective strategy for estimating the samples external to the measurement region in the planar spiral scanning is developed in this paper. It relies on the nonredundant sampling representations of the electromagnetic field and on the optimal sampling interpolation expansions of central type and uses the singular value decomposition method for extrapolating the outside samples. It is so possible to reduce the inevitable truncation error affecting the near-field reconstruction, thus giving rise to a more accurate far-field prediction. Numerical examples assess the effectiveness of the proposed technique.
Copyright 2006 The University of Salerno. Reprinted from AMTA 2006 Conference
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Reflections in anechoic chambers can limit the performance and can often dominate all other error sources. This paper will show the results of a new technique developed by NSI to suppress reflections and improve performance in anechoic chambers. The technique, named Mathematical Absorber Reflection Suppression (MARS), is a post-processing technique that involves analysis of the measured data and a special filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical range. It has also been applied to extend the useful frequency range of microwave absorber in a spherical near-field system. The paper will show typical improvements in pattern performance, and will show validation of the MARS technique using data measured on an antenna in a conventional anechoic chamber.
As an RF cable is moved during data acquisition, its insertion loss will often change. Techniques have been published that measure and compensate those changes in insertion loss. Each of these techniques, however, requires stable access to both the signal source and the receiver at one end of the cable bundle. This requirement poses a challenge when trying to compensate a moving RF cable between a receive antenna and a mixer where there are additional axes below the mixer. This paper will show measured results of a new technique developed by MI Technologies to do similar compensation where the source and receiver are at opposite ends of the moving (or otherwise changing) cable bundle. The technique was developed for transmission efficiency measurements on radomes, but also has applicability for quiet-zone field probing or any other scenario where a strong signal is always being received. It requires the use of multiple identical RF cables in the cable bundle, and measures multiple cable combinations to determine the cable characteristics.
Spherical near-field measurements have become a common way to assess performance of a wide variety of antennas. Published reports on range error assessments for spherical near-field ranges however are not very common. This is likely due to the perceived additional complexity of the spherical near-field measurement process as compared to planar or cylindrical measurement techniques. This paper will establish and demonstrate a simple procedure for characterizing the performance of a spherical near-field range. The measurement steps and reporting can be largely automated with careful attention to the test process. We will summarize the process and document the accuracy of a spherical near-field test range at NSI using the same NIST 18 terms commonly used for planar near-field measurements.
The increasing numbers of microwave instruments and devices that include IEEE 802.3 interfaces are influencing range design and capabilities, as well as the ability to remotely locate GPIB instruments. The major benefits of LAN based instrumentation systems are increased flexibility in instrument location and increased capabilities over long distances compared to GPIB based ranges. This paper discusses the relative merits of LAN based microwave test instrumentation ranges. Several example range designs are included that demonstrate how LAN based instrumentation can increase range flexibility and reduce costs in range implementation.
The IsoFilterTM Technique is a novel method of isolating the radiation pattern of an individual radiator from among a composite set of radiators that form a complex radiation distribution. This paper demonstrates that the technique is viable and applicable to cases where the individual radiator of interest is near the boundary of the minimum sphere that encloses the entire collection of sources.
This paper describes a novel method termed the IsofilterTM Technique, of isolating the radiation pattern of an individual radiator from among a composite set of radiators that form a complex radiation distribution. This technique proceeds via three successive steps: A spherical NFFF transform on an over-sampled data set, followed by a change of coordinate system followed in turn by filtering in the domain of the spherical modes to isolate a radiating source. The end result is to yield an approximate pattern of the individual radiator largely uncontaminated by the other competing sources of radiation.
This paper describes traditional antenna measurements and the relationship to the Over-the- Air (OTA) measurements specified by the Cellular Telecommunications & Internet Association (CTIA). It discusses the differences, the likenesses, and the importance of providing a system that can provide both traditional antenna measurements and CTIA OTA measurements. It will address the processes of providing a complete turn-key system – including chamber – that will meet CTIA certifications. Further, this paper shows the unique flexibility and features that the 700S-90 provides for meeting the customer’s needs, for a wide-variety of applications.
We are in the era of wireless communications and devices. The antennas that enable these technologies are electrically small and can be challenging to test and analyze. Yet, the industry is becoming more standardized, and so too are the tests and certifications being adopted to validate these antennas. These antennas must undergo “antenna measurements” to characterize such information as far-field patterns and gain. Additionally, hand-held devices, such as cell phones, must satisfy requirements of the Over-the-Air (OTA) performance tests as specified by the Cellular Telecommunication and Internet Association (CTIA). These tests require a measurement system that can accurately collect data on a spherical surface enclosing the AUT. This system also has to provide the appropriate data analysis capabilities and has to be constructed from dielectric materials to minimize reflections.
A hemi-spherical near-field test system with added far-field capability is described. The facility has been constructed for the characterization of automotive antennas. The test system consists of an 11m tall dielectric gantry, a 6.5m diameter in-ground turntable and a 28m-diameter radome enclosure. Special software required to compensate for the reflectivity in the facility and the hemi-spherical truncation was developed and forms an integral part of this test system. The characteristics of this facility are described in this paper and measured data is presented.
In making accurate measurements of antenna gain one must correct for the impedance mismatches between (1) the signal generator and transmitting antenna, (2) between the receiving power sensor and the receiving antenna and (3) between the signal generator and receiving power sensor. This is true for both far-field gain measurements and near-field gain measurements. It has recently come to our attention that there is a lack of clarity as to the form the mismatch factor should take when correcting near-field measured data. We show that a different form of impedance mismatch factor is to be used with the voltage domain equations of near-field than has been used with the power domain Friis transmission equation.
MI Technologies has developed and constructed two of a new generation of spherical near-field ranges that expand the regime of measurement requirements to which the spherical near-field technique can be applied. This evolutionary design permits measurements to be made on large apertures -- up to 10.0 m (32 ft) and at frequencies up to 60 GHz under conditions of constant gravity loads over 4π SR of coverage. Here we report on developments leading to a spherical near-field scanning system that has been built and realized for apertures up to 3.66 m (12 ft) and frequencies between 2.0 and 45 GHz which corresponds to an electrical size of 550 wavelengths.
This paper describes the first experience gained with a new carbon composite compact range reflector (C³R²). The reflector’s backup structure is made entirely of carbon fiber reinforced composite material. An outstanding advantage of this design is its superior mechanical and thermal-environmental stability. This yields improvement in the overall performance. We have revised the process by which compact range reflectors are designed and modeled, making use of professionally authored software. We describe the results of electromagnetic field probe measurements made at the factory. Special attention is given to new results at W-band – in the 75 to 100 GHz regime.
Precise mechanical alignment of motion axes of both cylindrical and spherical near-field systems is critical to producing accurate data. Until recently the only way to align these types of systems was to employ traditional optical tooling (i.e. jig transits, theodolites). Alignment by these methods is difficult, time consuming, and requires specialized training. More recently, laser trackers have been used for this type of alignment. Unfortunately, these devices are expensive and demand an even higher level of operator training.
This paper describes the use of low cost alignment tools and techniques that have been developed by Nearfield Systems, Inc. (NSI) that greatly simplify the alignment process. Setup and alignment can be performed in a very short period of time by technicians that have been given minimal training. Suitable optical alignment procedures when followed by the use of electrical alignment techniques [7] yield sufficient alignment accuracy to permit testing up to Ku-band.
A large compact range facility required a foam column for RCS testing where the center of the quiet zone was six meters above the floor level. The RCS measurement after vector background subtraction, had to be accurate down to a –50 dBsm level from 1.5 GHz to 40 GHz. A foam column was constructed from a single billet of material. The foam column was evaluated as to its RCS level in both whole body and ISAR imaging modes. This paper describes the specification, construction and RCS evaluation of this column in the compact range facility. The column was evaluated at single frequencies and with RCS images from 2 GHz to 36 GHz using a gated CW radar. Data is presented that shows the effects of the column on the response of a calibration sphere and the response of the column itself. A study of the foam column imaging response used as the background for vector background subtraction is also described. Targets in the –60 dBsm range were successfully imaged with vector background subtraction of the foam column.
Reflections in antenna test ranges can often be the largest source of measurement errors, dominating all other error sources. This paper will show the results of a new technique developed by NSI to suppress reflections from the radome and gantry of a large hemi-spherical automotive test range developed for Nippon Antenna in Itzehoe, Germany. The technique, named Mathematical Absorber Reflection Suppression (MARS), is a post-processing technique that involves analysis of the measured data and a special filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical near-field test range. It has also been applied to extend the useful frequency range of microwave absorber in a spherical near-field system in an anechoic chamber. The paper will show typical improvements in pattern performance and directivity measurements, and will show validation of the MARS technique using data measured on antennas in a conventional anechoic chamber.
This paper describes a Composite Near-Field Scanning Antenna Range for frequency bands that extend from X-Band in the microwave frequency regime through W-Band in the millimeter-wave regime – i.e. 8.2 through 110 GHz. We show some of the initial checkout data using pyramidal standard gain horns and compare the patterns to theory.
This paper describes a novel optical boresighting and alignment system used to mechanically align antennas on a compact antenna range at the North Island Naval Air Depot in San Diego, CA. The antenna range has a 5-axis (roll/upper slide/azimuth/elevation/lower slide) positioner used to measure various airborne antennas for production testing. The video alignment system implemented on this range uses two video cameras outfitted with telephoto lenses, one on the roll stage and the other on an antenna-mounting fixture. The system has been demonstrated to yield an accuracy of ±0.005 degrees. Prior to the start of testing the positioner is commanded to a “0” position and the cameras focus on a fixed optical target to provide the operator with a quick visual confirmation that the positioner is accurately aligned prior to testing. The video alignment system described has numerous advantages over other mechanical alignment techniques, is low cost, easy to use, and can be adapted to a variety of testing configurations.
A high speed receiver and beam controller has been introduced to address the growing need for faster antenna measurements. This article describes the Panther 6000 system and will show how it can benefit the antenna range operator by allowing more complex CW and pulsed antennas to be tested in less time.
Many of today’s high performance antennas are broadband, multi-beam, multi-port and polarization-selective devices. Technology improvements, including micro-electro-mechanical system (MEMS) devices and novel adaptive algorithms, are enabling increasingly complex antennas for satellite communications, radar, telecom, wireless and automotive applications. New and complex architectures are also being implemented, including adaptive arrays, multiple-reflector and digital beamforming antennas.
Copyright 2004 Horizon House Publications, Inc. Reprinted from icrowave Journal, January, 2004 Issue.
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The reflection coefficient of an antenna impacts the power transmitted by the antenna. Accurate characterization of this parameter is important in a communication or radar system. This paper discusses an implementation whereby a reflectometer is located near the antenna under test in an antenna range albeit far from the receiver. By placing the reflectometer near the antenna, the measurement uncertainty intrinsic to long cable runs can be minimized.
This paper describes the development, testing and evaluation of a new, automated system for calibration and AUT alignment of a planar near-field scanner that allows the calibration system to remain in place during AUT measurement and which can be used to support AUT alignment to the scan plane. During scanner calibration, probe aperture position measurements are made using a tracking laser interferometer, a fixture that positions the interferometer retro reflector at a precise location relative to the probe aperture and a probe roll axis that maintains the proper orientation between the retro reflector and the interferometer as the probe position is moved. Aperture scan path information is used to construct a best-fit scan plane and to define a Cartesian, scanner-based coordinate system. Scan path data is then used to build a probe position error map for each of the three Cartesian coordinates as a function of the nominal position in the scan plane. These error maps can be used to implement software-based corrections (K-corrections) or they may be used for active Z-axis correction during measurements. By using a set of tooling points on the antenna mount, an AUT coordinate system is measured with the interferometer. The system then directs an operator through a set of AUT adjustments that align the AUT with the planar near-field scanner to a desired accuracy. This paper describes the implementation and testing of the system on an actual planar scanner and AUT test environment, showing the improvement in effective scanner planarity.
This paper describes the methodologies and processes used for the development, installation, alignment and qualification of a Compact Range Rolled Edge Reflector purchased by the MIT Lincoln Laboratory and installed at their test facility located at Hanscom Air Force Base. The Ohio State University, under contract to MIT Lincoln Laboratory, performed the electromagnetic design and analysis to determine the desired surface shape and required mechanical accuracy of various zones of that surface. The requirement for operation over a very broad frequency range (400 MHz to 100 GHz) resulted in a surface specification that was both physically large (24 ft × 24 ft) and included extremely tight tolerance requirements in the center section.
The mechanical design process will be described, including the generation of a solid “Master Surface” created from the “cloud” of data points supplied by The Ohio State University, verification of the “Master Surface” with The Ohio State University, segmentation of the reflector body into multiple panels, design, fabrication and factory qualification of the structural stands, panel adjustment mechanisms, and panels. Results of thermal cycling of the reflector panels during the fabrication process will be presented.
The processes used for installation of the reflector and the alignment of each panel to the “Master Surface” will be presented and discussed. Final verification of the surface accuracy using a tracking laser interferometer will be described. Color contour plots of the reflector surface will be provided, illustrating the final surface shape and verifying compliance to the surface accuracy requirement.
This paper describes a new measurement system for testing antennas used in wireless communication system. A two-axis dielectric AUT positioner is used to reduce pattern perturbations, a vector network analyzer is employed for the receiver, and a broadband source antenna is utilized. RF test system design considerations are discussed including chamber design size. Software requirements that are unique to the needs of the wireless communication community are covered. The system is capable of making both far-field and spherical near-field measurements. A wide variety of antenna types can be tested using this system including handsets, laptop computers, and base station antennas.
Geometries for measuring radome characteristics can usually be split into two categories. The first category always has the antenna inside the radome pointing along the range axis. The second category has the antenna maintaining a fixed relationship with respect to the radome during each scan of data.
A facility can generally be designed to minimize measurement errors in one of the two geometries, but not both. Many facilities that permit collection of data in both geometries would benefit from the ability to dynamically capture data that lead to measurement errors, then compute and remove the associated errors.
This paper discusses some of the primary error contributors in a dual-geometry radome measurement system, and suggests some mechanisms for capturing and potentially removing those errors.
A combination of analysis and simulation are used to estimate the amplitude and phase errors in the hologram calculated from planar near-field data. The antenna hologram is modeled as the sum of a continuous function that essentially represents a correctly aligned antenna and one or more step function discontinuities located at elements that are misaligned. The spectrum of the step function can be calculated exactly from the assumed amplitude, phase, pulse width and location of the element. This spectrum is then filtered in k-space to simulate the effect of truncating the measurement plane. It is well known that the primary result of truncation is the filtering of real and evanescent plane waves beyond an angle defined by the antenna and measurement plane geometry.
Using the results of the analysis, a script program was developed for the NSI2000 software that would calculate the spectrum from the input parameters, perform the filtering and calculate the hologram using the Fast Fourier Transform. The change in the amplitude of the reconstructed hologram pulse is then used to determine the error that results in the calculated element amplitude and/or phase. Sample curves are generated to illustrate the technique.
This paper presents the results of a laboratory simulation of an outdoor telematic antenna test site that employs spherical near-field scanning to determine the far fields of telematic antennas mounted on vehicles.
In this paper, we describe a new method for improving the true-position accuracy of a very large, spherical near-field measurement system. The mechanical positioning subsystem consists of 10-meter diameter, 180° circular-arc scanner and an MI Technologies MI-51230 azimuth rotator and position controller.
The principle components of the error correction method are the error measurement system, the position correction algorithm, and a pair of very high precision, mechanical error correction stages. Using a tracking laser interferometer, error maps are constructed for radial, planar and elevation errors. A position correction algorithm utilizes these discrete-point error maps to generate error correction terms over the continuous range of the elevation axis. The small position correction motions required in the radial and planar directions are performed using the mechanical correction stages. Corrections to the position of the elevation axis are made using the primary elevation axis drive.
Results are presented that show the geometry of the spherical scanning system before and after error correction. It is observed that the accuracy of the radial, planar and elevation axes can be significantly improved using the error correction system.
The testing of modern automotive antennas is often accomplished by using spherical near-field test facilities. This paper presents sensitivity analysis data for errors introduced in the probe position of such a test system. The purpose of this study is to establish mechanical design goals for the gantry structure required in the implementation of the probe positioning system. Far-field error levels are calculated to demonstrate the effect of the probe positioning anomalies.
Copyright 2004 IEEE. Reprinted from ANTEM 2004 Conference.
