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Near Field

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.

Recent Advances in Anechoic Chamber Characterization using Spherical Near-Field Imaging
J. Mantovani,C. Sirles, R. Howland, November 2011

Anechoic chamber characterization typically requires measuring the level of extraneous signals within an arbitrarily defined quiet zone volume. From this data, measurement uncertainty due to the presence of extraneous signals can be quantified for various test scenarios. For the anechoic chamber designer, however, it is equally important to determine the magnitude and source of the extraneous signals so that they can be minimized or controlled. This paper discusses improvements in Spherical Near-Field Chamber Imaging as applied to anechoic chamber design and characterization. Measurement system improvements to improve image resolution are described. Data sampling requirements to eliminate processing artifacts is discussed. Critical sampling probe characteristics limiting UHF measurement capabilities are outlined. Test data on an outdoor range and on a large anechoic chamber are presented and discussed.

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.

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.

Probe Performance Limitation due to Excitation Errors in External Beam Forming Network
L. Foged,A. Giacomini, R. Morbidini, November 2011

New developments in ortho-mode junctions (OMJ) and probe technologies has enabled near field probes with up to 1:4 bandwidth, while maintaining the high performance standards of traditional narrow band probes [1–3]. The new probe technologies are based on inverted ridge structures providing four symmetrical feeding points for external balanced feeding. The inverted ridge structure stabilizes the frequency dependence of the OMJ while the external balanced feeding is a crucial feature to achieve the desired high performances. This paper briefly review the theory of balanced feeding and derive performance guide lines on the external beam forming network for achieving high port-to-port isolation and matching on a wide bandwidth with the inverted ridge probe technology. The relationship between excitation errors in the balanced feeding scheme and the spherical mode index µ.1 content of the probe is also investigated and upper bounds on acceptable excitation errors are derived.

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.

COMPARISON OF NEAR-FIELD MEASUREMENT METHODS AT NIST
Michael Francis,Douglas Tamura, Katherine MacReynolds, November 2010

ABSTRACT A comparison of the planar, spherical, and cylindrical near-field techniques was completed at the National Institute of Standards and Technology (NIST) for a Ku-band Cassegrain reflector antenna. This paper discusses measurement results and reports on the sources and magnitudes of the uncertainties for the three near-field techniques. We conclude that measurement results from the three different near-field methods agree within the expected uncertainties.

Some Detailed Implementation Aspects of an Automated Error Assessment Scheme for Antenna Spherical Near-Field Measurements
Patrick Pelland,Daniel Janse van Rensburg, Derek McNamara, Leili Shafai, Shantnu Mishra, Minya Gavrilovic, November 2010

This paper elaborates on certain aspects of a new measurement process that permits an assessment of spherical near-field (SNF) measurement errors based on a set of practical tests that can be done as part of any SNF measurement. It provides error bars for a measured radiation pattern in an automated fashion.

A Large Spherical Near-Field Arch Scanner for Characterizing Low Frequency Phased Arrays
Jeff Fordham, November 2010

The overall measurement system details are presented, along with mechanical accuracies achieved for the scanner system. Details of the chamber and host facility are described. Finally, the paper concludes with measurements of a UHF-band Standard Gain Horn using the system. The challenges and benefits of such a system will be highlighted.

Microwave Imaging System Incorporating an Array of Optically Modulated Probes for Rapid and Low-Perturbation Near-Field Measurements
Hamidreza Memarzadeh,Jean-Jacques Laurin, Raman Kashyap, November 2010

This communication addresses the design and implementation of a low-perturbation and high dynamic range near-field (NF) imager with increased measurement speed. The imager is equipped with an array of optically modulated scatterer (OMS) probes, each incorporating a commercial-off-the-shelf photodiode chip and a minimum scattering antenna, i.e. short dipole. In the OMS probes, transmission of modulating optical signals is performed using an optical fiber coupled to the photodiode, which is invisible to microwave signals. The imager measurement speed is also improved as the OMS array eliminates the delays associated with probes translations, in addition to fast switching of modulating light between the probes. Fast switching is accomplished by an array of fiber-pigtailed laser diodes. Improved dynamic range and linearity in the NF imager are achieved by adding a carrier canceller within the imager receiver front-end, eliminating the carrier signal and leaving the sidebands intact. This canceller also improves the isolation between input/output ports of the imager providing a potential for higher signal amplification. The performance assessment of the NF imager, including its linearity and result accuracy is made by comparisons with a known field distribution.

