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Far Field

Patrick Pelland,Scott Caslow, Gholamazera Zeinolabedin Rafi, November 2013

Abstract — 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.

High Gain Antenna Back Lobes from Near-Field Measurements
George Cheng,Yong Zhu, Jan Grzesik, November 2013

Abstract -We propose a method of utilizing near-field spherical measurements so as to obtain the back lobes of high gain antennas without sacrificing the accuracy of the far-field, high-gain main lobe prediction. While a spherical scan is perfectly adequate to gauge the relatively broad back lobes, it is in general inadequate to capture the required details of a sharp forward peak. We overcome this difficulty through recourse to our Field Mapping Algorithm (FMA), which latter allows us to assemble planar near-field data based upon the spherical measurements actually acquired. In particular, planar data of this sort on the forward, main-lobe side offers the standard route to predicting the desired, high-gain, far-field pattern. Our spherical-to-planar FMA near-field data manufacture showed excellent agreement with direct planar near-field measurements for a slot array antenna, each one of them, naturally, underlying a common, far-field, high-gain pattern.

Mechanical and Electrical Alignment Techniques for Plane-polar Near-field Test Systems
Michael Carey,Patrick Pelland, Stuart Gregson, Naoki Shinohara, November 2013

This paper will describe newly developed mechanical and electrical alignment techniques for use with plane-polar near-field test systems. A simulation of common plane-polar alignment errors will illustrate, and quantify, the alignment accuracy tolerances required to yield high quality far-field data, as well as bounding the impact of highly repeatable systematic alignment errors. The new plane-polar electrical alignment technique comprises an adaptation of the existing, widely used, spherical near-field electrical alignment procedure [8] and can be used on small, and large, plane-polar near-field antenna test systems.

Laboratory Tests on the Near-field to Far-field Transformation with Spherical Spiral Scan Optimized for Long Antennas
Francesco D'Agostino, Flaminio Ferrara, Jeffrey Fordham, Claudio Gennarelli, Rocco Guerriero, Massimo Migliozzi, October 2013

In this communication, the experimental verification of a probe compensated near-field - far-field (NF-FF) transformation with spherical spiral scanning particularly suitable for elongated antennas is provided. It is based on a nonredundant sampling representation of the voltage measured by the probe, obtained by using the unified theory of spiral scans for nonspherical antennas and adopting a cylinder ended in two half-spheres for modelling long antennas. Its main characteristic is to allow a remarkable reduction of the measurement time due to the use of continuous and synchronized movements of the positioning systems and to the reduced number of required NF measurements. In fact, the NF data needed by the classical NF-FF transformation with spherical scanning are efficiently and accurately reconstructed from those acquired along the spiral, by employing an optimal sampling interpolation formula. Some experimental results, obtained at the Antenna Characterization Lab of the University of Salerno and assessing the effectiveness of such a NF-FF transformation technique, are presented.

Near-Field – Far-Field Transformation With A Planar Wide-Mesh Scanning: Experimental Testing
Francesco D’Agostino, Ilaria De Colibus, Flaminio Ferrara, Claudio Gennarelli, Rocco Guerriero, Massimo Migliozzi, October 2013

This communication deals with the experimental validation of an efficient near-field - far-field (NF-FF) transformation using the planar wide-mesh scanning. Such a scanning technique is so named, since the sample grid is characterized by meshes wider and wider when going away from the center, and makes possible to lower the number of needed measurements, as well as the time required for the data acquisition when dealing with quasi-planar antennas. It relies on the use of the nonredundant sampling representation of electromagnetic fields based on the use of a very flexible modelling of the antenna under test, formed by two circular "bowls" with the same aperture diameter but eventually different bending radii. A two-dimensional optimal sampling interpolation formula allows the reconstruction of the NF data at any point on the measurement plane and, in particular, at those required by the classical NF-FF transformation with the conventional plane-rectangular scanning. The measurements, performed at the planar NF facility of the antenna characterization laboratories of Selex ES, have confirmed the effectiveness of this nonconventional scanning, also from the experimental viewpoint.