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In this paper two planar near-field scan plane reduction techniques are considered and results are presented. It is shown how truncation based on field intensity contours, instead of simple geometric truncation can in some cases improve the efficiency of the truncation process. Both techniques are applied to measured data sets and it is shown how these methods can be used to reduce data acquisition times while also assessing the impact of the total acquisition surface reduction on the far-field radiation pattern integrity.
Spherical near-field measurements require an increased level of sophistication and care to achieve accurate results. This paper will demonstrate an automated set of self-comparison tests, which can be used for establishing and optimizing a spherical system's performance. An over-determined set of measurements can help to qualify positioner alignment, range reflection levels, truncation effects, and additional parameters of interest. These results will help in optimizing the test configuration to achieve accurate near-field measurement results.
Characterization of radome performance involves measuring the radome-induced microwave signal attenuation and change in propagation direction. Transmission efficiency (TE) is a measurement of the percentage of microwave energy passing through the radome. Beam deflection (BD) is a measurement of the change in the direction of propagation of the energy as it passes through the radome.
Typical instrumentation configurations include one or two receivers interfaced with a monopulse antenna through a tracking loop. Tracking is mechanized at either the transmit or receive location.
The difficulty with these configurations is the complexity required to implement, coordinate, monitor, and measure tracking with sufficient speed and accuracy. MI Technologies has developed and tested a relatively inexpensive and simple dualreceiver method that provides high accuracy and fast test time. A radome measurement system of this configuration is installed and has been successfully tested for The Boeing Company.
At a Department of Defense antenna measurement laboratory, an important measurement is the accurate measurement of gain for circularly polarized antennas. An additional requirement is that a wide population of engineers and technicians that do not spend a significant amount of time using the facility make the measurements as they test the antennas for their projects. The objective was to create a highly automated, accurate test structure that was easily used by visiting engineers to make high quality measurements. Consistency of results across the user population was a paramount requirement. This paper describes the instrumentation and software used to meet this objective.
In this paper, we describe the process used to align a large spherical near-field test system. The probe positioner consists of a cantilevered arc design with a probe path radius of five meters and a scan angle of 180°. The AUT positioner consists of an MI Technologies Model 51230 azimuth positioner with a high-precision encoder. The system is aligned using an SMX Tracker 4000 tracking laser interferometer.
Alignment into a spherical system is achieved by initially defining two cylindrical systems; a primary probe positioner based system and a secondary, AUT positioner based system. Sources of mechanical error in each of these systems are identified and techniques used to control these error sources are described.
Probe calibration is a prerequisite for performing high accuracy near-field antenna measurements. One convenient technique that has been used with confidence for years consists of using two auxiliary antennas in conjunction with the probe-to-be-calibrated. Inherent to this technique is a calibration of all three antennas. So far the technique has mostly been applied to calibrate for polarization and gain characteristics. It is demonstrated how the technique can be extended to also measure an antenna’s phase-versus-frequency characteristic.
A new spherical near-field probe positioning device has been designed and constructed consisting of a large 5.0 meter fixed arc. This arc has been installed in a near-field test facility located at Alenia Marconi Systems on the Isle of Wight, UK. As part of the near-field qualification, testing was performed on a ground based radar antenna. The resultant patterns were compared against measurements collected on the same antenna on a large outdoor cylindrical near-field test facility also located on the Isle of Wight. These measurements included multiple frequency measurements and multiple pattern comparisons.
This paper summarizes the results obtained as part of the measurement program and includes discussions on the error budgets for the two ranges along with a discussion on the mutual error budget between the two ranges.
Probe position errors, specifically the uncertainty in the theta and phi position of the probe on the measurement sphere, are one of the sources of error in the calculated far-field and hologram patterns derived from spherical near-field measurements. Until recently, we have relied on analytical result for planar position errors to provide a guideline for specifying the required accuracy of a spherical measurement system. This guideline is that the angular error should not result in translation along the arc of the minimum sphere of more than λ/100.
As a result of recent simulation and analysis, expressions have been derived that relate more specifically to spherical near-field measurements. Using the dimensions of the Antenna Under Test (AUT), its directivity, the radius of the sphere (the minimum sphere) enclosing all radiating surfaces and the frequency we can estimate the errors that will result from a given position error. These results can be used to specify and design a measurement system for a desired level of accuracy and to estimate the measurement uncertainty in a measurement system.
This paper describes the installation and implementation of a Cylindrical Near-field Test Facility at Chelton Radomes Ltd, Stevenage, (formerly British Aerospace Systems and Equipment Ltd.), in the UK for the testing of large radome/antenna combinations. Test site commissioning and validation activities to determine measurement accuracy & repeatability for the radome performance parameters of transmission loss and boresight error, are discussed. Test data from actual measurements are presented.
Modern radome measurements often involve scanning the radome in front of its antenna while the antenna is actively tracking an RF signal. Beam deflections caused by the radome are automatically tracked by the antenna and its associated positioning system, which is typically a two-axis (pitch & yaw) gimbal. The motion required to accurately track the beam can be very demanding of the gimbal. High structural stiffness, zero drivetrain backlash, and extremely accurate angle measurement are all necessary qualities for radome beam deflection measurement. This paper describes a new, advanced, two-axis gimbal that embodies those qualities.
A measurement technique for Effective Isotropic Radiated Power (EIRP) using planar near-field scanning has been demonstrated earlier. In this paper I show how we at MI Technologies have implemented using the spherical near-field method the measurement of EIRP and a vector phasor quantity analogous to Effective Area that we call Antenna Receive Response. This technique is applicable to all antennas, including active antennas.
The recent launch and successful orbiting of the EO-1 Satellite has provided an opportunity to validate the performance of a newly developed X-Band transmit-only phased array aboard the satellite. This paper will compare results of planar near-field testing before and after spacecraft installation as well as on-orbit pattern characterization. The transmit-only array is used as a high data rate antenna for relaying scientific data from the satellite to earth stations. The antenna contains distributed solid-state amplifiers behind each antenna element that cannot be monitored except for radiation pattern measurements. A unique portable planar near-field scanner allows both radiation pattern measurements and also diagnostics of array aperture distribution before and after environmental testing over the ground-integration and pre-launch testing of the satellite. The antenna beam scanning software was confirmed from actual pattern measurements of the scanned beam positions during the spacecraft assembly testing. The scanned radiation patterns on-orbit were compared to the near-field patterns made before launch to confirm the antenna performance. The near-field measurement scanner has provided a versatile testing method for satellite high gain data-link antennas.
In this paper I demonstrate how our current technology now very readily permits a standard of accuracy and utility to be realized, that was formerly available only in research laboratories. This is accomplished with standardly available positioning equipment and standardly available software. Accurate alignment of the range is enabled by a tracking laser interferometer. This composite near-field scanning antenna range has afforded us the opportunity to compare readily, far-field results from the classic planar, cylindrical and spherical coordinate systems. Comparison data are presented.
The existing planar near-field antenna test range at Alcatel Space (ASP) in Toulouse has recently been enlarged and the frequency bandwidth increased to 18.5 GHz to allow for the testing of large fully integrated space flight antennas.
This upgraded test range, including a specific reconfigurable reflector antenna support tool, will be described. The range assessment method, carried out with a Ku-band single reflector antenna, will be outlined: error budgets obtained with the NIST 18-term method as well as absolute gain measurement and inter-comparisons with compact antenna test range (CATR) measurements will be presented.
Typical applications for L-Band and C-Band antennas will be presented with error budgets. Comparisons with simulated data will further demonstrate the range performance.
We at MI Technologies have employed the Hansen error analysis developed at the Technical University of Denmark (TUD), as a starting point for new system layouts. Here I expand it in two ways: the approach to mechanical errors, and the approach to system design.
I offer an alternative approach to the analysis of mechanical uncertainties. This alternative approach is based upon an earlier treatment of spherical coordinate positioning analysis for far-field ranges. The result is an appropriate extension of the TUD uncertainty analysis.
Also, the TUD error analysis restricts its attention to three categories of errors: mechanical inaccuracies and receiver inaccuracies and truncation effects. An error analysis for a spherical measurement system should desirably contain entries equivalent to the 18-term NIST table for planar near-field. In this paper, I offer such an extended tabulation for spherical measurements.
Traditional data collection strategy for antenna measurement is to perform a step and scan operation. This method moves a particular axis while holding all other source and AUT axes in a fixed location. Modern radome measurements require the coordinated motion of two or more axes due to the desired measurements, the radome testing geometries or a combination of both. An example would be transmission efficiency testing of a radome associated with a tracking antenna. In this measurement scenario, the antenna azimuth and elevation axes must maintain an orientation along the range axis while the radome is moved in front of the antenna. The axis coordination could be linear or non-linear in nature.
This paper describes the concept of coordinated motion and the needs for coordinated motion in radome measurements that have been identified. Additional potential applications for coordinated motion in radome measurements are described. Two methods of coordinated motion that have been implemented in instrumentation are described. They are geared motion, which is a linear master/slave relationship between two axes and generalized coordinated motion where the relationship of axes motion is described via linear or non-linear equations.
Wide frequency bandwidth feeds are used in compact ranges when multi-octave bandwidth operation of the range is desired. Dual-ridge or quad-ridge horns have been widely used in RCS applications as well as in antenna measurement applications to achieve wide band operation. This selection is made to take advantage of the lower cost of quad-ridge horns vs. other options.
In designing a compact range, one primary concern is the beamwidth of the feed over the operating band. This affects the amplitude taper across the quiet zone of the range. Another primary concern is the movement of the phase center vs. frequency of the feed. This directly affects the phase taper across the quiet zone as a result of de-focusing of the reflector.
Here we present measured data of the beamwidth and phase center movement vs. frequency of a wide-band quad-ridge feed designed to operate from 2.0-18.0 GHz. Measured and predicted quiet zone performance data over this bandwidth are presented with the feed installed in a Model 5751 compact antenna test range having a 4-foot quiet zone.
When a dual port probe is used for near-field measurements, the amplitude and phase difference between the two ports must be measured and applied to the probe correction files so that the measurements and calculations will have the same reference. For dual port linear probes, the measurement of this “Port-to-Port” ratio is usually accomplished during the gain or pattern measurements by using a rotating linear source antenna.
When a dual port linear probe is used to measure a circularly polarized antenna, the uncertainty in this Port-to-Port ratio can have a significant effect on the determination of the cross polarized pattern. Uncertainties of tenths of a dB in amplitude or 1-3 degrees phase can cause changes in the cross polarized pattern of 5-10 dB.2 3 The paper will present a method for measuring the Port-to-Port ratio on the near-field range using a circularly polarized antenna as the AUT (Antenna Under Test). The AUT does not need to be perfectly polarized nor do we need to know its correct polarization. The measurements consist of two separate near-field scans. In the first measurement the probe is in its normal position and in the second it is rotated about the Z-axis by 90 degrees. A script then calculates the Port-to-Port ratio by comparing the cross-polarization results from the two measurements. Uncertainties in the Port-to-Port ratio can be reduced to hundredths of a dB in amplitude and tenths of a degree in phase. Measurements were taken at TRW’s Large Horizontal Near-field Antenna Test Range.
Z-position errors are generally the largest contributor to the uncertainty in sidelobe levels that are measured on a planar near-field range. The position errors result from imperfections in the mechanical rails that guide the motion of the measurement probe and cause it to deviate from an ideal plane. The deviations (,)zxyδcan be measured with precise optical and/or laser alignment tools and this is generally done during installation and maintenance checks to verify the scanner alignment. If the measurements are made to a very small fraction of a wavelength in Z and at intervals in X and Y approximating one half wavelength, the sidelobe uncertainty can be estimated with high confidence and is usually very small. For Z-error maps with lower resolution the resulting error estimates are generally larger or have lower confidence.
This paper describes a method for estimating the Z-position error from a series of planar near-field measurements using the antenna under test. Measurements are made on one or more planes close to the antenna and on other planes a few wavelengths farther away. The Z-distance between the close and far planes should be as large as the probe transport will allow. The difference between the holograms calculated from the close and far measurements gives an estimate of the Z-position errors. This approach has the advantage of using the actual AUT and frequency of interest and does not require specialized measurement equipment.
Choosing the proper antenna range configuration is important in making accurate measurements and verifying antenna performance. This paper will describe the steps involved so the antenna engineer can select and specify the best antenna range configuration for a given antenna. It will describe the factors involved in choosing between near-field systems versus far-field systems, and the different scan types involved. It will explain the advantages of each type of antenna range and how the choices are affected by such factors as aperture size, frequency range, gain, beamwidth, polarization, field of view, sidelobe levels, and backlobe characterization desires. This paper will help the antenna engineer identify, understand, and evaluate the applicable characteristics and will help him in specifying the proper antenna range for testing the antenna.
Back projection techniques have been used extensively in planar near-field ranges and to a lesser degree in spherical near-field ranges. Recently a back projection technique allowing back projection from spherical near-field data onto a planar surface has been published and implemented. This paper explores this technique further through the presentation of measured data for a large microstrip array antenna. The results demonstrate how the technique can be used to investigate anomalies in the feed structure of the array.
This paper describes two methods that can be used to measure the leakage signals in quadrature detectors, predict the effect on the far-field pattern, and correct the measured data for leakage bias errors without additional near-field measurements. One method is an extension and addition to the work previously reported by Rousseau1. An alternative method will be discussed to determine the leakage signal by summing the near-field data at the edges of the scan rather than summing below a threshold level. Examples for both broad-beam horns and narrow-beam antennas will be used to illustrate the techniques.
This paper describes the experience of using a tracking laser interferometer to align and characterize the planarity of a planar near-field scanner. Last year, The Aerospace Corporation moved their planar near-field antenna range into a new larger room with improved environmental controls. After this move, the near-field scanner required careful alignment and characterization. The quality of the scanner is judged by how accurately the probe scans over a planar surface. The initial effort to align the scanner used a large granite block as a planarity reference surface and cumbersome mechanical probe measurements. However, a tracking laser interferometer was used for the final alignment and characterization.
The laser interferometer was included as part of an alignment service purchased from MI Technologies. The tracking laser interferometer emits a laser beam to a mirrored target called an SMR (Spherically Mounted Retroreflector). Encoders in the tracker measure the horizontal and vertical angles while the laser interferometer measures the distance. From these measurements, the three-dimensional SMR location is determined. The laser has the ability to very accurately (within about 0.001 inch) measure the location of the scanning near-field probe.
This paper includes a description of the mechanical alignment of the scanner, the tracking laser interferometer measurements, and the final planarity characterization.
This paper describes a large aperture, 650 GHz, planar near-field measurement system developed for field of view characterization of the Earth Observing System Microwave Limb Sounder (EOS MLS). Scheduled for launch in 2003 on the NASA EOS Aura spacecraft, EOS MLS is being developed by the Jet Propulsion Laboratory to study stratospheric chemistry using radiometers from 118 to 2500 GHz. The combination of a very high operating frequency and a 1.6-meter aperture, coupled with significant cost and weight restrictions, required a new look at near-field scanner design approaches. Nearfield Systems Inc. (NSI) developed a planar scanner that provides a planar accuracy of 4 microns RMS over the entire 2.4 x 2.4 meter scan area. This paper presents an overview of this system including the sub-millimeter wave RF subsystem and the ultrahigh precision scanner. Representative measurement results will be shown.
The need to measure the boresight pointing direction of radar antennas to a high degree of accuracy yields a requirement for excellent positioning accuracy on near-field antenna ranges. Evaluation of this requirement can be accomplished by a full and complete sensitivity analysis.
Alternatively, to gain an understanding of the effects of errors more simply, one can approach the question of accuracy required in the setup, by use of a physical model and straightforward physical reasoning. The approach starts with the assumptions of a collimated wave with planar phase fronts and the premise that the boresight direction of such a sum beam is along the normal to the phase fronts. A sensitivity analysis of the simple trigonometric boresight relationship between mechanical boresight and phase front normal, shows how accurate the receiver and the positioner must be to achieve a given boresight determination. Such an approach has been known for many years as it regards planar scanning; and, the results are known to be applicable.
In this paper this consideration is extended to spherical scanners to arrive at estimates of the mechanical positioner accuracies and electrical receiver accuracies needed to make boresight measurements of radar antennas with spherical near-field ranges.
A usual way of performing pattern-measurements on circularly polarized antennas is by measuring the linear components of the field and mathematically converting those to the left-hand and right-hand circular components. These synthesized circular components are sensitive for a number of factors: The exact orthogonality of the measured linear components, the measurement-accuracy of both phase and amplitude of the measured linear components, the polarization-pureness (or the accuracy of the description of the polarization-characteristics) of the probe, etc. This paper analyzes these factors, using a computer-model. An indication on the requirements to be imposed on the measurement-equipment is provided.
