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Far Field

Comparative Probe Parameter Error Analysis For Planar Near-Field Measurements With A Novel Approach For Reduced Probe-Aut Interaction
M. Ayyaz Qureshi, Carsten H. Schmidt, and Thomas F. Eibert, November 2012

Far-field uncertainty due to probe errors in planar near-field measurements is analyzed for the fast irregular antenna field transformation algorithm. Results are compared with the classical technique employing two dimensional Fast Fourier Transform (2D FFT). Errors involving probe's relative pattern, alignment, transverse and longitudinal position, interaction with AUT etc. have been considered for planar measurements. The multiple reflections error originating from the interaction of the probe and the AUT tends to deteriorate the radiation pattern to a greater extent. Therefore, a novel technique which utilizes near-field measurements on two partial planes is presented to reduce the multiple reflections between the probe and the AUT.

An Experimental Validation Of The Near-Field - Far-Field Transformation With Spherical Spiral Scan
F. D'Agostino , F. Ferrara , J.A. Fordham, C. Gennarelli, R. Guerriero, M. Migliozzi, November 2012

This work concerns the experimental validation of a probe compensated near-field – far-field transformation technique using a spherical spiral scanning, which allows one to significantly reduce the measurement time by means of continuous and synchronized movements of the positioning systems of the probe and antenna under test. Such a technique relies on the nonredundant sampling representations of the electromagnetic fields and makes use of a two-dimensional optimal sampling interpolation formula to recover the near-field data needed to perform the classical spherical near-field – far-field transformation. The good agreement between the so reconstructed far-field patterns and those obtained via the classical spherical near-field – far-field transformation assesses the effectiveness of the approach.

Exact Solutions In Antenna Holography Using Planar, Spherical, Or Cylindrical Near-Field Data
George G. Cheng, Yong Zhu, and Jan Grzesik, November 2012

We present exact solutions to antenna holography problems based on planar, spherical, or cylindrical nearfield data. Full field distribution information in the source region is determined exactly, from two tangential field components over a planar, spherical, or cylindrical surface. Stated in so many words, all three components of both electric and magnetic fields in the antenna aperture are obtained exactly from two-component near-field data. Conventional antenna holography relies upon back transformation for planar near-field data, and upon optimization schemes for both spherical and cylindrical near-field data. It is both acknowledged and accepted that the back transform is only an approximate solution due to its far-field nature, whereas optimization algorithms are vulnerable to convergence instability and, moreover, are computationally intensive. Our approach tackles holography by solving an inverse scattering problem, with exact solutions derived on the basis of three common types of near-field data. A mapping algorithm is proposed herein which determines the field everywhere, in both interior and exterior regions, based on a single-slice nearfield data capture. It provides exact antenna holography solutions analytically, with the full electric and magnetic fields disclosed throughout the source region. The field mapping algorithm is a direct, closed-form solution which is numerically straightforward and efficient. Verification is carried out and demonstrated by analytic examples and numerical simulations, as well as by hardware measurements. Nine test examples are given. Analytic examples include dipole arrays deployed across planar, spherical, and cylindrical regions, and a narrow azimuthal slot on a conducting sphere. The simulation example exposes the structure of a slotted array antenna based upon its near-field data as generated by a commercial software package. The hardware measurements address themselves to a concrete embodiment of that same slotted array antenna, an elongated sector antenna, and to a patch antenna. Excellent agreement is found in all test cases.

Outdoor Far-Field Antenna Measurements System For Testing Of Large Vehicles
Doug Kremer, Alan Morris, Rachel Blake, Todd Park, John Proctor, November 2012

The Electronic Proving Ground's Antenna Test Facility at Fort Huachuca, Arizona has some of the most interesting testing structures in the world. These structures include a wooden Arc measurements system with a 23 m radius, a 30 m tower, and a compact range with an 18 m quiet zone. All of these structures are outdoors and support testing from UHF to mm frequencies on antenna systems mounted on large land and air vehicles. This paper describes the ranges supported by these structures (some of which were built in the late 1960’s) and the efforts made to keep these ranges current. This paper also describes an economical approach to arc range design which moves the arc instead of the vehicles. This paper discusses plans to build one of these systems outdoors at EPG within a limited budget.

Outdoor Far-Field Antenna Measurements System For Testing Of Large Vehicles
Doug Kremer, Alan Morris, Rachel Blake, Todd Park, John Proctor, November 2012

The Electronic Proving Ground's Antenna Test Facility at Fort Huachuca, Arizona has some of the most interesting testing structures in the world. These structures include a wooden Arc measurements system with a 23 m radius, a 30 m tower, and a compact range with an 18 m quiet zone. All of these structures are outdoors and support testing from UHF to mm frequencies on antenna systems mounted on large land and air vehicles. This paper describes the ranges supported by these structures (some of which were built in the late 1960’s) and the efforts made to keep these ranges current. This paper also describes an economical approach to arc range design which moves the arc instead of the vehicles. This paper discusses plans to build one of these systems outdoors at EPG within a limited budget.

