This paper presents a comparative investigation of two versatile error-mitigation techniques, applicable to general antenna near-field measurement scenarios with echo signals of unknown origin. Both techniques are based on spatial filtering of the measured field, taking advantage of a priori knowledge of the antenna's size. The first approach takes advantage of the spatial-filtering properties of the spherical-wave expansion of the measured field. The second approach is based on the reconstruction of equivalent currents, and implements the spatial filtering as a direct consequence of the selected size and shape of the reconstruction surface. The investigation was performed using measured data on two different horns in both planar and spherical near-field scanning geometries. The presence and levels of echo pollution in the measurements were controlled by introducing known scattering objects in the anechoic chambers, and comparing with reference situations without disturbance.

Echo Suppression by Spatial-Filtering Techniques in Advanced Planar and Spherical Near-Field Antenna Measurements [AMTA Corner]

Multipath propagation limits the obtainable accuracy of antenna measurements in non-anechoic environments. For reliable results of the antenna radiation pattern, appropriate postprocessing of the acquired data should be performed in order to separate the line-of-sight signal from the multipath signals. In this paper, a measurement technique which facilitates the removal of multipath contributions during data postprocessing is described. This is achieved by measuring additional field data with multiple probe antennas such that enough information is available to resolve the direct contributions of the antenna under test and the multipath signals. The data so acquired is then processed by a well adapted plane wave based near-field far-field transformation algorithm, with the echo sources integrated by means of auxiliary sources. The performance of the technique is demonstrated using simulated and true echoic measurements for spherical and planar measurement geometries.

A Multi-Probe Antenna Measurement Technique With Echo Suppression Capability

The far-field pattern of an antenna under test is determined from the near-field measurements in a highly reflective environment. The proposed post-processing effectively combines time gating and spatial filtering of the radiated fields. Time gating utilizes frequency diversity, while spatial filtering makes use of probe diversity and source localization. Different probe locations are used to form a virtual array, which focuses the fields at the antenna volume. The possible scatterers are illuminated as little as possible. The weighting coefficients for the virtual array are found by solving a field synthesis problem. The effects of time gating and spatial filtering are analyzed separately. A measurement setup is described in a highly reflective chamber involving fully spherical measurements with probes in up to 120 different positions and with 1001 frequencies. Time gating yields better results, but a combination of both methods is necessary to achieve an estimated accuracy of −30 dB for the considered S-band and −40 dB for the considered Ku-band antenna.

Near-Field Antenna Pattern Measurements in Highly Reflective Environments

This paper presents a time domain antenna measurement technique by using a low cost software defined radio receiver. The technique aims to resolve measurement challenges derived from antennas where the reference signal is not accessible. The phase reconstruction implemented in this work is based on calculating the Fast Fourier Transform of the time domain signal to estimate the power spectrum and the relative phase between measurement points. In order to do that a reference antenna is used to retrieve the phase, providing a full characterization in amplitude and phase of the electric field and allowing source reconstruction. The results demonstrate the potential of this technique for new antenna measurement systems and reveal some of the limitations of the technique to be optimized, like the undesired reflections due to the interactions between the probe and the reference antenna.

Evaluation of Software Defined Radio Receiver for Phaseless Near-Field Measurements

The operating principles of RFID antennas should be considered differently than it is applied in the classical theory of radio communication systems. The procedure of measuring the radiation pattern of antennas that could be applied to RFID transponders operating in the UHF band is seldom discussed correctly in the scientific literature. The problem consists in the variability of the RFID chip impedance that strongly influences measurement results. The authors propose the proper methodology for determining the radiation pattern with respect to an individual transponder as well as an electronically tagged object. The advantage of the solution consists in the possibility of using components of different measuring systems that are available in typical antenna laboratories. The proposed procedure is particularly important in terms of parameter validation – the identification efficiency and costs of an RFID system implementation can be evaluated properly only on the basis of real values of considered parameters.