Holographic back-projections of planar near-field measurements to a plane have been available for some time. It is also straightforward to produce a hologram from cylindrical measurements to another cylindrical surface and from spherical measurements to another spherical surface1-7. In many cases the AUT is approximately a planar structure and it is desirable to calculate the hologram on a planar surface from cylindrical or spherical near-field or far-field measurements. This paper will describe a recently developed spherical hologram calculation where the farfield pattern can be projected on any plane by specifying the normal to the plane. The resulting hologram shows details of the radiating antenna as well as the energy scattered from the supporting structure. Since the hologram is derived from pattern data over a complete hemisphere, it generally shows more detail than holograms from planar measurements made at the same separation distance.
Modern base station antenna stresses antenna measurement instrumentation because of their narrower beamwidths and multi-sector characteristics. More complete testing must be performed preferably without appreciably increasing measurement time. MI Technologies has met this challenge by using increased automation and by designing its instrumentation specifically for antenna measurements. Earth station antennas require good polarization, gain, sidelobe, and multiband performance. The choice of an antenna test range to test an antenna is dependent on many factors, such as the directivity of the antenna under test (AUT), frequency range and desired test parameters. Care must be taken in the design of an antenna range to ensure that the performance parameters are measured with sufficient accuracy [1-5]. Often the mechanical features of the antenna (size, weight and volume) can have as much influence on the selection of an antenna range as do the electrical performance factors.
Simulated data is presented for a planar array to demonstrate the limitations of planar near-field back projections. It is well known that the result obtained in this way is of limited resolution and accuracy and these limitations are further illustrated through the data presented here. The impact of probe to AUT separation distance is shown as well as the correspondence between array excitation perturbations and that detected through the back projection technique. Results are shown for a simple iterative array excitation adjustment process. The purpose of this paper is to provide guidelines for the application of the planar near-field back projection technique.
Widespread deployment of cellular phones and use of wireless devices such as personal digital assistants, in-vehicle installs of Global Positioning System (GPS) receivers, and the upcoming deployment of mobile satellite digital audio has sprung a revitalized interest in faster, more affordable measurement techniques for antennas. This paper presents information on several new Spherical Near-field antenna measurement ranges developed by ATDS-Howland.
Accurate gain measurements using any measurement technique require a calibrated gain standard, and the uncertainty in the gain of the standard is usually the largest term in the error analysis. To reduce the uncertainty, gain standards are often calibrated using a three- antenna measurement technique and the resulting gain values are generally certified to have an uncertainty of approximately 0.10 dB1-11. For near-field measurements, the gain standard may be the probe that is used to obtain the near-field data or it may be a Standard Gain Horn (SGH). Since the calibration of the gain standard is time consuming and often costly, it is desirable to verify that the gain of the standard is stable over long periods of time.
This paper will describe tests to verify the gain stability of the standard and will also illustrate the terms in the error analysis that have the major effect on the uncertainty of any near-field gain measurement. With proper attention to the major error terms, the stability of the gain standard can be verified to approximate the original calibration uncertainty.
Microwave holography is a popular method for diagnosis and alignment of phased array antennas. Holography, commonly known in the near-field measurement community as “backtransformation”, is a method that allows computation of the primary (aperture) fields from the secondary (far-zone) fields. This technique requires the far-zone fields to be known over a complete hemisphere and adequately sampled on a regular spaced grid in K-space.
The holography technique, while known to be mathematically valid, is subject to errors just as all measurements are. Surprisingly, very little work has been done to quantify the accuracy of the procedure in the presence of known measurement errors. It is unreasonable to think that the amplitude and phase of the array elements can be trimmed to better than the uncertainty of the back-transformed amplitude and phase. This makes it difficult for an antenna engineer to determine the achievable resolution in the measurement and calibration of a phased array antenna.
This study reports the results of an empirical characterization of known errors in the holography process. A numerical model of the near-field measurement and holography process has been developed and many test cases examined in an effort to isolate and characterize individual errors commonly found in planar microwave holography. From this work, an error budget can be developed for the measurement of a specific antenna.
The complexity of modern antennas has resulted in the need to increase the productivity of ranges by orders of magnitude. This has been achieved by a combination of improved measurement techniques, faster instrumentation and by increased automation of the measurement process. This paper concentrates on automated measurement systems, and describes the requirements necessary to make such systems effective in production testing, and in research and development settings. The paper also describes one such implementation – the MI Technologies Model MI-3000 Acquisition and Analysis Workstation - that was designed specifically to comply with these requirements.
The paper discusses several important factors that must be considered in the design of automated measurement systems, including: (1) Enhancing range productivity; (2) Interfacing with instrumentation from a large number of suppliers; (3) Providing a uniform front-end for the measurement setup and operation that must be largely independent of the choice of the hardware configurations or the type of range (near-field or far-field); (4) Making the test results available in a format that simplifies transition to external commercial and userprogrammed applications; (5) Providing powerful scripting capability to facilitate end-user programming and customization; (6) Using a development paradigm that allows incremental binary upgrades of new features and instruments. The paper also discusses computational hardware issues and software paradigms that help achieve the requirements.
This paper describes error analysis and measurement techniques that have been developed specifically for cylindrical near-field measurements. A combination of analysis and computer simulation is used to show the comparison between planar and cylindrical probe correction. Error estimates are derived for both the probe pattern and probe polarization terms. The planar analysis is also extended to estimate the effect of probe position errors. The cylindrical measurement geometry is very useful for evaluating the effect of room scattering from very wide angles since scans can cover 360 degrees in azimuth. Using a broad beam AUT and scanning over a large y-range provides almost full spherical coverage. Comparison with planar measurements with similar accuracy is presented.
Toshiba Corporation, working with Nearfield Systems Inc., has developed a fully digital antenna measurement system for digital beam-forming (DBF) antennas. The DBF test facility is integrated with the large 35m x 16m vertical near-field range installed at Toshiba in 1997, and includes the NSI Panther 6500 DBF Receiver as the primary measurement receiver. The DBF system was installed in March 1999 and has been used extensively to test and characterize a number of complex, high performance DBF antennas.
A DBF antenna typically incorporates an analog-to-digital (A/D) converter at the IF stage of the transmit/receive (T/R) module. The digital IF signals are transferred to a digital beam-forming computer, which digitally constructs, or forms, the actual antenna pattern, or beams. Since the interfaces to the DBF antenna are all digital, the usual microwave mixers and down-converters are incompatible.
The NSI Panther 6500 is designed to interface directly with DBF antennas and allows up to 8 channels of I and Q digital input (16 bits each) with 90 dB dynamic range per channel. The NSI DBF receiver solves the DBF interface problem while providing enhanced performance over conventional microwave instrumentation.
Antenna measurement systems have unique requirements, which must be properly evaluated and understood in order for the antenna engineer to be successful in specifying an RF system that meets his needs. Antennas are characterized by specific operating and performance parameters that will determine the requirements for a measurement system. Aperture size, frequency range, bandwidth, side-lobe nulls, beamwidth, and polarization characteristics are a just few of the more important parameters. As with most engineering problems, system performance often requires a trade-off of equally important, but conflicting characteristics. Sensitivity and measurement time are well-known examples of this trade-off. Other examples include local vs remote mixers, receiver speed vs sensitivity, range size vs system dynamic range, and there are many others. The antenna engineer must be able to identify his most important system performance parameters in order to make compromises with confidence, since they are inevitably required. Once the system performance requirements have been determined, the antenna engineer can begin to select equipment, cables and components with the desired performance characteristics for his range.
This paper will describe the process for analyzing requirements, performing system trade-offs and specifying equipment and components for several antenna measurement system types.
This paper will detail the implementation and results of a gain calculation performed on standard gain horns (SGHs) in the LS and XN microwave bands. The three-antenna method was used to ensure the highest accuracy possible, and extensive efforts were made to minimize the error budget. The measurement was performed in a large anechoic chamber, with the receive and transmit antennas placed 4.6 meters high in opposing corners. The resulting fifteen meters of aperture separation (approximately 10D2/λ for LS band and 15D2/λ for XN band) eliminated all measurable aperture interactions and greatly reduced multipath interference from chamber reflections Rigorous analysis of the error terms proved this method to be both accurate and reliable. Typical values of measured error terms will be presented.
The majority of precision spherical positioner alignment techniques used today are based on procedures that were developed in the 1970’s around the use of precision levels and auto-collimation transits. Electrical alignment techniques based on the phase and amplitude of the antenna under test are also used, but place unwanted limitations on accurately characterizing an antenna’s electrical/mechanical boresight relationship. Both of these techniques can be very time consuming. The electrical technique requires operator interpretations of data obtained from amplitude and phase measurements. The autocollimation technique requires operator interpretations of optically viewed measurement data. These results are therefore typically operator dependent and the resulting error quantification can be inaccurate.
MI Technologies has recently developed a mechanical alignment technique for Spherical Near-Field antenna measurements using a tracking laser interferometer system. Once the laser system has been set-up and stabilized in the operational environment; the entire spherical near-field alignment may be completed in a few hours, as compared to the much more lengthy techniques used with level/transit or electrical techniques. This technique also simplifies the quantification of the errors due to the inaccuracy of the alignment.
This paper will discuss the effect of the alignment error on results obtained from spherical near-field measurements, and the procedures MI Technologies developed using a tracking laser interferometer system to obtain the precision alignment needed for a spherical near-field measurement.
Nearfield Systems, Inc. in Carson, California delivered a vertical 23’ by 22’ (7.0m x 6.7m) near-field test range to Raytheon Electronic Systems in Sudbury, Massachusetts. This planar and cylindrical measurement system is capable of characterizing antennas of various physical sizes in continuous wave or in pulse mode from 800MHz to 50GHz. The near-field measurements are computer controlled and capable of multiple frequency, multiple beam and multiple polarization in AUT transmit or receive modes. The precision robotic system uses a Data Acquisition Controller running NSI software to provide four-axes for probe positioning and three-axes for antenna positioning. The RF subsystem is based on the HP 8530A microwave receiver, HP 83630B RF source, HP 83621A LO source and HP 85309A LO/IF Distribution Unit. The test range was evaluated using the NIST 18-term error analysis on a 45GHz 54” diameter left-hand circular polarized reflector antenna.
This paper addresses the sensitivity of the cylindrical near-field technique to some of the critical alignment parameters. Measured data is presented to demonstrate the effect of errors in the radial distance parameter and probe alignment errors. Far-field measurements taken on a planar near-field range are used as reference. The results presented here form the first qualitative data demonstrating the impact of alignment errors on a cylindrical near-field measurement. A preliminary conclusion is that the radial distance accuracy requirement may not be as crucial as was stated in the past. This paper also shows how the NSI data acquisition system allows one to conduct such parametric studies in an automated way.
TRW, working with Nearfield Systems Inc., is building a state-of-the-art near-field antenna measurement system to test the Astrolink payload antenna system. Astrolink is the next generation broadband satellite network that will deliver high speed Internet connections to the business desktop. TRW is building the Astrolink on-board communications payload which includes the antenna system. For this multi-reflector antenna payload, TRW is building a 40 ft. x 30 ft. horizontal near-field measurement system to operate from 1 to 50 GHz using NSI’s high speed Panther receiver and Agilent Technologies high speed VXI microwave synthesizers. The system will be capable of performing conventional raster scans, as well as directed plane-polar scans tilted to the plane of a specific reflector. Completion of the range is scheduled for the first quarter 2001.
This paper will describe the near-field antenna measurement system that will test the Astrolink antenna payload and provide an overview of the specifications and test requirements for this test system. This paper will also describe the tilted plane-polar scanning capability, the 1 to 50 GHz RF subsystem, and the facility plans and progress.
This paper presents an accurate and portable method for RF testing of AN/SPY-1 Antenna Arrays on Navy ships. With four antennas per ship, the usual methods for RF testing are time consuming and very costly. Currently, the most thorough and accurate method of testing is to remove an array and ship it to the original equipment manufacturer's near-field facility. A Portable Planar Near-Field Scanner System (PPNFSS) was developed by Nearfield Systems Inc. for the Naval Surface Warfare Center-PHD to perform RF testing without removing the array from the ship. The system consists of a portable robotic scanner, optics, microwave subsystem, environmental/anechoic enclosure and active thermal control system. The system was designed to mount to various array/ship configurations with severe envelope and environment constraints. The design is modular to allow packaging in ruggedized transit cases and a 48 ft. shipping container.
Microwave Instrumentation Technologies (MI Technologies) in cooperation with Hollandse Signaalapparaten B.V. (Signaal) and the Royal Netherlands Navy has designed and produced a compact antenna test range to specifically address the unique testing requirements imposed in the testing of active phased array antennas. The compact range was built specifically to test Signaal’s new Active Phased Array Radar (APAR) prior to introduction into various naval fleets throughout the world. This reversible Compact Antenna Test Range (CATR) allows antenna testing in both transmit and receive modes. The measurement hardware is capable of testing both CW and pulsed waveforms with high dynamic range. In addition to conventional antenna pattern measurements the system is capable of measuring EIRP, G/T and G/NF, as well as providing analysis software to provide aperture reconstruction. A special Antenna Interface Unit (AIU) was designed and built to communicate with the Beam Steering Computer which controls the thousands of T/R modules which make up the APAR antenna system. A special high power absorber fence and other safeguards were installed to handle the transmit energy capable of being delivered from the APAR antenna system.
The impact of flex cables on mmwave planar near-field measurements is considered. It is demonstrated how to estimate measurement errors due to these effects and an example is presented. The techniques shown here allows one to evaluate existing antenna test facilities to assess their suitability for mm-wave testing and also shows that flex cables provide very accurate results despite their imperfections.
Copyright 1999 European Space Agency. Reprinted from 1999 ESA ESTEC Workshop on Antenna Measurements.
This material is posted here with permission of the European Space Agency (ESA). Such permission of the ESA does not in any way imply ESA endorsement of any of NSI-MI Technologies' products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the ESA.
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By definition, all of today’s wireless communication systems contain one key element, an antenna of some form. This antenna serves as the transducer between the controlled energy residing within the system and the radiated energy existing in free space. In designing wireless systems, engineers must choose an antenna that meets the system’s requirements to firmly close the link between the remote points of the communications system. While the forms that antennas can take on to meet these system requirements for communications systems are nearly limitless, most antennas can be specified by a common set of performance metrics Each of these characteristics are evaluated using a suitable antenna test range.
The results of theoretical calculations or measurements on antennas are generally given in terms of the vector components of the radiated electric field as a function of direction or position. Both the vector components and the direction parameters must be defined with respect to a spherical coordinate system fixed to the antenna. Along the principal planes there is no ambiguity about the terms such as vertical or horizontal component, but off the principal planes the definition of directions and vector components depends on how the spherical coordinate system is defined. This paper will define four different spherical coordinates that are commonly used in measurements and calculations, suggest a terminology that is useful to distinguish between them, and define the mathematical transformations between them. One important application of these concepts arises when comparing or combining measurement results from two antenna orientations. In this case, the axis of rotation dictates the preferred coordinate system and vector components. Measured results will be shown to illustrate the proper choice of coordinates for each situation.
Copyright 1999 European Space Agency. Reprinted from 1999 ESA ESTEC Workshop on Antenna Measurements.
This material is posted here with permission of the European Space Agency (ESA). Such permission of the ESA does not in any way imply ESA endorsement of any of NSI-MI Technologies' products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the ESA.
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The maintenance, test, and repair workload for the Air Force’s AN/MSQ-118 satellite ground-based receiving communication system has been transferred from the closing McClellan Air Force Repair Facility in Sacramento, California to Tobyhanna Army Depot located in Tobyhanna, Pennsylvania. The workload requires the support of four maintenance shops and two planar near-field ranges. The shops are the antenna repair, power supply repair, low-noise amplifier (LNA) repair, and radome repair shops. The near-field ranges are a 4’ x 4’ planar scanner used for antenna diagnostics and an 8’ x 8’ planar scanner used for certification of the repaired antenna-under-test (AUT).
This paper will bring the AMTA community up to date on the status of the new Tobyhanna Antenna Repair Facility, focusing on the techniques and methods used to quantify the alignment and performance characteristics of the planar near-field antenna measurement system used for certification. With the relocation complete, test data obtained at both locations will be analyzed and compared to show differences between the baseline measurements taken at McClellan Air Force Base versus those taken at Tobyhanna Army Depot.