On The Development Of 18-45 Ghz Antennas For Towed Decoys And Suitability Thereof For Far-Field And Near-Field Measurements
Matthew Radway, Nathan Sutton, Dejan Filipovic, Stuart Gregson, Kim Hassett, November 2012

The development of a wideband, high-power capable 18-45 GHz quad-ridge horn antenna for a small towed decoy platform is discussed. Similarity between the system-driven antenna specifications and typical requirements for gain and probe standards in antenna measurements (that is, mechanical rigidity, null-free forward-hemisphere patterns, wide bandwidth, impedance match, polarization purity) is used to assess the quad-ridge horn as an alternative probe antenna to the typical open-ended rectangular waveguide probe for measurements of broadband, broad-beam antennas. Suitability for the spherical near-field measurements is evaluated through the finite element-based full-wave simulations and measurements using the in-house NSI 700S-30 system. Comparison with the near-field measurements using standard rectangular waveguide probes operating in 18-26.5 GHz, 26.5-40 GHz, and 33-50 GHz ranges is used to evaluate the quality of the data obtained (both amplitude and phase) as well as the overall time and labor needed to complete the measurements. It is found that, for AUTs subtending a sufficiently small solid angle of the probe’s field of view, the discussed antenna represents an alternative to typical OEWG probes for 18-45 GHz measurements.

Controlling the Far-Field Resolution in Near-Field Antenna Characterization
A. Capozzoli,C. Curcioi, A. Liseno, November 2011
Planar Near-Field Measurement Error Analysis for Multilevel Plane Wave Based Near-Field Far-Field Transformation
M.A. Qureshi,C. Schmidt, E. Thomas, November 2011

This paper describes the behavior of the antenna radiation pattern for different planar near-field measurement errors superimposed on the near-field data. The disturbed radiating near field is processed using multilevel plane wave based near-field far-field transformation to determine the far-field. Errors like scan area truncation, transverse and longitudinal position inaccuracy of measurement points, and irregular sample spacing are analyzed for an electrically large parabolic reflector at 40 GHz. The error behavior is then compared with the standard transformation technique employing 2D Fast Fourier Transform (FFT) using the same near-field data. In order to exclude the effect of any other measurement or environmental error, electric dipoles with appropriate magnitude profile and geometrical arrangement are used to model the test antenna.

Large Size, Light Weight, Broadband RF Lens for Far-Field Antenna Measurement
L. Matytsine,P. Lagoiski, S. Matitsine, November 2011

Large size, light weight, broadband convex RF lens was developed to meet far-field requirements for antenna measurements. The Lens was fabricated from low loss, low density meta-materials and has diameter of D=2 m, focusing distance 2.4m and weight of just 50 kg with operational frequency 0.8 to 6 GHz. The lens is able to produce a plane-wave zone with an approximate size of 0.7D, allowing a 2m diameter lens to test antennas up to 1.4m in relatively small anechoic chamber. Another possible application of large size, lightweight RF lens is RCS measurements that include bi-static measurements. Results of quiet zone measurements for different frequencies are presented.

Evaluation of Multilevel Plane Wave Based Near-Field Far-Field Transformation Employing Adaptive Field Translations
C. Schmidt,T. Eibert, November 2011

The radiation pattern of an antenna can be deter­mined accurately by near-field measurement and transformation techniques. Low numerical complexi­ty, full probe correction capabilities, and high accura­cy of the transformed far-field pattern are important features of near-field transformation algorithms. The multilevel plane wave based near-field transformation algorithm achieves an efficient full probe correction by plane wave representations of antenna and field probe and realizes the low numerical complexity by hierarchical grouping of measurement points. Field translations are carried out to the boxes on the coars­est level and are further processed to the measure­ment points by disaggregation and anterpolation. Dis­aggregation is a simple phase shift of the plane waves and anterpolation reduces the sampling rate corre­sponding to the spectral content of the plane wave spectra on the various levels. The accuracy of the transformation is influenced by several variables where the number of buffer boxes between antenna and measurement point groups is crucial. A higher accuracy due to more buffer boxes can be achieved at the cost of increased computation time. Adaptive field translations structure the measurement setup such that individual groups are transformed with the re­quired accuracy at lowest costs. A detailed investiga­tion for a planar near-field measurement will be shown.

