Accurate Planar Near-Field Results Without Full Anechoic Chamber

Authors: Greg Hindman, Stuart Gregson, Allen Newell

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.

Advanced Positioner Control Techniques in Antenna Measurements

Author: Jacob Kunz

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.

Advanced Waveform Generator For Integrated Phased Array Testing

Authors: David S. Fooshe, Kim Hassett, William Heruska, John Butler, Patrick Fullerton

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.

Advances in Instrumentation and Positioners for Millimeter-Wave Antenna Measurements

Authors: Bert Schlüper and Patrick Pelland

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.

Application of Spherical Near-Field Uncertainty Analysis to Positioner Design

Author: Steven R. Nichols

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.

Assessing and Quantifying the Effects of Planar Mathematical Absorber Reflection Suppression Technique

Authors: Greg Masters, Stuart Gregson, Allen Newell, Greg Hindman

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.

Behaviour of Orthogonal Wave Functions and Their Application to the Correction of Antenna Measurements

Authors: S F Gregson, A C Newell, G E Hindman

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.


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