2013 Technical Papers

Advances in Automated Error Assessment of Spherical Near-Field Antenna Measurements

Authors: Patrick Pelland, Greg Hindman, William Heruska, Allen Newell

Publication: EuCAP 2013

Copyright Owner: IEEE

Over the years, spherical near-field (SNF) antenna measurements have become increasingly popular for characterizing a wide variety of antenna types. The SNF configuration allows one to measure data over a sphere surrounding the antenna, which provides it a unique advantage over planar and cylindrical near-field systems where measurement truncation is inherent. Like all antenna measurement configurations, SNF systems are susceptible to a number of measurement errors that, if not properly understood, can corrupt the antenna’s far-field parameters of interest (directivity, beamwidth, beam pointing, etc.). The NIST 18-term error assessment originally developed for planar near-field measurements [1] has been adapted for SNF systems [2] and provides an accurate measure of the uncertainty in a particular SNF measurement. Once particular measurement errors are known, steps can be taken to reduce their impact on far-field radiation patterns. When manually assessing all 18 terms of the NIST uncertainty budget this procedure becomes tedious and time consuming.

This paper will describe an acquisition algorithm that allows one to analyze all 18 error terms or a subset of those in automated fashion with minimal user intervention. Building upon previous research toward developing an automated SNF error assessment algorithm [3, 4], this new procedure will automatically generate tabulated and plotted uncertainty data for directivity, beamwidth and beam pointing of a particular farfield radiation pattern. Once measurement uncertainties are known, various post-processing techniques can be applied to improve far-field radiation patterns. Results will be shown for three antennas measured on large phi-over-theta SNF scanners.

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An Electronically Controlled Polarization Generator at Ka-band

Author: Steven R. Nichols

Publication: EuCAP 2013

Copyright Owner: IEEE

As part of a target simulator, a linearly polarized signal was required with a variable tilt angle that could be controlled electronically and changed at a 1 kHz rate. The signal simulates the effect of rapid polarization changes that a missile might encounter in real time during flight.

Tilt angles can be varied by adjusting the amplitude of the vertical and horizontal inputs to an orthomode transducer. To produce good cross-polarization, independent phase adjustments are also required. However, microwave components available in the 33.4 – 36 GHz operating range were inadequate to achieve the desired performance.

A novel approach was developed to downconvert the input signal to a lower frequency range and use vector modulators available in the lower band to produce the appropriate phase and amplitude changes in each path, then upconvert back to the desired operating frequency to drive the orthomode transducer. A device was built and tested using this approach.

A calibration and measurement procedure was developed to determine the vector modulator input settings that produced the most accurate tilt angles and best cross-polarization performance. By iteratively measuring cross-polarization and tilt angle, then adjusting the vector modulator controls, a tilt angle accuracy of +/-1 degree was achieved with a cross-polarization of -25 dB, exceeding the required performance.

By implementing the architecture described, both the phase and amplitude of the horizontal and vertical signals to the orthomode transducer can be controlled. In addition to linearly polarized signals, other types of polarization signals can also be generated, including left-hand and right-hand circular, as well as the general case of elliptical polarization.

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Antenna Measurements using Modulated Signals

Author: Roger Dygert

Publication: AMTA 2013

Copyright Owner: NSI-MI Technologies

Antenna test engineers are faced with testing increasingly complex antenna systems, one of these being the AESA (Active Electronically Steered Array) antennas used for cell communications, jammers, and radars. Often these antennas have integrated electronics and RF components that are an intricate part of the antenna, and as a result must be tested with the waveforms generated by the antenna itself. One cannot simply inject an unmodulated continuous wave signal. These antennas require new measurement techniques which are compatible with their broadband waveforms.

The reference channel of a measurement receiver can be used to collapse the spectrum of the modulated signal into a single CW measurement. Done properly all the energy in the signal is captured with noise and interference being dispersed, resulting in no loss of DR (dynamic range) over a CW measurement. A receiver employing this technique can capture all the energy in modulated and pulsed signals wielding wide dynamic range measurements. Phased locked loops (PLL) are not used as they can preclude such measurements.