OEWG Probe Pattern Comparisons between NPL Measurements, EM-Model and Analytical Model
C. Dempsey, November 2011

This paper compares 3 sets of far-field patterns of an S-Band Open Ended Waveguide (OEWG). The sources for the data are measurements from NPL, an EM-Model and the commonly used NIST analytical model. Both co-polarized (co-pol) and cross-polarized (x-pol) patterns are compared. Results indicate that accuracy improvements are possible by utilizing an EM-Model in certain applications. These applications as well as the pros and cons of doing this are discussed. Understanding the differences between these 3 independent sets of data enables near-field range engineers to better understand the directional dependence of probe correction accuracies over the majority of the forward hemisphere. Information and insight gained from this comparison, along with specific AUT requirements, better equips the near-field range user to address probe correction concerns and ultimately to determine if a calibrated probe solution is required for their unique testing scenario.

On-Axis Field Intensities of Circular Aperture Antennas in the Near and Far Field
M. Paquay, November 2011

When specifying a Near Field scanner, intended to measure radiating systems under operational conditions, one of the requirements is the power flux density that the Near Field system and the absorbers on it have to with-stand. Today's trend is to use an EM-solver to calculate field intensities of (aperture) antennas. The advantage of these solvers is that they can handle any geometry but the disadvantages are that they can only handle limited dimensions and use approximations. Analytical solutions are not only more elegant and accurate but they also provide insight in the field behavior. For symmetrical cases, it is clear that the maximum field intensities will appear on the symmetry axis. The only (nearly identical) expressions in the literature are from Rudduck and Chen [1] and Yaghjian [2]. These analytical expressions describe the on-axis electrical field intensity of a circular aperture with uniform illumination. Rudduck and Chen have derived their equation via a Plane Wave Spectrum approach. Unfortunately, Yaghjian provided this version without reference or background about the derivation. It turns out that the expressions of [1,2] need a (minor) correction. Besides that, uniform illumination is not a very realistic case. This paper will also present an analytical expression for a tapered illumination. Graphs will be provided of the equation of [1,2], the corrected formula for the uniform illumination case and the new expression for the tapered illumination case.

Optimization of an Array to Create a Plane Wave in a Chamber with Partially Reflective Walls
E. Walton,J. Holderle, November 2011

Far field measurements of ground vehicle antennas in anechoic chambers often require the creation of a plane wave by near field hemispherical probing with associated mathematical transformations to the far field/plane wave result. Direct far field measurements can be done to save time when the frequency is low enough. This paper discusses a method of extending the frequency band where direct measurements can be done by synthesizing a plane wave using a small array of antennas. The use of an array to create a plane wave in an anechoic chamber usually results in errors due to the reflections from the walls of the chamber. The technique to be described in this paper is to model the wall reflections and the array antenna characteristics and to use optimization techniques to derive an antenna placement and power distribution scheme to optimize the plane wave. Several optimization techniques will be described and results from testing in a 1.2 meter long sub-scale chamber model will be shown. Improvements in the far field measurements will be discussed.

Laboratory Tests on the Direct Cylindrical NF-FF Transformation Using an Effective Modelling for Long AUTs
F. D'Agostino,F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, J. Fordham, November 2011

ABSTRACT This work deals with the experimental validation of a direct near-field – far-field transformation with cylin­drical scanning for electrically long antennas, which requires a minimum number of measurements. Such a transformation is based on a nonredundant sampling representation making use of a flexible source model-ling suitable to deal with electrically long antennas and allows the evaluation of the antenna far field di­rectly from the acquired near-field data without inter­polating them. The good agreement between the so recovered far-field patterns and those obtained via the classical cylindrical near-field – far-field transforma­tion assesses the effectiveness of the approach.

Laboratory Tests on the Direct Cylindrical NF-FF Transformation Using an Effective Modelling for Long AUTs
F. D'Agostino,F. Ferrara, C. Gennarelli, R. Guerriero, M. Migliozzi, J. Fordham, November 2011

ABSTRACT This work deals with the experimental validation of a direct near-field – far-field transformation with cylin­drical scanning for electrically long antennas, which requires a minimum number of measurements. Such a transformation is based on a nonredundant sampling representation making use of a flexible source model-ling suitable to deal with electrically long antennas and allows the evaluation of the antenna far field di­rectly from the acquired near-field data without inter­polating them. The good agreement between the so recovered far-field patterns and those obtained via the classical cylindrical near-field – far-field transforma­tion assesses the effectiveness of the approach.