Near-Field Testing of Defocusing Methods for Phased-Array Antenna
Philip Brady,Derrick Mauney, November 2010

The Georgia Tech Research Institute (GTRI) analyzed a phased-array antenna for the purpose of testing phase-only defocusing methods. The array is defocused with the objective of broadening its beam at the cost of lower antenna gain. A design for the beam-steering computer is accomplished which adds the capability of focusing a beam, steering in azimuth and elevation, and performing beam defocusing using only element phase. Widening of the beam is accomplished using only 180° phase shifts in the elements, and it is compared with widening accomplished using gradual phase tapers. The antenna is measured in a near-field range to obtain amplitude and phase information as a function of each element in the array. Near-field testing of the antenna is also used to verify the capability of the beam-steering computer; two-dimensional antenna patterns and near-field hologram projections are compiled to prove this functionality. A software model is designed to mimic the behavior of the phased array antenna in its operational modes; it is also used to predict antenna gain and beamwidth prior to near-field testing. Measured and modeled antenna patterns are compared using focused and defocused modes. Metrics are performed on the near-field data to infer statistics of the individual phase shifters and on the computed far-field patterns to characterize the entire antenna. The defocusing methods under analysis are phase-only methods, due to the inability to control amplitude weighting of elements in this antenna. One method discussed uses only 180° shifting of elements in the antenna to achieve a desired beamwidth. This is compared with another method which gradually spoils the beam by applying a phase taper across the aperture. The results from near-field testing compare the defocusing methods and characterize the relationships between gain, beamwidth, and sidelobe levels for both defocusing methods.

EXONERATION OF PERFORMING TOTAL RCS MEASUREMENTS IN THE NEAR FIELD
Victorya Kobrinsky, November 2010

Very often far field conditions are violated at high frequencies RCS measurements and in real life scenarios. People go to great lengths to carry out these measurements in the far field. They make large investments to build suitable compact ranges, or long outdoor ranges. Others make extensive efforts to correct the near field measurements to the far field values. This paper suggests that those elaborate measures are superfluous, as far as the total RCS is concerned. Although near field measurements clip the high peaks, they broaden their shoulders compensating for the loss. Simulations and actual measurements show that the accumulative distribution of RCS values in the near field is equal or slightly higher than the distribution of these values in the far field, until one looks for very high 90th percentiles. Thus, for detection and survivability estimates the near field measurements provide a close upper bound.

A HIGH PERFORMANCE STATE OF THE ART PLANAR HYBRID SCANNER
Uri Shemer,Arnaud Gandois, November 2010