A Method for Measuring the Radiation Pattern of UHF RFID Transponders

A new method to reduce the unwanted reflection effects in antenna measurements is presented. The proposed method can be applied to measurements performed in semi-anechoic chambers, outdoor systems or even anechoic environments with a poor reflectivity level to remove reflected components and to retrieve the results one would obtain in an ideal anechoic chamber. The method is based on spatial filtering over the plane where the antenna under test (AUT) is placed. To calculate the field in this plane, a diagnostic technique is employed. However, in contrast to classic application (error source identification), the reconstructed field has to be obtained in a zone larger than the antenna dimensions. Thus, it is possible to identify virtual sources out of the antenna aperture, which appear due to the presence of reflections (image theory). Then, by cancelling the virtual and unwanted sources, the related reflected components can be suppressed. Finally, by employing this filtered reconstructed field, a new radiation pattern similar to the one obtained in a fully anechoic environment is calculated. To verify the effectiveness of the method, three examples are presented.

New Reflection Suppression Method in Antenna Measurement Systems Based on Diagnostic Techniques

Cylindrical near-field systems are appropriate measurement systems for large L-band RADAR antennas, because they can be measured on their azimuthal positioner, and the probe can be easily translated through a vertical linear slide. Thus, for large antennas, the mechanical aspects of the measurement systems are important and the errors in this mechanical part can affect to the far-field radiation pattern. This work presents the design and validation process of the near-to-far-field transformation algorithm applied to an outdoor L-band RADAR antenna cylindrical system. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 2527–2531, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23709

/pdf/cylindrical-near-to-far-field-transformation-system-for-2fk1nlysac.pdf

Cylindrical near‐to‐far‐field transformation system for radar antennas: Design, validation, and application

This document presents two measurement systems for testing the new large radar array antennas developed by the Spanish company INDRA Sistemas. The first one is a linear measurement system for testing the array rows. The second one is an outdoor cylindrical near field facility for the complete array test. Different techniques and improvements applied to them are reviewed and results are shown.

http://ieeexplore.ieee.org/iel5/5497865/5504894/05505574.pdf

Large radar antennas measurement systems

This paper is focused on explaining the radiation test in temperature performed on the Engineering and Qualification Model of the Medium Gain Antenna Radiofrequency (MGA-RFA) Assembly of ESA's BepiColombo mission. The goal of this program is to observe and study Mercury and its surroundings in a very demanding environment in terms of temperature and radiation. The MGA is an X-band two-axis steerable horn, which provides bidirectional communications between spacecraft and Earth as backup of the High Gain Antenna and also operates as primary communication link at several mission stages or conditions. The paper presents the measurement set-up for the qualification campaign of the antenna, where it was necessary to characterize the antenna in a representative thermal environment, and the results obtained from this test. Results of test up to 150°C show how gain and radiation pattern shapes are slightly affected by thermal stress, but without jeopardizing mission requirements. In addition, by analyzing correlation of this test with RF analysis in the same thermal conditions, it becomes possible to accurately extrapolate the MGA-RFA behavior up to temperatures of more than 500°C. This fact allowed the successful space qualification of this model.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/13B7B5F12972445E77A0CD39A8815CB0/S175907871700099Xa.pdf/div-class-title-measurement-of-bepicolombo-mission-medium-gain-antenna-parameters-under-realistic-thermal-conditions-div.pdf

Measurement of BepiColombo mission medium gain antenna parameters under realistic thermal conditions

Cylindrical near field systems are appropriate measurement systems for huge L-band RADAR antennas, because the antenna can be measured on its azimuthal positioner and the probe can be easily translated through a vertical linear slide. So for large antennas the mechanical aspects of the antenna measurement systems are important and the errors in this mechanical part can affect to the far field radiation pattern. This paper presents an error estimation tool to analyze the most important errors in one cylindrical acquisition system and the effect of these errors in the calculation of
the far field radiation pattern. This studyhas been performed to improve the cylindrical systemand to evaluate the error budgetof the Antenna Under Test (AUT). The simulator calculates the far field froman array of dipoles over a ground plane (Antenna UnderTest model) and compares the ideal result with the electric field obtained using the cylindrical near to farfield transformation algorithms. The results achieved are the variation in the principal patterns of the far field, RMS errors in side lobes and maximum errors in side lobes.
Random and deterministic source of errors have been considered.

Fernando Martin

Papers

New Reflection Suppression Method in Antenna Measurement Systems Based on Diagnostic Techniques

Cylindrical near‐to‐far‐field transformation system for radar antennas: Design, validation, and application

Large radar antennas measurement systems

Measurement of BepiColombo mission medium gain antenna parameters under realistic thermal conditions

Error estimations in cylindrical near field system for large radar antennas