High-speed receivers for near-field antenna and RCS measurements have traditionally been one-of-a-kind, expensive, difficult to interface and lacking in software support. Advances in digital signal processing, computer technology and software development now provide the means to economically solve these problems. NSI offers a high speed receiver subsystem, the Panther 6000 series, that allows multiplexed beam and frequency measurements at a rate of 80,000 independent amplitude and phase measurement points per second. The Panther 6000 receiver directly digitizes the 20 MHz IF test and reference input channels, and includes a high speed beam controller (HSBC) to sequence the measurement process. The HSBC receives an input trigger to initiate a measurement sequence of user-defined frequencies and beam or pol states.
NSI also offers a multi-channel all-digital receiver subsystem, the Panther 6500, to interface directly with Digital Beam Forming (DBF) antennas. The Panther 6500 allows up to 16 channels of I and Q digital input (16 bits each) with 90 dB dynamic range per channel. The alldigital DBF receiver reduces the cost, complexity and performance limitations associated with conventional instrumentation in DBF antenna measurement applications.
All Panther series receivers are fully integrated with the NSI97 antenna measurement software and operate with existing microwave sources, mixers and IF distribution equipment.
The angular coverage of planar near-field measurements is limited by the size of the scan plane, and the “region of validity” is defined by the angle between the edge of the AUT and the edge of the scan plane. In some applications, results are required over a larger angular region than is possible with the available scanner. The angular coverage can be increased by rotating the antenna and repeating the measurement. The results of the two measurements are then combined. Successful combination depends on using both the coordinate system and vector components that are appropriate for the antenna rotation.
In general for a single antenna orientation, any coordinate system can be used with any vector components, but when combining or comparing patterns for two antenna rotations, the axis of rotation must be the polar axis and the vector components must correspond to that coordinate system. Measurements results will be used to demonstrate the proper choice of coordinates and components and to illustrate potential problems that may arise.
MI Technologies has developed a technique to achieve very high accuracy cross-polarization measurements using a single reflector compact range. The technique, known as the "Error Correction Code Algorithm" (ECCA) leverages the "ideal" performance of a single parabolic reflector when the feed axis is aligned to the parabola axis. ECCA mathematically corrects for the amplitude taper induced by the feed axis alignment.
Historically, ‘conventional’ compact range polarization purity has been limited to »-30 dBi. The ECCA technique, however, lowers the cross-polarization error to »-48 dBi. This performance has been verified in two separate inter-range measurement comparisons with the National Institute of Standards and Technology. The results of these tests prove ECCA is an extremely accurate technique for low cross-polarization measurements and provides a lower cost, superior performance alternative to dualreflector systems when low cross-polarization measurements are required.
Compact antenna test ranges intended for low crosspolarization antenna measurements require the use of feeds with polarization ratios typically greater than 40 dB across the included angle of the quiet zone as well as across the frequency band of interest. The design for a series of circular corrugated aperture feeds to meet these requirements is presented. The feeds are based on a circular waveguide OMT covering a full waveguide frequency band with interchangeable corrugated apertures to cover three sub-bands. In order to validate the design of this series of scalar feeds, high accuracy cross-polarization data was collected. The primary limiting factor in the measurement of the polarization ratio was the finite polarization ratio of the source antennas. A technique for correcting for the polarization ratio of the source is presented along with measured data on the feeds. The technique begins with the accurate characterization of the linear polarization ratio of the standard gain horns using a three antenna technique, followed by pattern measurements of the feeds, and ends with the removal of the polarization error due to the source antenna from the measured data. Measured data on these feeds is presented before and after data correction along with data predicted using the CHAMP® moment method software.
Papers were presented at the last two AMTA meetings reporting on the effect of rotator system alignment on the results of spherical near-field measurements. When quantifying the effect of non-intersection errors on the AUT directivity, these two papers presented very different results. One AMTA paper1 and an earlier study at The Technical University of Denmark2 found that the directivity error was extremely sensitive to non-intersection errors while the other AMTA paper3 found a very small sensitivity. During the past year, scientists at the Technical University of Denmark, The National Institute of Standards and Technology, and Nearfield Systems Inc. have been working together to determine the reasons for these differences. It now appears that the major reason for the difference is due to the method used to acquire data on the sphere. Theta scans that pass through the pole, or equivalently, phi spans of 180 degrees, produce both plus and minus phase errors that tend to cancel in the on-axis direction. Theta scans that do not pass through the pole, or equivalently phi spans of 360 degrees, produce phase errors of the same sign over the sphere which are concentrated in the on-axis direction. Examples of measurements and recommendations for using this information in spherical measurements will be presented.
Nearfield Systems Inc. (NSI) has delivered the world’s largest vertical near-field measurement system. With a 30m by 16m scan area and a frequency range of 1GHz to 50GHz, the system consists of a robotic scanner, laser optical position correction, computer and microwave subsystems. The scanner and microwave equipment are installed in an anechoic chamber 40m in length by 24m in width by 25m in height. The robotic scanner controls the probe positioning for the 33m by 16m vertical scanner using X, Y, Z and polarization axes. The optical measurement package precisely determines the X and Y axes position, alignment errors along the X and Y axes, and Z-planarity over the XY scan plane.
This paper describes a new multi-purpose planar & cylindrical near-field antenna test facility installed at the Royal Netherlands Navy (RNN). In this paper an overview is given of the initial list of requirements that was generated and the process of selecting the best type of measurement facility to address these. A description of the facility is given and an outline of the accuracy of the planar/cylindrical near-field scanner is presented. The paper contains details of the extensive validation program and measured data demonstrating the performance of the system.
A simulation tool used during the design of nearfield ranges for phased array antenna testing is presented. This tool allows the accurate determination of scanner size for testing phased array antennas under steered beam conditions. Estimates can be formed of measured antenna pointing accuracy, side lobe levels, polarization purity, and pattern performance for a chosen rectangular phased array of specified size and aperture distribution. This tool further allows for the accurate testing of software holographic capabilities.
Airborne Doppler Velocity Sensors require precise boresight information in determining a Doppler solution. Far-field ranges have been extensively used to provide this boresighting capability. This paper discusses an empirical investigation to determine the feasibility of using near-field techniques to fulfill the boresighting requirement.
The accuracy of the probe antenna pattern used for probe-corrected near-field measurements is critical for maintaining high accuracy results. The probe correction is applied differently in the three standard near-field techniques – planar, cylindrical, and spherical. This paper will review the differences in sensitivity to probe correction for the three techniques and discuss practical aspects of probe correction models and measurements.
Concise mathematical relations have been derived for Planar Near-Field measurements that quantify the effects of x, y and z-position errors on antenna parameters such as gain, sidelobe level, pointing, and cross polarization. Because of the complexity of the theory, similar relations for spherical near-field measurements have not been developed. The requirements for the spherical coordinate system are generally defined in terms of the alignment parameters such as orthogonality and intersection of axes, θ-zero, xzero and y-zero rather than individual errors in θ, φ and r. Mechanical, optical and electrical techniques have been developed to achieve these alignments. This paper will report on the development of methods to estimate the antenna parameter errors that will result from spherical alignment errors for typical antennas.
The results of theoretical calculations or measurements for antennas are generally given in terms of the vector components of the radiated electric field as a function of direction or position. Both the vector components and the direction parameters must be defined with respect to a coordinate system fixed to the antenna. Along the principal planes there is no ambiguity about the terms such as vertical or horizontal component, but off the principal planes the definition of directions and vector components depends on how the spherical coordinate system is defined. This paper will define four different spherical coordinates that are commonly used in measurements and calculations, and propose a terminology that is useful to distinguish between them, and define the mathematical transformations between them. These concepts are essential when the results of different measurements or calculations are compared or when an antenna’s orientation is changed. Both mathematical and graphical representations will be presented.
This paper discusses the design, fabrication, installation, and testing of a Scientific-Atlanta Model 5702 Compact Range used for radar system testing. The unique feature of this compact range is that it provides a plane wave target source for automated closed loop radar system testing. Techniques employed for meeting and verifying stringent specifications such as phase and amplitude gradients over the quiet zone are discussed. Results from closed loop testing of radar systems in the compact range are also presented.
A large horizontal near field measurement facility has been validated and commissioned at Lockheed Martin’s Sunnyvale, CA facility. The new measurement facility will be used for characterizing antennas for a variety of satellites over a frequency range of 1 – 26.5 GHz. A horizontal near field scanner with a 14m x 7.8m (46’ x 26’) effective scan area has been designed to allow for 9.8m (32’) of vertical clearance permitting zenith oriented satellites to be easily positioned within the range and tested in an efficient manner. The facility will soon support the measurement of antennas that are in a vertical orientation. This is accomplished with a novel add-on that allows vertical planar near field scanning on the same range. The vertical scanner has an effective coverage area of 13.6m (45’) horizontal x 9m (30’) vertical. The system is being used to test commercial communications satellites.
When a phased array antenna consists of a number of complex subarrays, efficient and accurate testing of the subarrays is essential for overall project success. This paper presents a flexible system for testing various parameters of a subarray of an active aperture phased array antenna including S-parameters, noise figure, spurs, oscillations, and peak and average power. Testing is done for both CW and a variety of pulsed signals. A system block diagram is presented and system architecture explained. Timing diagrams are included for testing multiple states (which correspond to antenna beams), channels, and frequencies. Measured verification results are presented.
NSI recently delivered a Turnkey Near-field Antenna Measurement System (TNAMS) to the Naval Surface Warfare Center – Crane Division (NSWC-CD) in Crane Indiana. The system supports characterization and calibration of the Navy's active array antennas. TNAMS includes a precision 12' x 9' vertical planar near-field robotic scanner with laser optical position measurement system, dual source microwave instrumentation for multiple frequency acquisition, and a wide PRF range pulse mode capability. TNAMS is part of the Active Array Measurement Test Bed (AAMTB) which supports testing of high power active arrays including synchronization with the Navy's Active Array Measurement Test Vehicle (AAMTV), now under development. The paper summarizes the hardware configuration and unique features of the pulse mode capability for high power phased array testing and the TNAMS interface to the AAMTV and AAMTB computers. In addition, range test data comparing antenna patterns with various pulse characteristics is presented.
Andrew Corporation, founded in 1937 and headquartered in Orland Park, Illinois, has evolved into a worldwide supplier of communication products and systems. To develop a superior, high performance line of base station products for a very competitive marketplace, several new antenna measurement systems and upgrades to existing facilities were implemented. This engineering project developed an indoor test range facility incorporating design tool advantages from among Andrew Corporation’s other antenna test facilities. This paper presents a 22-foot vertical by 5-foot diameter cylindrical near-field measurement system designed by Nearfield Systems Incorporated of Carson, California. This system is capable of measuring frequencies ranging from 800 MHz to 4 GHz, omnidirectional and panel type base station antennas up to twelve feet tall having horizontal, vertical or slant (+/- 45 degree) polarizations. Far-field patterns, near-field data and even individual element amplitude and phases are graphically displayed.
Nearfield Systems Incorporated (NSI) provides antenna measurement systems to domestic and foreign, commercial and government customers with sophisticated requirements that demand custom solutions for RF, mechanical, thermal or software applications. NSI is continuously adapting existing designs to seek cost effective solutions for each customer’s demanding specification. This paper discusses numerous near-field scanner designs to meet a variety of applications. Presented are designs for several vertical planar scanners, horizontal scanners, tilted planar scanners, and special scanners designed to attach to structures to test antennas in-situ.
The mechanical rotator must be correctly aligned and the probe placed in the proper location when performing spherical near-field measurements. This alignment is usually accomplished using optical instruments such as theodolites and autocollimators and ideally should be done with the antenna under test mounted on the rotator. In some cases it may be impractical to place the alignment mirrors on the AUT or optical instruments may not be available. In these and other cases, it is desirable to check alignment with electrical measurements on the actual AUT and probe. Such tests have recently been developed and verified. Appropriate comparison and analysis of two near-field measurements that should be identical or have a known difference yields precise measures of some rotator and probe alignment errors. While these tests are independent of the AUT pattern, judicious choice or placement of the antenna can increase the sensitivity of the test. Typical measurements will be presented using analysis recently included in NSI software.
Scientific-Atlanta has recently begun work on a large 55 ft.(W) x 45 ft.(H) compact range reflector. The reflector is a Model 5738 with a 45 ft. focal length and a 38 ft. diameter by 38 ft. long cylindrical quiet zone. Due to the large size of the reflector, it is necessary to form the surface as several large, independent sections and assemble and align the reflector at the installation site. The 5738 reflector is shown in Figure 1 with the 38 ft. quiet zone superimposed.
The independent and predictable behavior of large sections proves to be very beneficial for performing an electrical alignment of the reflector based on field probe phase data. This paper discusses the required alignment tolerances and analytic tools developed to predict the effects on quite zone performance due to alignment errors in the sections of the reflector.
Far-field range testing has been the standard at the Southwest China Research Institute of Electronic Equipment (SWIEE) and at other facilities in mainland China. SWIEE has recently commissioned a new spherical near-field measurement system from Nearfield Systems Inc. (NSI) and Hewlett Packard (HP) to improve its antenna measurement capability. The nearfield system provides significant advantages over the older far-field testing including elimination of weather problems with outdoor range testing, complete characterization of the antenna, and improved accuracy. This paper will discuss the antenna types at SWIEE tested with the NSI/HP near-field system, and the results being achieved.
Accurate EIRP measurements are possible to make on a near-field range but require great care and attention to detail. NSI has recently implemented a near-field test range for the Globalstar satellite program which makes automated EIRP and gain measurements. Automation for this program is extremely important since the production cycle requires testing many antenna systems per month, each of which has two antennas with 16 separate beams per antenna. Among the various range measurements, EIRP is the key parameter of the Transmit antenna’s performance. This paper reviews the measurement theory of EIRP measurements and presents some of the results of this automated activity.
Use of a compact range for testing high power antennas is generally limited to testing the antennas at low power levels. In most cases, this is adequate, but for antennas where the management and dissipation of power is a key test parameter, the antenna and transmitter must be tested at the design power level. If this testing is to be performed in a compact range, it is important that the energy be captured and safely dissipated because allowing the energy to be incident on the absorber could result in destruction of the facility. The chamber under construction for Hollandse Signaalapparaten in Hengelo, Netherlands is designed to receive this energy in a specific region of air cooled absorber and to dissipate the heat into the chamber as an added load on the HV AC system.
Scientific-Atlanta has developed a new algorithm for obtaining high accuracy cross-polarization measurements from prime focus, single reflector, compact ranges. The algorithm reduces cross-polarization extraneous signals to levels that rival or exceed much more expensive dual reflector systems, but with the associated cost and simplicity of a single reflector system. This paper provides an overview of the new algorithm. It explains the limitations on conventional polarization measurements in single reflector systems and the methods for overcoming these limitations without error correction for some antennas. A method for determining if error correction is needed for a particular antenna is reviewed and the fundamentals of the error correction algorithm are explained. Preliminary test results are provided.
NSI recently completed installation of two large 7m x 7m horizontal planar scanners to support the Globalstar satellite program test activity. These systems were installed at Alcatel in France, and Alenia in Italy. These two systems are similar to the NSI system installed at Space Systems/Loral in Palo Alto, CA. described in previous AMTA papers. The companies are part of the Globalstar satellite consortium, committed to launching a constellation of satellites for mobile telephone communications. The paper will summarize the hardware configuration and the unique features of the two new test systems including high power phased array testing and the interface to the Globalstar payload for active antenna control and payload testing. In addition, range data comparing all 3 test ranges will be shown.
construction, installation and validation of two large horizontal near-field antenna measurement ranges. These new measurement systems are located in the existing HSC satellite factory building. These ranges will be used to measure various types of directive satellite antennas both at a unit level and at spacecraft level. The facility will accommodate mechanical integration of the test articles as well. This facility is the result of Hughes committing the time and money to create a state of the art antenna measurement facility that will be highly efficient and accurate. A detailed description of this facility's configuration, design and current status will be discussed herein.
This paper addresses some of the practical considerations and numerical consequences of vsing the Advanced Antenna Pattern Comparison (AAPC) method to improve the accuracy of antenna measurements in compact ranges. Two main issues an of particular importance:
These issues deal specifically with Kasa's circle-fitting procedure-an essential part of the AAPC method and provides useful checks for conditions commonly met with the use of this technique. We also briefly address the single interfering wave case that the AAPC method tacitly assumes.