Application of Mathematical Absorber Reflection Suppression to Direct Far-Field Antenna Range Measurements
S. Gregson,B. Williams, G. Masters, A. Newell, G. Hindman, November 2011

Mathematical Absorber Reflection Suppression (MARS) has been used successfully to identify and extract range multi-path effects in a great many spherical [1, 2], cylindrical [3, 4], and planar [5, 6] near-field antenna measurement systems. This paper details a recent advance that enables the MARS measurement and post-processing technique to be used to correct antenna pattern data from far-field or compact antenna test ranges (CATRs) where only a single great circle pattern cut is taken. This paper provides an overview of the measurement and novel data transformation and post-processing chain that is utilised to efficiently correct far-field, frequency domain antenna pattern data. Preliminary results of range measurements that illustrate the success of the technique are presented and discussed.

Cubical Surface Scanning for Near-Field Antenna Measurements Using Spherical Wave Expansion
A. Khatun,T. Laitinen, P. Vainikainen, November 2011

In this work we study the near-field antenna measurement using cubical surface scanning and related near-field to far-field (NF-FF) transformations. The cubical surface scanning is a fascinating idea because it can be realized using widely used planar scanning on six surfaces of a cube, and it provides the possibility to determine the complete 3-D pattern instead of the pattern in a limited angular region as in traditional planar scanning. The NF-FF transformation presented in this paper is based on spherical vector wave expansion (SWE). The most important issue of this paper is to introduce the azimuthal mode decomposition technique to be applied as a part of the NF-FF transformation allowing a reduction in the computational burden of the transformation.

Advances In Planar Mathematical Absorber Reflection Suppression
S. Gregson,A. Newell, G. Hindman, November 2011

When making antenna measurements, great care must be taken in order to obtain high quality data. This is especially true for near-field antenna measurements as a significant amount of mathematical post-processing is required in order that useful far-field data can be determined. However, it is often found that the integrity of these measurements can be compromised in a large part through range reflections, i.e. multipath [1]. For some time a technique named Mathematical Absorber Reflection Suppression (MARS) has been used to reduce range multi-path effects within spherical [2, 3], cylindrical [4, 5] and most recently planar [6, 7] near-field antenna measurement systems. This paper presents the results of a recent test campaign which yields further verification of the effectiveness of the technique together with a reformulation of the post-processing algorithm which, for the first time, utilises a rigorous spherical wave expansion based orthogonalisation and filtering technique.

Evaluating the Time Domain Performance of Spiral Antennas Using Near Field Measurements
M. Elmansouri,M. Radway, D. Filipovic, November 2011

Ultra wideband (UWB) systems use short pulses in order to achieve high data rate wireless communications and/or radar resolution. Thus, UWB antennas should be designed carefully, both in time and frequency domains, with the system performance in mind. Time domain characterization of an antenna can be performed first by measuring the frequency domain transfer function of a direct link consisting of two identical antennas. Then, the time domain response is obtained by post processing the frequency domain data using the Inverse Fast Fourier Transform (IFFT). This paper discusses frequency and time domain performance of four-arm equiangular and Archimedean spiral antennas operating in mode 2. The frequency domain transfer function is synthesized using complex far field information measured in a spherical near-field chamber from 2GHz to 12GHz. The synthesized approach is validated using simulation and direct link measurements. The quality of radiated pulses is evaluated in terms of fidelity factor over a full field of view, a task not trivial for the direct link measurements.

Beam-Steering Computer Design for Space-Fed Phased-Array Antenna
P. Brady,D. Mauney, J. Skala, November 2011

In this paper, a beam-steering computer design is explored for a large space-fed phased-array antenna. GTRI previously developed a beam-steering computer for a smaller phased-array antenna which accomplished spherical propagation focusing and multiple phase-only beam-broadening modes. In a subsequent effort, the beam-steering computer design was scaled for a large phased-array antenna to accomplish similar tasks. To verify the design, a series of far-field measurements was initiated to characterize the performance of the antenna by comparing with past measured near-field data and modeled results. One of the primary responsibilities of the beam-steering computer was the focusing of the spherical propagation wave front. A measurement technique is discussed to accomplish this focusing for the large space-fed phased-array antenna by correcting measurement errors in the spherical propagation routine of the beam-steering computer. Additional patterns were taken using the updated feed horn focal point for spherical propagation correction. By correcting the phase errors caused by spherical propagation defocusing in the original beam-steering computer, significantly better antenna performance was obtained, including higher peak gain, reduced nearby sidelobe levels, and removal of beam-pointing errors. Another important responsibility of the beam-steering computer was the ability to realize multiple antenna modes, including a focused pencil beam as well as defocused broadened-beam modes. A stochastic gradient descent algorithm was utilized to obtain several phase tapers to accomplish beam-broadening for the antenna modes. These modes were implemented in the beam-steering computer and tested on a far-field range. The antenna patterns were compared with modeled results and with previous measured data to ensure validity of the implementation.







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