A Straightforward Dynamic Range Error Analysis

Authors: Marion Baggett and Brett T. Walkenhorst
Publication: AMTA 2019
Copyright Owner: NSI-MI Technologies

The 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.

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CATR Quiet Zone Depth Variation

Authors: Marion Baggett and Brett T. Walkenhorst
Publication: AMTA 2019
Copyright Owner: NSI-MI Technologies

The 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.

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Measurement of RF Absorber at Large Angles of Incidence using Spectral Domain Transformations

Authors: Vince Rodriguez, Brett Walkenhorst, Jorgen Bruun
Publication: AMTA 2019
Copyright Owner: NSI-MI Technologies

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.

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Practical Considerations in Compressed Spherical Near-Field Measurements

Authors: Cosme Culotta-L´opez, Brett Walkenhorst, Quang Ton, Dirk Heberling
Publication: AMTA 2019
Copyright Owner: RWTH Aachen University

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.

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Recent Changes to the IEEE std 1502 Recommended Practice for Radar Cross-Section Test Procedures

Authors: Eric Mokole, L. J. Foged, Vince Rodriguez, Jeff Fordham, Vikass Monebhurrun
Publication: AMTA 2019
Copyright Owner: NSI-MI Technologies

Radar 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.

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Customized Spherical Near-Field Test Time Reduction for Wireless Base Station Antennas

Authors: E’qab Almajali, Daniël J. van Rensburg, Derek A. McNamara
Publication: IEEE Antennas and Wireless Propagation Letter, Vol. 18, No. 1, January 2019
Copyright Owner: IEEE

Effective 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.

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Copyright 2019 IEEE. Reprinted from IEEE Antennas and Wireless Propagation Letter, Vol. 18, No. 1, January 2019

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A Method for the Measurement of RF Absorber using Spectral Domain Transformations

Author: Vince Rodriguez, Brett Walkenhorst, Jorgen Bruun
Publication: APS 2019
Copyright Owner: IEEE

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.

You have requested a Reprint of an IEEE-AP-S Paper

Copyright 2019 IEEE. Reprinted from 2019 IEEE AP-S Symposium.

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.

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