Validation of the Polynomial for RF Absorber Reflectivity for the Prediction of Anechoic Chambers

Author: Vince Rodriguez
Publication: IEEE Transactions on Antennas and Propagation
Copyright Owner: NSI-MI Technologies

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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Attenuating Tunnels for Accessing Shielded Enclosures

Author: Vince Rodriguez
Publication: EMC Society of Australia Newsletter, June 2017, Issue 77
Copyright Owner: NSI-MI Technologies

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.

Digitally Reconfigurable Approach to Compact Antenna Test Range Design

Authors: C.G. Parini, R. Dubrovka, S.F. Gregson
Publication: EuCAP 2017

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 sectorshaped, 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.

Examination of the Effectiveness of Far-field Mathematical Absorber Reflection Suppression in a CATR Through Computational Electromagnetic Simulation

Authors: S.F. Gregson, C.G. Parini, A.C. Newell, G.E. Hindman
Publication: EuCAP 2017

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.

Thermal Noise Effects of a Simple Correlator for High Dynamic Range Measurements

Author: Brett T. Walkenhorst
Publication: EuCAP 2017
Copyright Owner: NSI-MI Technologies

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.

A Multi-Robot Large Antenna Positioning System for Over-The-Air Testing at NIST

Authors: David R. Novotny, Joshua A. Gordon, Michel S. Allman, Alexandra E. Curtin, Jeff R. Guerrieri, Kim Hassett, Quang Ton, George McAdams
Publication: AMTA 2017
Copyright Owners: NSI-MI Technologies, NIST | Communications Technology Laboratory

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.

Acquisition, Reconstruction, and Transformation of a Spiral Near-Field Scan

Author: Brett T. Walkenhorst
Publication: AMTA 2017
Copyright Owners: NSI-MI Technologies

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.

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