Antenna Diagnostic, Echo Suppression and Equivalent Sources Representation Capabilities of the Fast Irregular Antenna Field Transformation Algorithm
Raimund Mauermayer, Georg Schnattinger and Thomas Eibert, October 2013

The Fast Irregular Antenna Field Transformation Algorithm (FIAFTA) determines the equivalent sources of an antenna under test (AUT) from arbitrarily located sampling points of the antenna field. The application of Fast Multipole Method (FMM) principles to the formulation of the forward operator shows that the influence of the measurement probe is fully corrected based on its far-field radiation pattern. For antenna diagnostic purposes, equivalent surface current densities represent the unknown equivalent AUT sources. However, the FMM gives the possibility to settle the unknowns of the inverse problem in the ^k-space domain. The expansion of the appearing plane wave spectra in spherical harmonics leads to a compact representation of the equivalent plane wave sources. The forward operator is evaluated in a multilevel fashion similar to the Multilevel Fast Multipole Method (MLFMM). This enables to incorporate a priori knowledge about the geometry of the AUT in the antenna model by placing nonempty FMM boxes where sources are assumed.

Best-Fit 3D Phase-Center Determination and Adjustment
David J. Tammen, Scott T. McBride, Doren W. Hess, October 2013

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.

Plano-Convex Lens with Reduced Amplitude Variation
Tse Tong Chia and Serguei Matitsine, October 2013

We recently introduced large, lightweight, broadband plano-convex RF lens for close-range measurement of far-field antenna radiation pattern [1]. While the lens can drastically reduce the phase variation of the field across the transverse plane at a relatively short distance from the lens, the amplitude of the field in the same plane is affected by the diffraction from the circular edges of the lens, and to some extent by the transmitted field after internal reflections inside the lens. Furthermore, while the phase variation is minimal (within ±10°) and almost independent of the distance of the transverse plane from the lens, the field amplitude variation across the same plane increases with the distance of the plane from the lens. The amplitude variation reduces the useful size of the "quiet zone". To reduce the amplitude variation, we propose to incorporate "matching layers" around the lens. As we shall demonstrate in the paper, these matching layers help to reduce the aforementioned diffraction and internal reflections. As a result, the amplitude variation of the field across the transverse plane is reduced (to within ±1dB), thereby increasing the size of the "quiet zone". The matching layers are effective even for lenses as small as 6 in diameter.

Scattering Suppresion in a Combined Compact Range and Spherical Near-field Measurement Facility
Hammam Shakhtur, Rasmus Cornelius, Dirk Heberling, October 2013

Stray signals/scattering suppression techniques will be deployed to enhance measurements quality of a combined compact antenna test range (CATR) and spherical near-field (SNF) measurement facility. Spherical mode filtering and softgating techniques will be the focus of this paper. Using soft-gating the mutual effects between the CATR and SNF facilities will be shown and mitigated. The use of SNF decomposition to enhance the far-field measurements will be also shown. This contributes to a reduction of the costs arising from the need of absorbers to shield both facilities and cover the antenna's support structure.

Feasibility of Near-Field Pattern Characterization for V-band Antennas
Nathan Sutton, Daniël Janse van Rensberg, Matthew Radway, Kim Hassett, Jovan Filipovic, October 2013

This paper presents V-band radiation pattern characterization of both low- and high-directivity antennas. A fourarm micro-machined spiral antenna with monolithically integrated mode-forming network designed for dual circularlypolarized radiation represents the low-directivity antenna, while a standard gain horn is used for the highly directive antenna. All measurements were performed using an in-house NSI-700S- 30 system capable of spherical near-field measurements from 1-50 GHz and direct far-field measurements from 50-110 GHz. Complete comparisons of simulated, near- and far-field patterns show the feasibility of near-field measurements in V-band. Based on pattern comparison and measurement statistics conclusions are drawn about V-band near-field measurements.

Estimation of Far-Field Errors Due To Mechanical Errors In Spherical Near-Field Scanning
Michael Francis,National Institute of Standards and Technology, November 2012

ABSTRACT When the mechanical requirements are established for a spherical near-field scanner, it is desirable to estimate what effects the expected mechanical errors will have on the determination of the far field of potential antennas that will be measured on the proposed range. The National Institute of Standards and Technology (NIST) has investigated the effects of mechanical errors for a proposed outdoor spherical near-field range to be located at Ft. Huachuca, AZ. This investigation was performed by use of theoretical far-field patterns and introducing position errors into simulated spherical near-field measurements using software developed at NIST. Periodic and random radial and angular position errors were investigated. Far-field patterns were then calculated with and without probe-position correction to determine the effects of mechanical position errors. Periodic errors were found to have a larger effect than random errors. This paper reports the results of these investigations.