Modem pulsed phased array radar systems bring new challenges to antenna measurement. These antennas generally consist of hundreds of Transmit-Receive (TR) modules controlled via a beam steering computer to fonn the antenna beam. Attempting to operate these modules with a CW wavefonn will not only quickly damage the modules but will not properly characterize the antenna. The Navel Surface Warfare Center, Crane Division, recognized the need to add pulsed capability when specifying their latest antenna measurement system. Scientific Atlanta met these requirements by integrating their newly introduced Model 1795P Pulsed Microwave Receiver into their proven 2095 Microwave Measurement System to make the Model 2095P Pulsed Microwave Measurement System.
Compact ranges have found wide application for antenna measurements, RCS measurements, and, most recently, for spacecraft payload measurements. Each of these applications requires certain special features of the range optics, positioning systems, electronics, and software. The system design of a compact range measurement system for making all these types of measurements presents a number of challenges.
This paper will discuss the system aspects of the design of a multi-purpose compact range facility. Items of interest include the RF electronics design, the positioning system design, the optimization of the reflector and feeds and the specialized software design.
Portable scanners used for near-field antenna measurements are usually incapable of providing a large scan area with a high degree of probe position accuracy. This paper discusses a 4.5m x 2.0m portable scanner developed by NSI with a probe position accuracy on the order of 2 mils (0.050 mm) rms. An NSI patented optical measurement system measures the X, Y, and Z position, and provides real-time position correction capability. This lightweight, portable scanner combined with optical correction provides enhanced accuracy while reducing overall antenna measurement system costs and improving test chamber flexibility.
In an earlier paper ("System Engineering for a Radome Test System," John R. Jones, et al, AMT A, October 1994) the system level design of a compact range enhancement for the testing of the Triband Radome was presented. This paper will discuss the installation and testing of the radome measurement system in the compact range. The purpose of the radome measurement system is to determine (within close tolerances) boresight shift, transmission loss, antenna pattern changes and polarization effects caused by the radome. Unique features include novel coordinate transformation and correction by means of a laser autocollimator and data reduction algorithms. Also featured is the tracking subsystem which consists of a specially designed two-axis track pedestal, an autotrack controller, and three five-horn compact range feed arrays operating at X, K, and Q-bands. The performance of the triband radome measurement system in the compact range setting will be presented.
Hologram measurements are becoming more and more popular as a reliable method for identifying bad elements and the tuning of active phased array antennas. Relying on holographic data to adjust phase shifters and attenuators in these antennas can give undesired results if the accuracy of the data is poor. Often measurements can be improved if the error sources can be isolated and quantified. This paper presents an approach to producing a hologram accuracy budget based on the NIST 18-term error budget created for near-field measurements. A set of hologram accuracy terms is identified and data is presented showing the typical hologram accuracy that can be expected from a near-field scanner.
Planar near-field measurements are the usual choice when testing phased array antennas. NSI recently delivered a large state-ofthe- art near-field measurement system for testing a multi-beam, solid state phased-array antenna. The critical sidelobe and beam pointing accuracy specifications for the antenna required that special attention be paid to near-field system design. The RF path to the moving probe was implemented using a multiple rotary joint system to minimize phase errors. Additional techniques used to minimize system errors were an optical probe position correction system and a Motion Tracking Interferometer (MTI) for thermal drift correction.
This paper describes a 550 GHz planar near-field measurement system developed for flight qualification of the radio telescope carried onboard the NASA submillimeter wave astronomy satellite (SWAS). The very high operating frequency required a new look at many near-field measurement issues. For example the short wavelength mandated a very high precision scanner mechanism with an accuracy of a few microns. A new thermal compensation technique was developed to minimize errors caused by thermally induced motion between the scanner and spacecraft antenna.
The Spherical Near-Field measurement technique has been in existence for a number of years. The cost associated with this type of measurement system has often been assumed to be substantial. Herein is presented the system configuration for a low cost Spherical Near-field System whose design goals include the capability for production line testing while retaining simplicity in approach. NSI has been contracted to provide a Spherical Near-field antenna measurement system. This paper focuses upon the design considerations undertaken during the prototype development of that system.
This paper briefly describes the theory and application of a small near-field imaging system designed for EMC precompliance applications. This system produces EMI and EMS images of circuit cards, cables and related items. If extended by using a phase coherent receiver in a region of free space propagation, this same system can precisely measure the radiation pattern of directive antennas and image the multipath within an anechoic chamber or TEM cell.
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This paper details the evaluation of a major aerospace company's tapered anechoic chamber. Using an NSI 3' x 3' near-field scanner and software, the chamber was evaluated at 11 frequencies and two polarizations. SAR imaging techniques were used to map the chamber reflections. A new addition to the software provided the ability to map the difference between the measured phase front and the theoretical spherical phase front; the software also derives the x, y, and z phase centers of the source. Error estimates for all aspects of the evaluation will be discussed.
Earlier measurement results are reviewed to understand the result that cross-polarized patterns agree well when compared between a point-source compact range and spherical near -field scanning. By taking account of the symmetry of the aperture distribution, one can see how the cross-polarized pattern can be affected only moderately by the classic polarization feature of an offset reflector geometry.
Accurate, fast, and cost effective antenna test equipment is necessary to meet many programs' measurement requirements and schedule and budget constraints. Testing time may be significantly reduced by measuring multiple channels of data simultaneously. Further time savings are realized via the electronic storage of data, which allows easy pattern overlays and changes in the page setup parameters. Electronic storage of data also allows the user to accurately ascertain test parameters. Test data for a dual band, multi-channel antenna measured with the Scientific Atlanta 1590 Pattern Recorder and multi-channel 1795 Microwave Receiver is presented. This antenna has transmit and receive ports, multiple polarization capability, data and tracking channel outputs, and multiple frequency bands. The substantial savings in testing costs are estimated.
Large scanners used for near-field antenna measurements require careful attention to the design and fabrication process to maintain probe position accuracy1. This paper discusses the design, implementation, and results of a novel optical probe position tracking system used by NSI on a number of large near-field scanners. This system provides measurement of the probe X, Y and Z position errors, and real-time on-the-fly position correction. The use of this correction can significantly enhance measurement accuracy, and can reduce the cost of building large near-field scanners.
This paper will discuss the system level design of a radome test system implemented in a compact range. The system includes a tracking pedestal controlled by an autotrack controller, a measurement receiver, a unique five-feed arrangement for the compact range which accommodates both tracking and measurement functions, and a laser autocollimator for coordinate system referencing. Key elements of system design include the required coordinate system transformations, the mechanical design of the positioning system and its contribution to the tracking system, and the synchronization of the autotrack controller, the measurement receiver, and the autotrack controller, the measurement receiver, and the system controller. There aspects of system design will be discussed and measurement and analysis results will be presented.
The Three-Antenna gain method is commonly used on far-field ranges to determine an antenna’s absolute gain. This is especially true when no other calibrated antenna is available. This method has been used for years by calibration laboratories such as NIST to calibrate probes and gain standards for far and near-field ranges. In some cases the calibration is too costly or does not meet the schedule requirements of the near-field test range. An alternative is to calibrate the probe or gain standards directly on the near-field range. In this paper we present the results of a study done to show the accuracy of the Three-antenna gain method when used on a near-field range. An extensive error analysis is presented validating the utility of this method.
The Compact Antenna Range is becoming the method of choice for indoor testing of many types and sizes of antennas. Implementation of a compact range requires a suitable parent building structure in which to house the chamber. The chamber is located within the parent building and the compact range is then installed within the chamber. In some cases an existing building may not be available for the range and it may be difficult to acquire a new building due to local or proprietary requirements. Once a building has been located, many problems still exist with coordinating installation of the chamber and compact range within this building. Overcoming these problems can be both time consuming and inefficient in terms of cost.
This paper describes a Compact Antenna Range conceived and designed to be totally self-contained and truck transportable. The compact range consists of a complete anechoic chamber facility with self-contained electrical, lighting, HVAC and fire protection systems. The compact range provides a 3 foot test zone over the 5.8 to 94 GHz frequency range. Once completed and tested at the factory, the facility is transported and set in place at the users’ site.
Details are presented which describe the structural requirements of the chamber, the RF performance of the completed facility, and the transport and installation process. The integrated test positioner and an automatic feed changing mechanism are also described.
Test sequencing flexibility and high throughput are essential ingredients to a state-of-the-art near-field test range. This paper will discuss methods used by NSI to aid the operator through the near-field measurement process. The paper will describe NSI's expert system and customer applications of a unique test and processing sequencer developed by NSI for optimizing range measurement activities. The sequencer provides powerful control of the software functions including multiplexed measurements, data processing, and unattended test operations.
Scientific-Atlanta is presently developing a new family of high-speed, general purpose, on-wafer test systems which greatly reduces RF test times and thereby can alleviate test bottlenecks at MMIC foundries. The primary objective for the equipment developed on the Scientific-Atlanta MIMIC Phase 3 program is to reduce present MIMIC chip RF test times by an order of magnitude compared to test systems presently in use.
In the past, various companies have installed large permanent Near-field antenna measurements systems. In many instances, a test range has been constructed for a particular project or purpose. the conclusion of the project, the range may become dormant or under-utilized. In addition, a dormant range quickly becomes a potential source for spare parts. These factors combine quickly to render the once functioning range unless.
With the current industry emphasis on cost reduction, minimizing new capital purchases, and utilization of existing resources, an upgrade of a dormant test facility is a preferable path. NSI has recently upgraded an existing Near-field antenna measurement system at Hughes Space and Communications Co. Hereinafter referred to as Hughes S&C. This paper focuses upon the design considerations undertaken during the upgrade procedure.
Characterizing antennas under pulsed RF conditions has focused attention on a class of measurement challenges not normally encountered in CW measurements. The primary problems often include high transmit power, thermal management of the AUT, and a close interaction between the antenna and its transmitting circuitry. This paper presents instrumentation techniques for pulsed RF antenna measurements using the 1795P Pulsed Microwave Receiver as an example of a commercially available solution applicable to both active and passive apertures. Emphasis is given to measurement speed, dynamic range, linearity, single pulse versus measurements, pulse width, pulse repetition frequency (PRF), frequency coverage, system integration and automation, and suitability of equipment for antenna range applications.
Design and implementation of a large horizontal planar near-field system delivered to Space Systems / Loral for satellite antenna testing will be discussed. The 22' by 22' scan plane is 25' above the ground and employs real-time optical compensation for the X,Y, and Z corrections to the probe position. The system provides high-speed multiplexed near-field measurements using NSI's software and the HP-8530A microwave receiver. System throughput is enhanced through the use of a powerful and flexible test sequencer software module.
As antennas have become more sophisticated, the testing requirements have grown tremendously. Testing often adds significantly to the cost of the system. A need has developed for test equipment more advanced than the completely manual systems of the past and less expensive than the completely automated systems of today. An antenna pattern recorder which helps to minimize test time is presented. The instrument utilizes a user friendly touch screen which facilitates user interaction with the unit. The pattern recorder is capable of measuring up to five channels of data simultaneously as a function of angle, linear position, or time. The data is stored on electronic media and may be saved, retrieved, zoomed, plotted, analyzed by internal programs or exported for analysis by external programs. The user may customize the plot format for test reports, proposal information, and other data requirements.
Traditionally the testing of satellite antennas was characterized by an emphasis on accuracy of measurement and thoroughness of testing. This emphasis was required because of the high performance needed to accomplish the mission. As experience was gained in the construction of satellites, more emphasis was placed on achieving greater technical capability but at lower costs of testing. The net result is the requirement to test ever more efficiently without giving up accuracy.
In the meantime, test instrumentation for conducting antenna and satellite testing also was influenced by improvements in speed. Driven by the demands of near-field scanning and electronically controlled phased arrays, automatic instrumentation has afforded high speed measurement and fast data acquisition rates to the satellite industry.
This presentation offers some examples of performance in accomplishing high volume testing under the rigorous technical constraints imposed by the satellite industry. As an example of a high speed system, the Scientific-Atlanta Model 2095 will be used to illustrate the capability offered by today's technology. This system has found application in the facilities of five satellite manufacturers constructed within the past three years and is proven by its demonstrated application in satellite programs.
A dual-linearly polarized probe developed for use in planar near-field antenna measurements is described. This probe is based upon Scientific-Atlanta’s Series 31 Orthomode Feeds originally developed for spherical near-field testing. The unique features of this probe include dual-orthogonal linear ports, high polarization purity, excellent port-to-port isolation, an integrated coordinate system reference, APC-7 connectors, and a thin-wall horn aperture to minimize probe-AUT interactions. The probe was calibrated at the National Institute of Standards and Technology (NIST) and the calibration data consisting of the probe’s complete plane-wave spectrum receiving characteristic s'02(K) were imported directly into the Model 2095/PNF Microwave 02 Measurement System. This paper describes the dual-ported probe and its application in a planar near-field range.
The three cable method for removing the amplitude and phase variations of microwave cables due to temperature change and movement can offer a substantial improvement in antenna measurement accuracy. Implementation details of the method are provided for a planar near-field range. Items specifically addressed are range configuration, hardware requirements, data collection methodology, identification and assessment of error sources, and data requirements.
Traditional techniques for evaluating the performance of anechoic chambers, compact ranges, and far-field ranges involve scanning a field probe through the quiet zone area. Plotting the amplitude and phase ripple yields a measure of the range performance which can be used in uncertainty estimates for future antenna tests. This technique, however, provides very little insight into the causes of the quiet-zone ripple. NSI's portable near-field scanners and diagnostic software can perform quiet-zone measurements which will provide angular image maps of the chamber reflections. This data can be used by engineers to actually improve the chamber performance by identifying and suppressing the sources of high reflections which cause quiet-zone ripple.
Planar Near-Field scanning is becoming the method of choice for the testing of many types of antennas. These antennas include planar phased arrays, space deployable satellite antennas and other antennas either too large to move during the test or otherwise sensitive to the gravity vector. The planar scanner is a major component of the measurement system and must provide an accurate and stable platform for moving the RF probe across the test antenna’s aperture. This paper describes basic design requirements for a planar near-field scanner. Based on recent development activity at Scientific-Atlanta several design considerations are presented. Scanner parameters discussed include basic scanner concepts and geometry, scanner accuracy and stability, RF system including cabling and accuracy, load carrying requirements of the RF probe carriage, position and readout systems and drive and control systems. A scanner will be presented which incorporates many of the design features discussed.
Near-field range design includes the placement of RF absorber in the test area. Absorber placement depends highly on the antennas being tested. A common approach is to design an expensive low-reflection chamber around the near-field scanner. The chamber and the additional floor space can sometimes cost more than the near-field scanning system itself. Another approach seeks to identify multi-path reflection to minimize cost by optimally placing absorber to meet specific antenna test requirements. The result is a lower cost range using less floor space. This paper describes a technique of evaluating near-field range multipath.
This paper describes a novel planar near-field measurement system designed to test a beam-steered flat face phased array antenna. This system is unique in its ability to measure multiple beams during a single scan of the aperture. The system utilizes a very fast planar scanner with six foot by six foot of travel combined with fast beam-steering commands to significantly reduce the test time of multiple-beam phased array antennas. These features combined with software based on algorithms developed by the National Institute of Standards and Technology provide state of the art measurements of planar phased array antennas.
A novel technique has been devised that permits a length of cable to be measured in place and its transfer characteristic monitored as motion occurs. The scheme is to measure the cable in transmission as a member of a pair and to infer the characteristic of the cable from a set of three pair-wise measurements, in analogy to the well-known three-antenna technique. From the resulting knowledge of the signal cable’s characteristic one can correct the measured data to account for the changes in the cable through which the signal of interest was passed.
This paper reports on the design, fabrication and installation of the first large compact range whose reflector was machined in one piece. The overall size of this reflector is 30 feet high and 43 feet wide and it produces a test zone 18 feet high and 30 feet wide. It features a novel serrated edge design and a unique multi-feed system. This compact range was fabricated under contract to the U.S. Navy and is currently installed at the Pacific Missile Test Range at Pt. Mugu, California.
Rapid data acquisition is crucial in making comprehensive near-field scanning tests of electronically-steered phased array antennas. Multiplexed data sets can now be acquired very rapidly with high speed automatic data acquisition. To obtain high speed without giving up accuracy in probe position a feature termed subinterval triggering has been devised. To obtain simultaneously reliable thermal drift or tie scan data a feature termed block tie scans has been devised. This paper describes these two features that yield speed and accuracy in planar near-field scanning measurements.