An Indian Defense Research and Development Organization (DRDO) laboratory has commissioned a state-of-the-art indoor far-field antenna test facility in 2009. This facility supports highly accurate measurement of a wide range of antenna types over 1.12–40 GHz. Owing to the heavy usage of this range, it was decided to enhance the existing facility to include a Hybrid Planar Near-Field facility for high speed accurate antenna measurements with minimal changes to the existing chamber configuration. The scanner is implemented as a highly innovative Hybrid T-type scanner, with a Y-axis that consists of a Linear Multi-Probe array and a traditional single probe configuration. The linear Multi-Probe array consists of two sets of dual polarized probes each one covering a sub set of the full frequency range. In particular one set covers the 1.0-6.0GHz band (operational from 400MHz) and the other set covers the 6.0 to 18.0GHz band. The traditional Single Probe configuration includes a set of Open Ended Waveguide Probes to facilitate an operational frequency range of 1.12 – 40.0GHz, as in the existing Far Field system. The Hybrid scanner is placed along the sidewall opposite the door, on the DUT positioner side. The major benefit of this layout is that there is no need to change the basic design of the chamber and it is built according to the original plans. When the chamber is used in the far-field mode, the tower is moved to the end of the horizontal axis in the direction of the corner of the chamber. The tower sides that face away from the chamber corner are covered with absorbing material to reduce reflections from the tower. Assuming that the chamber is intended to measure directional antennas, the existence of the tower behind and to the side of the AUT is expected to introduce minimal interference. The high speed linear Multi-Probe array has typical measurement speeds ranging between 5 and 15 minutes at 5 frequencies and 2 polarizations. Instrumentation is based on an Agilent PNA E8362B. Software is based on the MiDAS 6.0 package for both Single Probe & Multi Probe modes. A Real-Time Controller (RTC), accompanied by a 4-port RF switch, facilitates multi-port antenna measurements, with the possibility of interfacing to an active antenna.

Extension Of The Mathematical Absorber Reflection Suppression Technique To The Planar Near-Field Geometry
Stuart Gregson,Allen Newell, Greg Hindman, Michael Carey, November 2010

Obtaining a quantitative accuracy qualification is one of the primary concerns for any measurement technique [1, 2]. This is especially true for the case of near-field antenna measurements as these techniques consist of a significant degree of mathematical analysis. When undertaking this sort of examination, room scattering is typically found to be one of the most significant contributors to the overall error budget [1]. Previously, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used with considerable success in quantifying and subsequently suppressing range multi-path effects in first spherical [3, 4] and then, cylindrical near-field antenna measurement systems [5, 6]. This paper details a recent advance that, for the first time, enables the MARS technique to be successfully deployed to correct data taken using planar near-field antenna measurement systems. This paper provides an overview of the measurement and novel data transformation and post-processing chain. Preliminary results of computational electromagnetic simulation and actual range measurements are presented and discussed that illustrate the success of the technique.

Assessment of Irregular Sampling Near-Field Far-Field Transformation Employing Plane Wave Field Representation
Carsten Schmidt,Elankumaran Kaliyaperumal, Thomas Eibert, November 2010

Near-field antenna measurements are accurate and common techniques to determine the radiation pattern of an antenna under test. The minimum near-field sampling rate is dictated by the electrical size of the antenna and usually equidistant sampling is applied for planar, cylindrical, and spherical measurements. Certain applications either rely on or benefit from near-field sampling on irregular grids. To handle irregular measurement grids near-field transformation algorithms like equivalent current methods or the multilevel fast multipole accelerated plane wave based technique are required which do not rely on regularly sampled data. In this contribution the plane wave based near-field transformation is applied to spherical, cylindrical, and “combined” near-field measurements employing irregular sampling grids. The performance is assessed by various simulated near-field measurement scenarios.

Design and verification of Galileosat Ground Station P-band Antenna
Lars Foged,Alessandro Rosa, Andrzey Baranski, Luc Duchesne, Luciano Paiusco, Thierry Blin, Ulrich Grunert, November 2010

A ground station antenna for Galileosat application operating in right hand circular polarization at P-band has been designed, manufactured, and tested. Other than stringent environmental requirements for typical ground station antennas the specification call for an antenna with very stringent requirements on pattern shape and symmetry and a very severe control on side and back lobes. In order to ease the requirement on the antenna positioner the antenna should have very compact size and low weight. The final antenna consists of an array of 7 medium gain, dual linear polarized yagi elements as shown in Figure 1. This paper describes the antenna design trade-off activity including the selection of the most suited antenna technology and manufacturing details. It also reports on the testing in the SATIMO SG-64 multiprobe spherical near field test range with considerations on the associated measurement uncertainty. The final acceptance of the antenna was based on measurements performed in CNES and SATIMO.







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