Estimation of Far-Field Errors Due To Mechanical Errors In Spherical Near-Field Scanning
Michael Francis,National Institute of Standards and Technology, November 2012

ABSTRACT When the mechanical requirements are established for a spherical near-field scanner, it is desirable to estimate what effects the expected mechanical errors will have on the determination of the far field of potential antennas that will be measured on the proposed range. The National Institute of Standards and Technology (NIST) has investigated the effects of mechanical errors for a proposed outdoor spherical near-field range to be located at Ft. Huachuca, AZ. This investigation was performed by use of theoretical far-field patterns and introducing position errors into simulated spherical near-field measurements using software developed at NIST. Periodic and random radial and angular position errors were investigated. Far-field patterns were then calculated with and without probe-position correction to determine the effects of mechanical position errors. Periodic errors were found to have a larger effect than random errors. This paper reports the results of these investigations.

Square Patch Antenna Design from Equivalent Circuit Models for MIMO Antenna Communications Application
Paul Oleski,US Air Force Research Laboratory, November 2012

Although the square patch antenna is a well known printed circuit antenna, there are gaps in the publications that prevented accurate design for practical dual polarization patch antennas. This paper describes (without gaps) the steps that allow rapid design of the dual polarized square patch antenna with typical commercial RF materials. Given a patch laminate material, the design process proceeds by using the Matlab program which is given in Appendix A. Typical values for a 5 GHz patch antenna are given. Dual polarization square patch antennas were constructed. Measurements show the two ports are well isolated, and they provide polarization diversity which is useful in our MIMO array development program. The scattering matrix of the two port antenna was measured with an Agilent PNA network analyzer. The antenna patterns were measured in our anechoic chamber and on our far field range. The pattern widths provide hemi­spherical coverage. The results which are given imply good efficiency for the antenna ports. When combined with the other patch elements in the MIMO array, robust communications are achieved for all look angles.

Computer Reconstructed Holographic Technique for  Phaseless Near-Field Measurements
Zhiping Li,BeiHang University, November 2012

A novel holographic near-field phaseless technique is presented. The measurement system is composed of the antenna under test, the reference antenna, the amplitude scanning measurement system and the holographic reconstructed algorithm. The interference amplitude of the antenna under test with the reference antenna is measured by the amplitude scanning system. The complex near field of the antenna under test is reconstructed by computer, where the measured interference is corrected by the multiplication with the virtual spherical reference wave and then filtered in Fourier Transformation domain (e.g. Plane Wave Angular Spectrum) or the back-projected image space. The reconstruction method is rigorous without traditional Fresnel Approximation. The novel technique requires the amplitude on one measurement surface and the computer reconstructed algorithm, while the previous phaseless technique depends on two measurement surfaces or extra hardware to provide Synthesized­Reference-Wave. The novel holographic measurement method and reconstruction algorithm could be used in many applications as for planar near field measurements for example. Simulated results are presented to demonstrate the complex field retrieval method and near-field to far field transformation.

Exploration of the Feasibility of Adaptive Spherical Near-Field Antenna Measurements
Vincent Beaulé,EECS, University of Ottawa, November 2012

The feasibility of using adaptive acquisition techniques to reduce the overall testing time in spherical near-field (SNF) antenna measurements is investigated. The adaptive approach is based on the premise that near-field to far-field (NF-FF) transformation time is small compared to data acquisition time, so that such computations can be done repeatedly while data is being acquired. This allows us to use the transformed FF data to continuously compute and monitor pre-defined decision functions (formed from the antenna specifications most important to the particular AUT) while data is being acquired. We do not proceed with a complete scan of the measurement sphere but effectively allow the probe to follow a directed path under control of an acquisition rule, so that the sampled NF datapoints constitute an acquisition map on the sphere (the geographical allusion being purposeful). SNF data acquisition can be terminated based on decision function values, allowing the smallest amount of data needed to ensure accurate determination of the AUT performance measures. We demonstrate the approach using actual NF data for several decision functions and acquisition rules.

Advanced Spherical Near-Field-To-Far-Field Software for Modern Computers
Randal Direen,DireenTech Inc, November 2012

The speed of spherical near-field scanning is increased significantly when measurements are not restricted to standard measurement locations, i.e., the locations that are equidistant in theta and in phi. Measurement positions can be chosen so that mechanical positioners perform scans with a continuous motion; this will decrease the time it takes to acquire data for near-field measurements. The issue then becomes transforming the data acquired with non-uniform spacing. This paper describes the development of a spherical near-field to far-field transform that can efficiently process data acquired on a non-uniform grid.

An Improved Antenna Gain Extrapolation Measurement
Jason Coder,National Institute of Standards and Technology, November 2012

An improved system for antenna gain extrapolation measurements is proposed. The improved method consists of a vector network analyzer, a pair of RF optical links, and a pair of waveguide mixers. This change in hardware equates to a system with better dynamic range and a simplified reference measurement. We present a detailed description of the new extrapolation measurement setup, discuss the advantages and disadvantages, and validate the new setup by measuring the gain of an antenna previously measured with a traditional extrapolation setup. After presenting the comparison, we will discuss applications of this measurement system that extend beyond extrapolation gain measurements (e.g., spherical near- and far-field pattern measurements).

Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction
Allen Newell,Nearfield Systems Inc, November 2012

The numerical analysis used for efficient processing of spherical near-field data requires that the far-field pattern of the probe can be expressed using only azimuthal modes with indices of µ = ±1. (1) If the probe satisfies this symmetry requirement, near-field data is only required for the two angles of probe rotation about its axis of . = 0 and 90 degrees and numerical integration in . is not required. This reduces both measurement and computation time and so it is desirable to use probes that will satisfy the µ = ±1 criteria. Circularly symmetric probes can be constructed that reduce the higher order modes to very low levels and for probes like open ended rectangular waveguides (OEWG) the effect of the higher order modes can be reduced by using a measurement radius that reduces the subtended angle of the AUT. Some analysis and simulation have been done to estimate the effect of using a probe with the higher order modes (2) – (6) and the following study is another effort to develop guidelines for the properties of the probe and the measurement radius that will reduce the effect of higher order modes to minimal levels. This study is based on the observation that since the higher order probe azimuthal modes are directly related to the probe properties for rotation about its axis, the near-field data that should be most sensitive to these modes is a near-field polarization measurement. This measurement is taken with the probe at a fixed (x,y,z) or (.,f,r) position and the probe is rotated about its axis by the angle .. The amplitude and phase received by the probe is measured as a function of the . rotation angle. A direct measurement using different probes would be desirable, but since the effect of the higher order modes is very small, other measurement errors would likely obscure the desired information. This study uses the plane-wave transmission equation (7) to calculate the received signal for an AUT/probe combination where the probe is at any specified position and orientation in the near-field. The plane wave spectrum for both the AUT and the probe are derived from measured planar or spherical near-field data. The plane wave spectrum for the AUT is the same for all calculations and the receiving spectrum for the probe at each . orientation is determined from the far-field pattern of the probe after it has been rotated by the angle .. The far-field pattern of the probe as derived from spherical near-field measurements can be filtered to include or exclude the higher order spherical modes, and the near-field polarization data can therefore be calculated to show the sensitivity to these higher order modes. This approach focuses on the effect of the higher order spherical modes and completely excludes the effect of measurement errors. The results of these calculations for different AUT/probe/measurement radius combinations will be shown.

Using Spherical Near-field Transforms to Determine the Effects of Range Length on the Measurement of Total Radiated Power
James Huff,The Howland Company, Inc., November 2012

Total radiated power (TRP) and total isotropic sensitivity (TIS) are two metrics most commonly used to characterize the performance of a wireless device. These integrated measurement parameters are not as sensitive to the measurement distance as a single point measurement such as an antenna gain measurement, but it is difficult to accurately quantify the effects of measurement distance on these two parameters. This paper presents a simple approach to quantifying the effects of measurement distance using spherical near-field transforms. Data is taken on a typical wireless device at different range lengths and transformed to the far-field using a spherical near-field transform. The total radiated power is then calculated for both the measured data and the transformed data. The difference in the two calculations shows the effect of a finite range length on the measurement. Measured results are presented for three different range lengths. For each of these range lengths the data is transformed to the far-field and the TRP is calculated.

Multi-port RF 16-Switch Controller at Air Force Research Laboratory Rome's Newport Test FacilityMulti-port RF 16-Switch Controller at Air Force Research Laboratory Rome's Newport Test Facility
Frances Rose,Air Force Research Laboratory, November 2012

The Air Force Research Laboratory (AFRL), Information Directorate, has served as an Air Force center for antenna measurements for over thirty years. AFRL's Newport Research Facility consists of multiple far field outdoor test ranges with 3-axis positioner towers. The range supports a wide variety of test activities including measurements on simple antennas, complex active phased arrays, avionics, communications and electronic countermeasure systems. The trend towards increasingly complex antenna systems led to a requirement for a faster, more adaptable data acquisition system. To support the changing test requirements, AFRL developed a multi-port 16-RF-switch controller as part of our data acquisition system. In a typical antenna test at Newport, multiple aircraft antennas are multiplexed into a single RF output through a programmable matrix of solid state RF switches. The switch controller is installed inside the aircraft under test which is mounted on a 3-axis positioner. We can then configure and reconfigure each aircraft for a variety of antenna tests at Newport.
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