A series of measurement were made to validate the performance of a Planar Near-Field (PNF) Antenna Test Range located at the Satellite and Aerospace Systems Division of Spar Aerospace Limited. These measurements were made as a part of a contract to provide Spar with a Model 2095 Microwave Measurement System with planar near-field software options and related instrumentation and hardware.
The range validation consisted of a series of self-tests and far-field pattern comparison tests using a planar array antenna provided by Spar that had been independently calibrated at another range facility.
This paper describes the range validation tests and presents some of the results. Comparisons of far-field patterns measured on the validation antenna at both the Spar PNF facility and another antenna range are presented.
This paper describes a software based receiver post processor that corrects circularity and gain errors in coherent receivers. The receiver post processor additionally provides range gating capabilities, signal quality estimation, mixer non-linearity detection and various display functions. This paper will concentrate primarily on the identification of circularity errors by the receiver post processor.
Performance trade-offs are Investigated between the use of clustered waveguide bandwidth feeds and the use of one multioctave bandwidth single aperture feed in a prime focus compact range for dual linear polarization. The results show that feed structure may be used for advantage for the particular test requirements of compact range systems for Radar Cross Section Measurement.
Portable near-field measurement systems can provide significant flexibility to both large companies seeking to increase their antenna test capabilities, and small companies looking for their first investment in a test range. There are many unique applications for portable near-field antenna measurement systems in addition to their use for standard antenna performance measurements. Some additional applications include flightline testing, anechoic chamber quiet zone imaging, and EMI testing.
Many of NSI's near-field systems have been portable designs, capable of being set up in a small lab or office and easily relocated. Key features required for a portable system are rapid setup, simplicity of use, low cost, and accuracy. This paper will be focused on practical experience with installing, calibrating, and operating portable near-field measurement systems. It will also cover tradeoffs in their design, and usage in a variety of applications.
An assessment of instrumentation error sources and their respective contributions to overall accuracy is essential for optimizing an electromagnetic field measurement system.
This study quantifies the effects of measurement receiver signal processing and the relationship to its transient response when performing measurements on rapidly varying input signals. These signals can be encountered from electronically steered phased arrays, from switched front end receive RF multiplexers, from rapid mechanical scanning, or from dual polarization switched source antennas.
Numerical error models are presented with examples of accuracy degradation versus input signal dynamics and the type of receiver IF processing system that is used. Simulations of far field data show the effects on amplitude patterns for differing rate of change input conditions. Criteria are suggested which can establish a figure of merit for receivers measuring input signals with large time rates of change.
This paper describes measurement system performance parameters that were considered during the design phase of a planar near-field measurement range for Spar Aerospace Limited. All aspects of the planar near-field measurement system are addressed. These include; instrument selection, scanner interface hardware, system controller/computer hardware, software for data collection, near-field to far-field transformation, data analysis, networking and system configuration. The Scientific-Atlanta Model 2095 Microwave Measurement System with its near-field options is used as the basis for meeting the Spar requirement. The various data collection parameters of the Model 2095 are described with special emphasis on how the factors relate to near-field requirements such as fixed grid sampling. Examples of typical test scenarios are presented as an aid in exploring detailed data collection system timing.
The experience with near-field scanning at Scientific-Atlanta began with a system based upon a analog computer for computing the two-dimensional Fourier transform of the main polarization component. When coupled with a phase/amplitude receiver and a modest planar near-field scanner this system could produce far-field patterns from near-field scanning measurements. In the 1970’s it came to be recognized that the same advances, which made the more sophisticated probe-corrected planar near field measurements possible, would enable conventional far-field range hardware to be used on near-field ranges employing spherical coordinates. In 1980 Scientific-Atlanta first introduced a spherical near-field scanning system based upon a minicomputer already used to automate data acquisition and display. In 1990, to meet the need of measuring complex multistate phased-array antennas, Scientific-Atlanta began planning a system to support the high volume data requirement and high speed measurement need represented by this challenge. Today Scientific-Atlanta is again pursuing planar near-field scanning as the method of choice for this test problem.
Probe correction is necessary in near-field measurements to compensate for non-ideal probes. Probe compensation requires that the probe’s far-field pattern be known. In many cases direct far-field measurements are undesirable, either because they require dismantling the probe from the near-field range set-up or because a far-field range is not available. This paper presents a unique method of deriving probe-correction coefficients by measuring a probe on a near-field range with an “identical” probe and taking the square root of the transformed far-field. This technique, known as the “Probe-square-root” method can be thought of as self-compensation. Far-field compensations are given to show that this technique is accurate.
This paper compares some of the features and capabilities of gated CW and pulse radars for RCS and imaging measurements. At the conceptual level, these two types of radars are very similar. The primary conceptual difference is that a pulse radar has a relatively high bandwidth receiver while a gated CW system has a relatively narrow bandwidth receiver.
The measures of performance of an RCS and imaging system include sensitivity, measurement time, clutter rejection, dynamic range and accuracy. Other considerations such as inter-pulse modulation may be important in some cases.
For some applications, typically where long ranges are involved, a pulse system has significant performance advantages. For many applications, the performance advantage of a pulse system is not significant, particularly when viewed in light of the large difference in cost. This is particularly true of Quality Assurance applications which are normally characterized by both short ranges and lower budgets. Typically, the price of a gated CW system is in the range of 1/4 to 1/2 the price of a comparable pulse system.
This paper discusses general similarities and differences in the fundamental operating characteristics of the two systems. Specific performance measures are discussed including system sensitivity, gate performance, clutter rejection, and measurement times. Other considerations such as pulse modulation are discussed. A summary of the various considerations is presented in order to give the reader an understanding of the applications for which a gated CW system is more appropriate.
Multiple mechanisms for the generation of extraneous signals exist in a compact range. These include edge diffraction, scattering from surface imperfections, direct feed radiation, and scattering from absorber or other objects in the range. The field quality in the quiet zone is the resultant of the direct signal and these multiple scattering mechanisms.
Since the scattering mechanisms are independent, their effects are often modeled independently and statistically combined to yield an estimate of quiet zone field quality. This paper examines the statistics of multiple independent extraneous signals in a compact range. It is shown that the amplitude ripple produced by an extraneous signal computed as the root sum of the squares (RSS) of the individual extraneous signals does not correctly predict the final quiet zone amplitude ripple.
Theoretical results for scattering from multiple thin gaps in the surface of a compact range are presented and statistical computer models are used to demonstrate the computation of the resultant compact range quiet zone.
Since the first commercial compact range was introduced in 1973, the compact range has become a very popular alternative to far-field ranges. In recent years larger and larger compact ranges have been built, increasing the size of antennas that may be tested and lowering the operating frequency. However little has been done in the other direction, to increase the operational frequency and to decrease the size of the compact range. This paper reports on the design and fabrication of a small compact range having a 1 foot test zone and operating at 95 GHz.
The compact range mechanically collimates electro-magnetic energy, thus creating a plane wave useful for testing antennas in a far field environment. Since this collimation can be achieved in a relatively small space, the tests can be performed in an environmentally controlled chamber. With the increasing use in both military and commercial applications of antennas operating at millimeter wavelengths, there is increasing need for small compact ranges operating in the 26.5 to 100 GHz frequency range. This paper describes the development of a small compact range with a two foot diameter test zone that operates from 26.5 GHz to greater than 100 GHz.
A major objective in the design of an RCS measurement facility is to obtain the greatest possible productivity (overall measurement efficiency) while maintaining the accuracy and sensitivity necessary for low radar cross section targets. This paper will present parameters affecting the total throughput rates of an indoor facility including instrumentation, target handling, and band changes - one of the most time consuming activities in the measurement process. Sensitivity and accuracy issues to be discussed include radar capabilities, feeds and feed clustering, compact range, background levels, and diffraction control.
An s-parameter measurement system and a procedure are described for making fast s-parameter measurements on multi-state devices. A sample test problem is considered and the application of the system and the procedure to this test problem is discussed. The important features of the system are described and timing measurements of system operation are presented.
State-of-the-art microwave antennas often incorporate the ability to operate in many operating states and over wide frequency ranges. Adaptive arrays and electronically scanned antennas may have programmable beam shapes, configurations or scan positions resulting in several thousand operational states. In addition, these antennas may also require testing at many frequencies. To be properly characterized, these antennas may require testing in all these states. The amount of data required to characterize these antennas, coupled with the requirement to maximize antenna range throughput and minimize costs, puts ever increasing demands on the test equipment designed to perform these measurements. This paper describes a new automated microwave measurement system utilizing a high speed measurement receiver and an 80386 PC based computer to rapidly test these new generation antennas. The recently introduced Scientific-Atlanta Model 2095 Microwave Measurement System incorporates a unique data acquisition coprocessor (DAC) for high speed test device control, instrument timing, frequency control, data buffering and transfer to the system controller. Antenna measurements on multi-beam and multi-port antennas can be made in a fraction of the time associated with other types of test systems. The various timing parameters of the Model 2095 are described with special emphasis on how these independent and variable factors inter-relate to each other. A method is presented to calculate total test time, given the test requirement and timing for state changes of the AUT. Examples of typical test scenarios are presented as a further aid in understanding system timing.
This paper presents an overview of the Integrated Radar Measurement System (IRMS) installed at the Air Force Radar Target Measurement Facility (RATSCA'D for AFSC/6585 TG/RX Holloman AFB, New Mexico.
The extraneous signals that perturb antenna patterns can be found and identified by a method known as "longitudinal translation at selected points". The method is usually applied to certain selected angular points on the antenna pattern. With this technique the composite pattern -- consisting of the direct-path signal and the reflection signal -- is measured at a series of translation distances along the axis of the antenna range. By utilizing both the amplitude and phase of the received signal, one can remove the signal that results from stray reflection and retain the desired direct path signal. The result is an improved and more accurate version of the pattern. In this presentation I review this technique as specifically applied to compact range antenna measurements, and apply it to several patterns, to demonstrate the method.
This paper will describe new developments in a gated-CW radar that has been designed to improve the productivity and sensitivity of RCS measurements. Improvement in data acquisition speeds result from the design of a fast synthesizer, a data acquisition co-processor and pulse modulator. Each of these new products have been specifically designed to take advantage of the high speed capabilities of Scientific-Atlanta’s Model 1795 Microwave Receiver. The RF sub-system has also been designed to permit continuous 2-18 GHz, full polarization data acquisitions. Critical RF components are now mounted at the feed in the chamber, improving the sensitivity and ringdown of the the system. Productivity in analysis activities has been improved by the use of a multi-tasking system controller which permits simultaneous use of the system for acquisitions, analysis, and plotting.
The effects of non-systematic receiver instrumentation errors on precision antenna measurements are investigated. A simple uncertainty model relating dynamic range to random perturbation effects on amplitude measurements is proposed. Examples of measurement uncertainty versus both input level and measurement speed are presented using data taken on modern measurement receivers. Data are compared w1th the model to estimate measurement uncertainty at various pattern levels and acquisition speeds. Equivalent dynamic range specifications are deduced from the measured data.
Increased productivity and higher resolution imaging capabilities are becoming of greater concern for RCS ranges. The ideal measurement scenario involves taking data on all desired frequencies for a target configuration in a single rotation. This could involve one or more frequencies in several bands, imaging data on more than one band or very high resolution imaging data covering several bands. Placing several feeds in a cluster at the focal point of an offset fed com-pact range can provide these capabilities. The effects of feed clustering such as beam tilt are discussed along with cluster sizes that provide little if any degradation in compact range performance. Experimental data is shown that gives an indication of the quality of data that may be obtained. The concepts are also applicable for outdoor ranges that have an array of antennas offset from range boresight.
In order to justify the expenditure for capital equipment such as a microwave receiver, it must be shown that the instrument provides the best value to the user. The best value for a microwave receiver for measuring today’s complex microwave antennas dictates that the receiver be versatile enough to adapt and operate over a diverse set of applications and performance specifications. Some important characteristics to consider when evaluating a microwave receiver’s value is measurement speed, frequency agility, number of measurement channels, remotability, dynamic range, ease of operation, and system integration.
This paper addresses the development and important characteristics of a high speed microwave receiver that was designed to provide users with maximum productivity, and therefore, the best value for a microwave antenna measurement receiver. Receiver characteristics such as acquisition speed, frequency agility, number of measurement channels, controls, interfacing, and versatility are discussed.
Implementing an antenna test range has traditionally been viewed as a major and costly undertaking, requiring significant long term facility planning, computer hardware interfacing, and software development. This paper describes a complete low cost, yet high accuracy portable near-field measurement system that was privately built for less than $2,000 and interfaced to a PC compatible computer.
The design and operation of this system, including the scanner, microwave hardware, and computer system will be described. This system has since been extended into a commercial product capable of providing rapid and accurate measurements of small to medium size feeds and antennas within a small office or lab space at significantly lower cost than standard antenna test techniques. The system has demonstrated an equivalent sidelobe noise level of less than -50dB, includes a probe corrected far-field transform and holographic back projections, and can output pattern cuts, contour plots, 3D plots, and grey scale images of antenna performance.
Compact range facilities designed for RCS measurements have exhibited a performance-limiting effect commonly referred to as "feed ringing". "Feed ringing" is a phenomenon in which energy is stored in or about the RF feed structure and is sustained for a sufficient period of time after the source is turned off such that its presence contaminates the true target return. This effect has placed severe constraints on the design of the RF feed for the compact range, particularly in regard to its operating bandwidth. This paper presents the design of a lossless, waveguide type RF feed suitable for compact range application with a demonstrated useful bandwidth approaching a full octave.
This paper describes techniques for coherently suppressing multipath and other error sources in planar near-field measurements. Of special interest is a simple, yet effective technique for suppressing axial multipath and mutual coupling between the near-field probe and an antenna. This is of particular value in the testing of low sidelobe antennas. Traditionally, self comparison tests with different separations between the probe and the antenna under test are used to identify the magnitude of multipath errors. What is not generally realized is that these tests can be used to produce a coherent estimate of the induced error, which can often be suppressed. A series of tests was performed with a small X-band phased array antenna, resulting in a reduction of the sidelobe noise background from a 25dB level to better than 50dB.
An overview of non-destructive real-time testing of missiles is discussed in this paper. This testing has become known as hardware-in-the-loop (HIL) simulation because it involves the actual missile hardware.
Measurements of the microwave reflectivity of materials is often performed with complex test setups using probes attached to a vector network analyzer. The lack of portability of these systems prevents the user from measuring reflective properties of surfaces that are not easily moved to an appropriate test facility. This paper describes a small, hand held microwave reflectometer which is designed to perform rapid reflectivity measurements in the field. The reflectometer consists of a tuneable Ku band source, a dual polarization sampling horn, a pair of crystal detectors, and a battery powered microcomputer.
Precise and complete measurements of advanced electromagnetic systems demand dramatically higher data acquisition speeds than those commonly attainable. Specific challenges include requirements for wideband measurements with arbitrarily spaced frequency steps. These types of measurements are often encountered in characterizing EW/ECM systems, radars, communications systems, and in performing antenna and RCS measurements.
The MI Technologies Model 1795 Microwave Receiver offers capabilities directly applicable to solving measurement problems posed by highly frequency agile systems. These problems include:
A technique is discussed which shows the application of the Model 1795 Microwave Receiver in its high frequency agility mode of operation. Measurement examples are presented showing the advantages gained compared to previous methods and instrumentation configurations.
This paper presents the productivity improvements that are possible in complex antenna measurements using state of the art instrumentation. The productivity improvement is calculated for a hypothetical antenna, and from this productivity improvement manufacturing cost reductions and payback times are derived.
In an earlier paper the virtual vertex compact range reflector was introduced and data from a specific design was reported. This paper describes the extension of the virtual vertex serrated edge concept to other reflectors that serve a wider range of application. Two new 12 ft focal length reflectors have been built that possess 3 ft and 6 ft diameter symmetric test zones. We describe the electromagnetic considerations and the mechanical design approach that has been used for these reflectors. We demonstrate the performance with field probe data showing the excellent surface accuracy of these units.
A variety of positioner control systems are available for making antenna and RCS measurements, but few can be upgraded economically as test facilities are expanded. Positioner control system components may include a controller, positioner motor drive unit, and a position indicator. Integration of these functional components into a single modular unit to operate the desired number of axes provides the basis for a positioner control system. Other desired features may include programmability, remotability, operation outdoors, and expansion capability.
