2018 Technical Papers

Controlling the Gain of Wide Band Open Quad Ridge Antennas

Author: German Cortes-Medellin

Publication: EuCAP 2018

Copyright Owner: IET

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.

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Further Refining and Validation of RF Absorber Approximation Equations for Anechoic Chamber Predictions

Author: Vince Rodriguez

Publication: EuCAP 2018

Copyright Owner: IET

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.

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Near-Field Antenna Measurements using a Lithium Niobate Photonic Probe

Authors: Vince Rodriguez, Brett Walkenhorst, and Jim Toney

Publication: EuCAP 2018

Copyright Owners: IEEE

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.

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Enhanced PNF Probe Positioning in a Thermally-Uncontrolled Environment using Stable AUT Monuments

Authors: John H. Wynne, Farzin Motamed, George E. McAdams

Publication: AMTA 2018

Copyright Owner: NSI-MI Technologies

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.

Imaging a Range’s Stray Signals with a Planar Scanner

Authors: Scott T. McBride, John Hatzis

Publication: AMTA 2018

Copyright Owner: NSI-MI Technologies

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.

Implementation of a Technique for Computing Antenna System Noise Temperature Using Planar Near-Field Data

Authors: A.C. Newell, C. Javid, B. Williams, P. Pelland, D. Janse van Rensburg

Publication: AMTA 2018

Copyright Owner: NSI-MI Technologies

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.