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Far Field
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.
Accuracy of Near Field Pattern Measurements Performed with Analytical Probe Models
F. Boldissar,A. Haile, November 2011
Calibration of probes for planer near field range measurements is generally required to obtain accurate cross-polarization (xpol) data; however, probe calibration is costly and time consuming. Using analytical models in place of calibration is generally much more cost effective, but may result in larger measurement errors. In a previous paper [1], we showed that simple models of copol probe patterns with zero xpol can give accurate measured results, provided that the probe xpol is much better, generally 10-15 dB better, than the Antenna Under Test (AUT). The next question is “Can a lower performing (and cheaper) probe be used if both the copol and xpol probe patterns are modeled?” In this paper, we compute AUT xpol measurement errors that result from probe xpol errors, and we compare far field AUT patterns processed using probe models with patterns processed with calibrated probe files.
Spherical Near-Field Measurements at UHF Frequencies with Complete Uncertainty Analysis
A. Newell,P. Pelland, B. Park, T. White, November 2011
A spherical near-field measurement range at Nearfield Systems Inc. has recently been used to measure gain, pattern and polarization of a multi-element helix array operating in the UHF band. Verification of gain performance over the operating band was of primary importance and so major efforts were made to obtain the best possible gain results and to quantify the gain uncertainty through a complete error analysis. A single element helix gain standard was first calibrated and the estimated uncertainty in this calibration was 0.35 dB. A double ridged horn was to be used as the probe for the spherical near-field measurements and so the patterns of the horn at all test frequencies were measured on the spherical range using identical horns as the AUT and the probe. From these measurements, probe pattern files were generated that could be used to perform the probe correction in the measurements of the helix gain standard and the multi-element array. The helix gain standard was then installed in a new spherical near-field range at NSI with the double ridged horn as the probe. The range used a specially designed phi-over theta rotator that could support and rotate the array and maintain the required position accuracy. The chamber was lined with 36 inch absorber. Spherical measurements were then performed and the data processed to provide the far-field peak amplitudes at each frequency that were necessary for gain measurements. The far-field peak values are equivalent to the far electric field for the gain standard and are compared to the same parameter for the multi-element array to produce the final gain results. The helix array was then installed in the spherical range and a series of measurements were performed to produce the far-field gain, pattern and polarization results and also to provide the data for the complete 18 term uncertainty analysis. The uncertainty in the gain measurements was 0.45 dB and the axial ratio uncertainty was 0.11 dB.
An Empirical Near-field to Far-Field Convergence Study for Antenna Measurements
P. Nelson,J. Henrie, November 2011
Pattern distortion due to finite range measurement of antennas in close proximity to electrically large metallic media is examined. The ubiquitous yet arbitrary 2D2/. distance requirement cannot be blindly applied to scenarios where antennas couple to nearby structures. A C-band standard gain horn antenna is analyzed near a circular metallic plate at 6 GHz using the commercial software FEKO. The near-fields are computed at various radii, which are set to multiples of D2/., where D is defined as the largest dimension of the complete structure. The radiating near-field patterns are normalized and compared to the far-field pattern. Results indicate that measurement at 2D2/. may not be necessary. Increasing fractions of D2/. results in a diminishing measurement error that may be tolerable, depending on the intended application.
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.


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