This paper will address the development of a modular positioner control system that provides users with a basic system that can economically be upgraded as changing test requirements dictate. Functional capabilities such as remotability will be highlighted. System configuration and integration will also be discussed.
In this presentation we consider the features and performance of a large serrated-edge compact range reflector. This is a straightforward innovation from earlier compact range reflectors. The virtual vertex reflector is a paraboloidal surface truncated to exclude the vertex. This layout provides the advantages of better use of reflector surface are, reduced feed blockage, and reduced feed backscatter. The design is made economical by the use of serrations.
The receiving patterns (both amplitude and phase) of two probes must be known and utilized to determine accurately the complete far field of an antenna under test from near-field measurements. This process is called probe correction. When the antenna to be tested is nominally linearly polarized, the measurements are more accurate and efficient if nominally linearly polarized probes are used. Further efficiency is obtained if only one probe which is dual polarized is used instead of two probes to allow for simultaneous measurements of both components. It should be noted, however, that a single-port probe can be rotated by 90. (in effect, the second probe) to obtain the second component. A procedure used by the National Bureau of Standards (NBS) for accurately determining the plane-wave receiving parameters of both single- and dual-port linearly polarized probes is described. Examples are presented, and the effect of these probe receiving characteristics in the calculation of the parameters for the antenna under test is demonstrated using the required planar near-field theory.
Copyright 1988 IEEE. Reprinted from 1988 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION. VOL. 36, NO. 6.
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of the IEEE does not in any way imply IEEE endorsement of any of NSI-MI Technologies' products or services. Internal or personal use of this
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The use of serrated edge treatment in the design of a compact range collimating reflector is one method of mitigating the effects of edge diffraction on quiet zone performance. In this note a physical optics analysis is applied to the serrated reflector. The computational procedure is described and several results are presented. In particular, computed results are presented for the Model 5755 compact range reflector and compared with experiment.
This paper will address low RCS target mounting systems. Structural and electromagnetic aspects will be considered. The 4:1 vs the 7:1 ratio ogival shell pylons will be evaluated with consideration given to structural integrity, electromagnetic scattering, and positioner size. Measured and analytic data will be used in these evaluations.
The results of an extensive error analysis on planar near-field measurements are described. The analysis provides ways for estimating the magnitude of each individual source of error and then combining them to estimate the total uncertainty in the measurements. Mathematical analysis, computer simulation, and measurement tests are all used where appropriate.
Copyright 1988 IEEE. Reprinted from IEEE Transactions on Antennas and Propagation, Vol 36, No. 6, June 1988.
This material is posted here with permission of the IEEE. Such permission
of the IEEE does not in any way imply IEEE endorsement of any of NSI-MI Technologies' products or services. Internal or personal use of this
material is permitted. However, permission to reprint/republish this
material for advertising or promotional purposes or for creating new
collective works for resale or redistribution must be obtained from the
IEEE by writing to pubs-permissions@ieee.org.
By choosing to view this document, you agree to all provisions of the
copyright laws protecting it
Equations are derived and measurement techniques described for obtaining gain, effective radiated power, and saturating flux density using planar near-field measurements. These are compared with conventional far-field techniques, and a number of parallels are evident. These give insight to the theory and help to identify the critical measurement parameters. Application of the techniques to the INTELSAT VI satellite are described.
Copyright 1988 IEEE. Reprinted from IEEE Transactions on Antennas and Propagation, Vol 36, No. 6, June 1988.
This material is posted here with permission of the IEEE. Such permission
of the IEEE does not in any way imply IEEE endorsement of any of NSI-MI Technologies' products or services. Internal or personal use of this
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This paper presents a conceptual overview of the instrumentation system and signal processing involved in dynamic RCS and Imaging measurement systems.
This paper describes recent advances in antenna measurement instrumentation for millimeter frequency applications. Application of a new, lightweight, programmable, ruggedized signal source at 40 and 60 GHz is outlined. An RF instrumentation system for millimeter frequency antenna range application is detailed. A millimeter-to-microwave converter is described which improves millimeter antenna range performance. System performance levels are predicted. Compact range configuration and operation at millimeter frequencies is detailed. Specific measurement examples are presented to demonstrate the measurement sensitivity which can be achieved.
Currently many new compact range facilities are being constructed for making antenna pattern measurements indoors. Limited suppression of stray signals -- due to range layout, confined surroundings and residual absorbing material reflectivity -- represents a limitation on the accuracy of the measurements made in these facilities. Time-gating of the compact range signal appears to be very attractive technique to reduce unwanted reflections.
The authors have carried out an experimental investigation of time gating in a compact range. it is demonstrated that time-gating can improve the uniformity of the aperture field by removing the feed backlobe radiation; and, it is demonstrated that time-gating can remove the effects on a pattern of certain room reflections and of feed backlobes.
When compared to conventional methods of reducing reflections based on placement of absorber, time gating appears equivalent. It does not appear however that time gating improves the conventional methods, except for measuring wide beamwidth antennas.
Over the years formulations have been developed which provide an implicit measure of transfer efficiency of the compact range. Reasonable accuracy has been demonstrated for both antenna and RCS measurement applications. In general, however, these formulations require specific design details pertaining to the collimating :reflector. In this note a more general formulation is examined in which efficiency is explicitly expressed in terms familiar to antenna engineers and which do not directly involve reflector parameters. Applications of this formulation are presented.
In any type of electromagnetic measurements, the ideas of "precision and accuracy" and "low cost" tend to be mutually exclusive. At Scientific-Atlanta, for instance, production testing of antenna products is conducted in low cost miniature "anechoic chambers" which are fabricated in-house. These "chambers" are actually mediumsized to large (64-200 cubic feet) rectangular boxes with absorber attached to their walls. They are usually equipped with single axis positioners at one or both ends, and their usefulness is limited to the measurement of axial ratio on low gain small antennas...
The difficulty of discussing the practical aspects of boresighting in general terms has been circumvented by writing two parallel sections - one pertaining to antennas with a fixed mechanical reference and one pertaining to antennas with adjustable optical - mechanical references
The reader is advised to concentrate first on the section that pertains most specifically to his particular antenna boresight problem. Either of Sections I or II may be read without prior knowledge of the other; they are partially repetitive when discussing overlapping material. Those antennas whose boresight reference is defined by touch points on the mounting structure are covered in Section I; those antennas having boresight telescopes are covered in Section II.
A way has been found to utilize the reflector return in a compact range as a source of continuous drift compensation. This is performed by translating receive polarizations 45 degrees with respect to the transmit polarizations to ensure returns in co- and cross-polarizations. An added benefit is the simplicity of alignment for the polarization calibration standard.
A method is presented for making fast multifrequency high accuracy polarization measurements using a digital computer. This paper will provide a brief review of the IEEE standard polarization definitions, their applicability to the three antenna method, and finally a fast two antenna method.
The fast two antenna method uses a dual polarized orthomode sampling antenna along with a standard antenna whose polarization is known. The dual polarized sampling antenna is calibrated before the test data is acquired using the polarization standard in two different orientations 90 degrees apart. Once the calibration data is acquired the dual polarized orthomode antenna is used as a sampling antenna for the AUT.
Since the sampling antenna is dual polarized the AUT polarization data can be obtained rapidly for many frequencies since neither antenna is required to rotate. This method has been used to acquire polarization data for over 500 frequencies in less than 20 seconds.
Measurements of the Electromagnetic Field in the quiet zone of Scientific-Atlanta's Model 5753 Compact Range are presented. The Model 5753 is believed to be the largest high frequency compact range yet built and measurements demonstrate a quiet zone exceeding 8 ft. high by 12 ft. wide. Both field probe measurements and pattern comparison measurements are presented in the operating frequency range of 1-94 GHz.
Prime focus fed paraboloidal reflector compact ranges are used to provide plane wave illumination indoors at small range lengths for antenna and radar cross-section measurements. The “quiet zone”, which is the region of space within which a uniform plane wave is created, has previously been limited to a small fraction of the reflector size. A typical quiet zone might be six feet by four feet for a ten foot radius reflector.
Two new feed antennas have been designed which can provide an increase in quiet zone size as defined by the amplitude taper in the quiet zone field. Design details for the feeds are presented and the performance of the feeds in a compact range is discussed.
Gating is a widely used technique of improving RCS measurements. However, the exact type of gating used has a dramatic effect on such parameters as dynamic range and clutter rejection. Time Domain Gating offers significant advantages over software gating as used in some network and spectrum analyzers. This paper explores a technique used by Scientific-Atlanta in CW and FMCW RCS measurements. With the adaptation of an external computer controlled hardware gating unit, existing RCS and antenna systems can be retrofitted for significant performance improvements.
In this paper we present a three-antenna measurement procedure which yields the polarization of an unknown antenna to an accuracy comparable to that of the improved method of Newell. The complete method is based on step-scan motion of the two polarization axes on which the antenna pairs are mounted. As a special case this step-scan procedure includes the usual single axis polarization pattern method of polarization measurement.
This three-antenna polarization measurement method can be readily automated and is carried out straightforwardly with the assistance of a minicomputer for data acquisition and data reduction. The data reduction method is based on conventional digital Fourier transform techniques and has the advantage of inherent noise rejection. It utilizes a large number of sample points which greatly over determine the parameters to be measured.
The method has been verified experimentally with measurements made on multiple overlapping sets of three antennas, as is conventional for this kind of procedure. The data are presented for broad-beam antennas of the type use as near field probe horns.
This paper briefly discusses the calibration techniques used in the Sandia National Laboratories Radar Cross-Section Test Range (SCATTER)a We begin with a discussion of RCS calibration in general and progress to a description of how the range, electronics, and design requirements impacted and were impacted by system calibrationa Discussions of calibration of the electronic signal path, the range reference used in the system, and target calibration in parallel and cross-polarization modes follow. We conclude with a discussion of ongoing efforts to improve calibration quality and operational efficiency. For an overview description of the SCATTER facility, the reader is referred to the article Sandia SCATTER Facilitv, also in this publication.
The accurate measurement of radar target scattering properties is becoming increasingly important in the development of stealth technology. This paper describes a low cost imaging Radar Cross Section (RCS) instrumentation radar capable of measuring both the amplitude and phase response of low RCS targets. The RCS instrumentation radar uses wide band waveforms to achieve fine range resolution providing RCS data as a function of range, frequency and aspect. With additional data processing the radar can produce fully focused Inverse Synthetic Aperture Radar (ISAR) images and perform near field transformations of the data to correct the phase curvature across the target region. The radar achieves a range resolution of 4 inches at S-band and a sensitivity of -70 dBsm at a 30ft. Range.
Lesson learned in the design of large, planar near-field ranges used at millimeter wavelengths are described. Specific issues include facility design, RF equipment, scanner design, dynamic position measurement, servo control and software requirements.
Over the long periods of time needed to acquire spherical near-field data, thermal drift of the system can cause errors in the measurement. The effect of thermal-drift can be removed, if it is monitored during the scanning process. This is accomplished by periodically returning the probe to the near-field peak during acquisition. The same point is re-measured upon each return; and the variations in phase and amplitude are used to produce a correction factor which is applied to each point in the near-field data file. This paper describes the return-to-peak method and the correction algorithm. Experimental results will also be presented.
One of the variables to be quantified when making antenna measurements is position. Without accurate and timely position information, the spatially dependent data cannot be correctly interpreted.
Scientific-Atlanta’s 1885 Positioner Indicator and 1886 Position Data Processor offer several improvements in providing position information which can enhance an antenna measurement system. New position indicating techniques have been implemented to allow a higher degree of accuracy and speed than previously attainable. These have been combined with advanced features for automatic system flexibility to create a high performance instrument for many applications. This paper describes the capabilities of these two instruments and how they can be used to improve system performance.
This paper describes the relationship of probe polarization correction to probepattern corrected and non-probe-pattern-corrected spherical near-field measurements. A method for reducing three-antenna polarization data to a form useful for polarization correction is presented. The results of three-antenna measurements and the effects of polarization correction on spherical near-field measurements are presented.
Spherical near-field testing of antennas requires the acquisition of a great volume of data. In general, to compute the far-field of the antenna under test in any direction requires the acquisition of data at sample intervals related to the size of the antenna under test over a spherical sampling surface completely enclosing the antenna under test. This data must also be sampled as a function of probe orientation. Even for the simplest possible case, two probe orientations (or two probes) must be used.
Acquisition of this much data takes a long time. The requirement that data be acquired as a function of probe orientation is particularly troublesome, since acquiring data for even two probe orientations doubles the time required for data acquisition. In 1979, F. H. Larsen' demonstrated the feasibility of using a dual- polarized, dual-ported spherical near-field probe. Such a probe makes possible the acquisition of spherical near-field data for two probe orientations in a single scan. The development of a broadband, dual-ported, dual-polarized antenna which could be used as a spherical near-field probe was the goal of the efforts reported herein.
A set of near-field measurements has been performed by combining the methods of non-probe-corrected spherical near-field scanning and gain standard substitution. In this paper we describe the technique used and report on the results obtained for a particular 24 inch 13 GHz paraboloidal dish. We demonstrate that the gain comparison measurement used with spherical near-field scanning gives results in excellent agreement with gain comparison used with compact range measurement. Lastly we demonstrate a novel utilization of near-field scanning which permits a gain comparison measurement with a single spherical scan.
The antenna measurement environment has changed substantially in the past few years. Designers are working with higher frequencies than were practical before and new techniques require both phase and amplitude data for design optimization. This has created new demands for positioner designs. This paper describes how modern mechanical design tools together with modern components were used to develop a n0w generation poitioner for antenna measurement.
Automated antenna testing has become economical with the MI Technologies Series 2080 Antenna Analyzer. Since its introduction last year, new computer hardware and software additions have enhanced the system performance.
This paper will provide a brief overview of the system and its enhancements. It is recognized that testing requirements differ and an automated system must be capable of adapting to a specific test. The Series 2080 has a flexible data base and display programs which permit special antenna testing. A discussion of meeting special test requirements and the cost benefits of automated testing will be made.
Spherical near-field testing and other specialized antenna measurements require precise range and positioner alignment. This paper presents a method based on optical techniques to conveniently measure and monitor both range alignment and the positioner axis orthogonality and intersection. The hardware requirements consist of a theodolite and a unique target mirror assembly viewable from either side.
Just as with far-field or compact ranges, it is important to evaluate spherical near-field ranges with electromagnetic field-probe measurements. Recall that the fundamental motion for utilizing the spherical near-field measurement technique is to permit antenna measurements to be made at short range lengths, relieved from the constraint of the far-field criterion. Just as the illumination function in the test zone of an ideal far-field range is a uniform planar wavefront, the ideal illumination function for a near-field range is a spherical wavefront from an elemental dipole. The field probe measurements provide a quantitative and qualitative assessment of the deviation of either a near-field or far-field range from ideal conditions.
In this presentation the results from a program of field probe measurements for an experimental spherical near-field range are reported. A comparison is made between stray signal effects assessed by field probing the near-field range and by repeating pattern measurements at different range lengths. The two methods show consistent conclusions on the extraneous signal level. Differences and similarities between a spherical near- field range and a compact range are illustrated with field probe measurement data that strikingly reveal the basic difference between the two techniques.
A strong emphasis is now being placed on techniques for reduction of radar cross-section, A missile or aircraft which is invisible to radar has an important strategic advantage. With this fact in mind, the user of a weapons system may place an upper limit on the radar cross-section that he will permit his missile or aircraft to have. The designer must then make use of "stealth technology" to reduce the cross-section to an acceptable level, In order to verify the design, radar cross-section measurements must be made. Thus the current emphasis on cross-section reduction leads to an important need for accurate and reliable methods of measuring radar cross-section.
In this article a method of radar cross-section measurement is described. It utilizes a compact range configuration, More widely used as a tool for measuring the radiation patterns of microwave antennas, the compact range can also be applied to the problem of radar cross-section measurement. A heuristic development which illustrates the principles of compact range operation is presented and the coupling equations for both the antenna measurement and the radar cross section measurement cases are derived.
MI Technologies was selected by the United States Air Force to design and install a complete turn-key test facility for depot maintenance support of the F-16 fighter aircraft. These facilities have been installed at Hill Air Force Base, Utah. Four complete facilities have been supplied, each consisting of a Series 2020 Antenna Analyzer and a Series 5750 Compact Antenna Range. Two facilities are configured for antenna testing and two for radome testing.
This paper describes the equipment furnished for this program. The hardware is discussed as well as the special software designed to perform specific radome and antenna tests.