A measurement receiver that uses a digital correlator to collapse the spectrum of modulated and pulsed signals will be presented. This paper will describe the technique used to do this and show measured results on example broadband signals.

Behaviour of Orthogonal Wave Functions And The Correction of Antenna Measurements Taken in Non-Anechoic Environments

Author: S.F. Gregson, A.C. Newell, G.E. Hindman

Publication: 2013 Loughborough Antennas and Propagation Confererence

Copyright Owner: IEEE

The measurement and post-processing mode orthogonalisation and filtering technique, named Mathematical Absorber Reflection Suppression (MARS) [1, 2], has been used extensively to identify and subsequently extract measurement artefacts arising from spurious scattered fields that are admitted when antenna testing is performed in non-ideal anechoic environments. Underpinning the success of the MARS postprocessing, and other mode orthogonalisation and filtering strategies [3], is the behaviour of the orthogonal vector wave (mode) expansions that are employed to describe the radiated fields and in particular their behaviour under the isometric coordinate translations that are central to the post processing. Within this paper, simulated and measured data will be used to illustrate the applicability of this measurement and post processing technique paying particular attention to the behaviour of the various modal expansions examining and confirming specific, commonly encountered, measurement conventions.

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Best-Fit 3D Phase-Center Determination and Adjustment

Author: Scott T. McBride, David J. Tammen, Ph.D, Doren W. Hess, Ph.D

Publication: AMTA 2013

Copyright Owner: NSI-MI Technologies

There are several applications in which knowledge of the location of the phase center of an antenna, and its twodimensional variation, is an important feature of its use. A simple example occurs when a broad-beam antenna is used as a feed for a reflector, where the center of the spherical phase fronts should always lie at the focal point of the paraboloidal surface. Here, the ability to determine the phase center of the feed from knowledge of its far-field phase/amplitude pattern is critical to the reflector’s design.

Previously published methods process a single cut of data at a time, yielding 2D lateral and longitudinal phase-center offsets. Eand H-plane cuts are thus processed separately, and will, in general, yield different answers for the longitudinal offset. The technique presented here can process either one line cut at a time or a full Theta-Phi raster. In addition, multiple frequencies can be processed to determine the average 3D phase-center offset. The technique can merely report the phase-center location, or it can also adjust the measured phases to relocate the origin to the computed phase center. Example results from measured data on multiple antenna types are presented.

Combination Planar, Cylindrical, Far-Field and Dual Spherical Near-Field Test System for 0.2-110GHz Applications

Author: Patrick Pelland, Scott Caslow, Gholamreza Zeinolabedin Rafi

Publication: AMTA 2013

Copyright Owner: NSI-MI Technologies

Nearfield Systems Inc. (NSI) has been contracted by the Department of Electrical and Computer Engineering of the University of Waterloo to install a unique antenna test system with multiple configurations allowing it to characterize a wide variety of antenna types over a very wide bandwidth. The system employs a total of 10 positional axes to allow near-field and far-field testing in various modes of operation with great flexibility. A 4 m x 4 m planar near-field (PNF) scanner is used for testing directive antennas operating at frequencies up to 110 GHz with laser interferometer position feedback providing dynamic probe position correction. The PNF’s Y-axis can also be used for cylindrical near-field (CNF) testing applications when paired with a floor mounted azimuth rotation stage. A single phi-over-theta positioner permits both spherical near-field (SNF) testing from L-band to W-band and far-field testing down to 0.2 GHz. This positioner is installed on a translation stage allowing 1.8 m of Z-axis travel to adjust the probe-to-AUT separation. In addition, a theta-over-phi swing arm SNF system is available for testing large, gravitationally sensitive antennas that may be easily installed on a floor mounted rotation stage. In order to ensure system and personnel safety, a complex interlock system was designed to reduce the risk of mechanical interference and ease the transition from one configuration to another. The system installation and validation was completed in March 2013. We believe that this facility is unique in that it encompasses all commonly used near-field configurations within one chamber. It therefore provides a perfect environment for the training of young engineers and could potentially form the baseline of future academic test facilities. This paper will outline the technical specifications of the scanner and discuss the recommended applications for each configuration. It will also describe the details of the safety interlock system.