A new generation programmable, phase-amplitude measurement receiver has been developed which advances the state-of-the-art of antenna pattern measurements. The new receiver features microprocessor-based control and data processing systems resulting in improved performance and versatility.
In principle, spherical near-field scanning measurements are performed in the same way as conventional far-field measurements except that the range length can be reduced. This provides a natural advantage to scanning in spherical coordinates over other coordinate systems due to the ready availability of equipment. However, special considerations must be given to near-field range design because of the necessity for phase measurement capability, mechanical accuracy and the need to handle large quantities of data.
Based on experience with spherical near-field measurements carried out during verification testing of a spherical near-field transformation algorithm, we discuss the practical aspects of constructing a near-field range. In particular we will consider range alignment procedure, engineering of the RF signal path and times for data collection and processing.
The mechanical alignment of a reflector antenna involves both the reflector shape and also the relative orientation of the feed and subreflector. The requirements for alignment are derived from the system requirements for antenna functional performance, including pointing...
The compact antenna range has been recognized as an effective means of testing microwave antennas. Antennas which normally require long outdoor ranges for testing can be tested under far field conditions at an indoor facility, using the compact range.
The compact range operates on the principal that a parabolic reflector will transform an incident spherical wave into a collimated plane wave in its near zone. The plane wave produced is suitable for testing antennas, thus simulating far field electromagnetic criteria in the near zone. The typical compact range is housed in a room approximately 20 feet wide, 40 feet long and 20 feet high.
The performance of the compact range has been well documented and specified over a frequency range of 3.95 GHz to 18.0 GHz. Now, through recent testing performed at Scientific-Atlanta, the compact range can be specified for operation up through 60.0 GHz.
This paper describes the tests that were performed, discusses the results of these tests and establishes performance specifications for operation at these millimeter frequency bands.
Near-field antenna measurement techniques offer an alternative to conventional far-field antenna measurement techniques. Of the various coordinate systems used for near-field measurements, the spherical coordinate system provides the most natural extension from the conventional far-field characterization of an antenna to a more general characterization for arbitrary range lengths.
This paper describes the Scientific-Atlanta Model 2022, a user-oriented implementation of a spherical near-field antenna measurement system. An example of typical system usage is provided. System capabilities and performance are described. Key concepts required to understand and use the spherical near-field method are discussed.
The advantages and disadvantages of near-field antenna testing in relation to conventional far-field testing are considered. The particular merits of spherical near-field testing as compared to other forms of near-field testing are discussede Antenna testing situations which provide the most likely candidates for the spherical near-field measurement technique are described.
The testing of microwave antennas or the measurement of radar backscatter usually requires that the antenna or target under test be illuminated by a uniform plane electromagnetic wave; however, the creation of such a wave is difficult. In practice, a uniform plane wave is approximated.
The conventional procedure for approximating a uniform plane wave is to locate a transmitting source antenna at such a distance that the incident wave can be considered to be planar. When the source antenna is located 2D2/λ away from the test antenna (where D is the largest dimension of the test antenna aperture and λ is the wavelength), the spherical wavefront emitted from the source will produce a maximum phase taper of ϖ/8 at the edge of the test antenna aperture. For most applications, such a phase taper is acceptable.
In a compact range [1-5], on the other hand, the incident plane wave is created by a range reflector and feed in the immediate vicinity of the test antenna. The basic principle of operation is illustrated in Figure 1. The diverging rays from the point-source feed are collimated by the range reflector, and a plane wave is incident on the test antenna or target. The incident wave has a very flat phase front but the feed-reflector combination introduces a small (but acceptable) amplitude taper across the test zone.
The principal advantage of a compact range is its small size; this allows it to be indoors and free from adverse weather effects. In research and development laboratories, a compact range can be located convenient to the design engineers. In manufacturing or rework facilities, a compact range can be located near an assembly line for use in final testing and adjustment. By placing the range in a shielded room, one can eliminate interference from external sources and provide a test site that is secured against monitoring by outside parties.
Near field antenna measurements have long been of interest to the antenna community and of particular interest to those in the design and measurement of antennas. Efforts in this area using analog computers for data reduction were already under way in the late 19SO's. These applications were limited, primarily due to the limitations of the analog computer. Two planar near field probe positioners were built by Scientific-Atlanta during this period and delivered; one to Martin Denver and one to the Georgia Institute of Technology. These units were used for development on planar near field measurements. The unit at Martin Denver .was-also used by the Bureau oL Standards. ExperimentaLwork at Georgia Tech led to Dr. Joy's thesis on spacial sampling and filtering.1 This work on sampling was· particularly important because it gave an understanding of the required data density for meaningful transformation by digital computer. Numerical integration is a time and core intensive process and it was the utilization of the Fast Fourier Transform in the early 1970 1 s that made the digital computer a viable approach to the problem.
Antenna pattern recorders are used to plot the relative signal strength of an antenna under test as a function of the angular position of the antenna. The signal plotted is obtained from the output of a receiver or directly from a microwave detector. The position information is normally obtained from synchro transmitters geared to the test posi ti oner axis. Typical antenna pattern recorders are electromechanical devices which employ servo systems to drive the recorder axis. A chart drive servo system positions the recording paper as a function of the angular position of the antenna. A pen servo system positions a recording pen in response to the amplitude of the input signal.
During the past two years the compact range has emerged from the research laboratory into the field for use by the e11gineer in antenna and radar cross section measurements. During the development of the compact range, experience has been gained in how to handle many of the problems identified in the early work. This paper describes successful results in the areas of improvement of surface accuracy and the design of the reflector edges; also reported are antenna measurements made at 30 GHz using the compact range.
Copyright 1977 IEEE. Reprinted from Antennas and Propagation Society International Symposium, 1977.
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The absolute gain of wide-beam antennas may be accurately measured using the method described. Both the theoretical and practical aspects of gain calibration on a ground reflection antenna range are presented. The measurement procedures developed were used to calibrate a log-periodic antenna at selected frequencies from 250 to 400 MHz. Measured data at 300 MHz is tabulated and error contributions are analyzed, yielding a measurement accuracy of f0.27 dB with a 95 percent confidence interval.
Copyright 1973 IEEE. Reprinted from IEEE Transactions on Antennas and Propagation.
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In many cases, it is impractical or impossible to make antenna pattern measurements on a conventional far-field range; the distance to the radiating far field may be too long, it may be impractical to move the antenna from its operating environment to an antenna range, or the desired amount of pattern data may require too much time on a far-field range. For these and other reasons, it is often desirable or necessary to determine far-field antenna patterns from measurements made in the radiating near-field region; three basic techniques for accomplishing this have proven to be successful. In the first technique, the aperture phase and amplitude distributions are sampled by a scanning field probe, and then the measured distributions are transformed to the far field. In the second technique, a plane wave that is approximately uniform in amplitude is created by a feed and large reflector in the immediate vicinity of the test antenna. And in the third technique, the test antenna is focused within the radiating near-field region, patterns are measured at the reduced range, and then the antenna is refocused to infinity. Each of these techniques is discussed, and various advantages and limitations of each technique are presented.
Copyright 1973 IEEE. Reprinted from IEEE Transactions on Antennas and Propagation.
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Compact range techniques for measuring the gain patterns of full-size microwave antennas and for making radar reflectivity measurements are described. The basic principle of this technique is the use of a large collimating device to generate a uniform plane wave across the aperture of a target or antenna without requiring the normal far-field separation. Two different collimating devices were used in the investigation, a paraboloid with a point-source feed and a parabolic cylinder with a line-source feed generated by a large hoghorn. Pattern and gain measurements were made on both compact ranges using a 30-inch paraboloidal test antenna, and the measurements were compared with similar ones made on conventional outdoor ranges. Radar cross-section patterns as a function of aspect angle were measured for various size standard targets and compared with theoretically calculable radar cross-section patterns. The results which have been achieved are very encouraging. They demonstrate that the performance of compact ranges at theX-band is comparable to that of outdoor ranges..
Copyright 1969 IEEE. Reprinted from IEEE Transactions on Antennas and Propagation.
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In this paper, we explore various methods of measuring noise power in RF systems. We discuss some of the systems commonly used to make such measurements, then analyze each system’s architecture to obtain the measurement bias and uncertainty for six different combinations of noise models (real and complex noise) and methods of averaging (linear power, magnitude, and logarithmic). Equipped with this knowledge, users can correct for post-measurement biases and determine the number of averages required to yield a desired uncertainty level.
Copyright 2021 IEEE.IEEE Instrumentation & Measurement Magazine, October 2021.
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In its current form, IEEE Std 149, "IEEE Standard Test P rocedures for Antennas" [l], is a marginally useful document. While the standard is a good source for interesting and pertinent information, the document has not undergone a significant update since 1979. The instrumentation section, for example, describes in detail a chart recorder, an instrument that was common in antenna measurements in the 1970s and that can be found in the product catalogs of the era (see Fig ure 1). The basics of antenna measurement Portable Pattern Recorders Series 1410 have not changed and the underlying theory has not ch anged as the physics behind it are still based in Maxwellian electromagnetics, the last of the classical physics fields. H owever, the document is mainly centered on outdoor ranges for the measurement of antennas (which represented the most common approach for antenna measurements at the time), and includes long discussions on elevated and ground reflection ranges. H owever, there are very limited discussions on anechoic chambers and on currently popular techniques, such as compact antenna test ranges (CATR) or near to far field measurements using mathematical transforms. These last two have become very important approaches in precision antenna measurements.
Copyright 2018 Same Page Publishing, Inc.
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Indoor antenna ranges must have the walls, floor, and ceiling treated with a radio-frequency (RF) absorber. The normal incidence performance of the absorber is usually provided by the manufacturer of the material—but the bistatic or offangle performance must also be known. Recently, a polynomial approximation was introduced to predict the reflected energy from a pyramidal absorber.
In this article, these approximations were used to forecast the quiet zone (QZ) performance of several anechoic chambers. The predictions were compared with full-wave analyses performed in CST Suite. In addition to the comparison of the predictions with analyses that use a different numerical method, comparisons were made against actual measurements of the QZ effectiveness of anechoic chambers. Measurements taken per the free-space voltage standing wave ratio (VSWR) method were compared with the estimates that use the polynomials presented in [1]. The results show that the polynomial approximations can be used to give a fairly accurate indication of the QZ performance of anechoic chambers.
Copyright 2018 IEEE. Reprinted from IEEE Antennas & Propagation Magazine, August 2018.
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The task of adequately specifying performance for an indoor anechoic chamber without driving unnecessary costs or specifying contradictory requirements calls for insight that is not always available to the author of the specification. While there are some articles and books13 that address anechoic chamber design, a concise compendium of reference information and rules of thumb on the subject would be useful. This article intends to be a helpful tool in that regard. It starts by recommending the proper type of range for different antenna types and frequencies of operation. Rules of thumb are provided to select the best approach for the required test or antenna type. The article concentrates on rectangular chambers. Simple approximations are used for absorber performance to generate a series of charts that can be used as a guide to specify performance and appropriate facility size.
Microwave Journal Copyright ©2016. Reprinted from January 2016 issue.
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The task of adequately specifying performance for an indoor anechoic chamber without driving unnecessary costs or specifying contradictory requirements calls for insight that is not always available to the author of the specification. Although there are some articles and books13 that address anechoic chamber design, a concise compendium of reference information and rules of thumb on the subject would be useful. This second part of the series intends to do that, concentrating on the sizing of compact ranges and chambers for near field systems. As was done in part one, simple approximations are used for absorber performance to generate a series of equations that help specify performance and size of facilities.
Microwave Journal Copyright ©2016. Reprinted from February 2016 issue.
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This paper describes a novel method, termed the IsofilterTM Technique, of isolating in the measured data the radiation pattern of an individual radiator from among a composite set of radiators that form a complex radiation distribution. This technique proceeds via three successive steps: A spherical NFFF transform on an oversampled data set, followed by a change of coordinate system followed in turn by filtering in the domain of the spherical modes to isolate a radiating source. The end result is to yield an approximate pattern of the individual radiator largely uncontaminated by the other competing sources of radiation. I.
Copyright 2010 IEEE. Reprinted from IEEE Antennas and Propagation Magazine (Volume: 52 , Issue: 1 , Feb. 2010).
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Today’s automobiles have a wide variety of RF systems with antennas on them for Sirius and XM radio, collision avoidance radars, the Global Positioning System (GPS) and other systems. Conventional test facilities can only perform terrestrial directed pattern measurements of the antenna on the automobile. Special test facilities are required when the automobile must communicate with a satellite as well as other ground systems. Table 1 provides a partial list of satellite-based wireless systems below 2.5 GHz in frequency. This paper discusses a system specially designed for making antenna measurements from the zenith to the horizon. In addition, some of the issues involved in making satellite band measurements such as Sirius/XM and GPS and terrestrial band measurements such as CELL800 and CELL1800 are reviewed.
Copyright Advantage Business Marketing. Reprinted from Electronic Component News Magazine, 2010.
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Incredible efforts are made by system designers to produce state-of-the-art radar and other RF based capabilities for our military. Modern radar systems are used for various purposes including, but not limited to: weather assessment; navigation; terrain following/terrain avoidance; weapons fire control; electronic warfare; enemy tracking, listening and identification, etc.
Dependant upon extremely high measurement precision, repeatability and accuracy, these radar systems all require protection from the elements. While many think about the exotic hardware and sexy looking screen shots produced by these sophisticated radar systems, most do not think about one extremely critical component of these systems: the radar dome or radome. When one considers the critical need for proper operation of these systems for our military, as well as the harsh conditions during conflicts, this component protects vital systems which can make the difference between survival and disaster.
The most well recognized radome is the one positioned on the nose of an aircraft or missile. However, many military applications, and new commercial applications, are positioning microwave based systems in other locations on the aircraft. These often require odd shapes in order to protect the RF system and to be sufficiently aerodynamic. Military radome testing is, not surprisingly, considerably more involved than for commercial applications.
The Radar Cross Section (RCS) of an object is defined as, “the area intercepting that amount of power which, when scattered isotropically, produces a return at the radar equal to that from the target.” In simpler terms, RCS is the projected area of a sphere that has the same radar return as the target. The unit of measure for an object’s RCS is “decibels per square meter,” or dBsm. The power received by a radar for a target indicates how well the radar can detect or track that target. For this reason, much research and effort has been put into reducing the “signature” of various aircraft, ships and other objects.
There are an increasing number of antennas on mobile platforms such as trucks, tanks, armored personnel carriers, UAV ’s , small ships and boats. This has resulted from the military C3I requirement to be able to communicate both tactical and strategic information in real time. Applications for such communication range from real time targ e ting to asset deployment. This on vehicle commutation capability to and from mobile platforms is referred to as telematics.
We are witnessing an explosive expansion of RF transmitting and receiving products due to consumer demand for wireless voice and data connectivity and the availability of cost-effective technology to produce such products and services. Developers of these new products are keenly interested in the pattern, gain and polarization of their products. Traditional antenna measurement equipment can provide the needed information but usually at a prohibitively high price since such test equipment was designed for general purpose, very high precision aerospace applications. Many of the features needed in aerospace antenna measurements are not required in wireless applications. For example, aerospace measurements are often made at 35 or 95 GHz while wireless communication devices often work at 800 or 1,900 MHz. In addition, phase and amplitude measurements are made in the aerospace applications while wireless measurements usually only require amplitude information.
During the past two years the compact range has emerged from the research laboratory into the field for use by the engineer in antenna and radar cross section measurements. During the development of the compact range, experience has been gained on how to handle many of the problems identified in the early work. This paper describes successful results on the areas of improvement of surface accuracy and the design of the reflector edges; also reported are antenna measurements made at 30 GHz using the compact range.
The absolute gain of wide-beam antennas may be accurately measured using the method described. Both the theoretical and practical aspects of gain calibration on a ground reflection antenna range are presented. The measurement procedures developed were used to calibrate a log-periodic antenna at selected frequencies from 250 to 400 MHz. Measured data at 300 MHz is tabulated and error contributions are analyzed, yielding a measurement accuracy of f0.27 dB with a 95 percent confidence interval.
Copyright 1969 IEEE.
This material is posted here with permission of the IEEE. Such permission
of the IEEE does not in any way imply IEEE endorsement of any of NSI-MI Technologies' products or services. Internal or personal use of this
material is permitted. However, permission to reprint/republish this
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IEEE by writing to pubs-permissions@ieee.org.
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