AMTA Paper Archive

 

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Far Field
General Technique of Antenna Phase Center Determination by NF or FF Measurements
D. Asatryan, November 2004
A problem of determination of an antenna phase center (PhC) usually is solved by different ways from a theoretical calculation to the near-field measurements of complex characteristics in the aperture of an antenna or the far-field measurements of the radiation-pattern phase. The present paper is devoted to a general technique of an antenna PhC determination by use of the known (or the measured) distribution of the complex characteristics in the antenna near zone or the phase pattern in the far zone. An algorithm of determination of the phase pattern evolute, based on the lowest moments of distribution, as well as a criterion for PhC existence, which is independent on the observation angle, are offered. A simple expression of PhC for an antenna with a quadratic phase distribution in the aperture is obtained. An error of PhC determination depending on both the error of observation angle and the error of measurement of the phase pattern is considered.
On the Number of Modes in Spherical Expansions
F. Jensen, November 2004
Since the early days of spherical near-field far-field transformations a recommendation for the necessary number of polar modes has been given by , being the wavenumber and or the radius of the minimum sphere. The almost explosive development in computer speed and storage capacity witnessed during the last two decades has made trans-formations of fields from antennas exceeding thou-sands of wavelengths feasible, and a closer investiga-tion of the above expression seems to be appropriate. An improved expression for the number of modes, N, related to the antenna size and the required accuracy will be developed. The impact of truncation of the modal expansion at a given level will be illustrated. This is especially important for measurements where noise is present, or where there is undesirable scatter-ing from objects.
An Automated Cylindrical Near-Field Measurement and Analysis System for Radome Characterization
M. Giles,S. Mishra, November 2004
The David Florida Laboratory (DFL) was contacted by the Canadian Department of National Defense (DND) to develop an accurate, reliable, more cost effective method of characterizing existing nose cone mounted radomes for the radar systems aboard aircraft such as CF-18. Traditionally, these measurements have been performed in a far-field (FF) [1] range using conventional positioning and measurement systems and specialized instruments such as a null seeker. Recently, the use of near field methods has been incorporated in radome measurement practices [2]. This paper describes one such adaptation of a cylindrical near-field facility (CNF) for radome measurements.
Near-Field Remote Calibration System with Minimal Sampling For Operationally Large Reflectors
W. Lippincott,M. Lundmark, R. Eisinger, T. Gutwein, November 2004
Accurate near-field calibration of a large 60 ft. diameter reflector can be accomplished with a minimal sampling technique. Near-field amplitude and phase is collected as the reflector scans across a receiving calibration tower. The near-field data is then transformed to a far-field pattern using a Fourier transform technique. Information on far-field EIRP, directivity, pointing, axial ratio and tilt, as well as encoder timing is obtained with accuracies comparable to anechoic chamber measurement techniques. The system was analyzed for sampling and multipath effects, as well as the effects of phase and amplitude stability. A spherical wave expansion technique was compared to a straight-forward summation technique for the Fourier transform.
A Magnitude and Phase Near-field Measurement Technique for Digital Circuits Emissions
P. Barriere,J. Laurin, Y. Goussard, November 2004
Abstract — A technique for near-field measurements over a digital PCB is presented. Phase measurement with a vectorial network analyzer (VNA) is possible with antennas but it is in principle not possible when the DUT has its own free-running oscillator. In order to get around this problem, a two-probe approach is proposed. While one of the probes is mobile the other one is fixed and collect the reference signal. An analogue circuit must be used to obtain the specifications in power and phase of the reference signal of the VNA. The collected near field above the test circuit allows us to clearly identify the hot spots and the constant phase areas. These results could be used to find problematic spots on a board or to extrapolate the far-field. This is of practical interest in EMC testing of digital devices.
Estimating Multiple Reflection Uncertainties in Spherical Near-Field Measurements
M. Francis,J. Guerrieri, K. MacReynolds, R. Wittmann, November 2004
We propose a simple method for estimating uncertainties due to multiple reflections between the test antenna and probe in near- field spherical-scanning measurements. To estimate uncertainties in far-field parameters, we measure the test antenna by scanning the probe over two spheres whose radii differ by a quarter wavelength (?/4). We compare this estimate to that obtained with a reduced data set (containing all values of ? but only a few values of f). In our example, we find that measuring only two f cuts suffices to obtain RMS uncertainties within 1 dB of those obtained using full-sphere data.
Using a Chirp Z-transform on Planar Near-Field Data to Expand a Portion of the Far-Field with Increased Resolution and No Interpolation
D. Thompson, November 2005
This paper describes the use of a two-dimensional chirp z-transform (2D-CZT) to efficiently concentrate a large number of sample points in a single portion of the far zone without interpolation. This work presents the equivalence of transforms calculated from measured near-field data using both the 2D-CZT and 2D-fast Fourier transform (FFT). The paper also shows that the 2D-CZT is computationally more efficient than a zero-padded FFT when one requires a high resolution over a small area of the pattern.
Development of a Hemispherical Near-Field Range with a Realistic Ground - Part 2
E. Walton,C. Buxton, G.F. Paynter, J. Snow, T-H. Lee, November 2005
This paper will discuss the development of a VHF/UHF near field test range for the case where there are reflections from a realistic ground surface. We will show the results of a direct computation algorithm where a far field pattern is computed using plane wave synthesis. The performance of a C++ program that implements this algorithm will be discussed.
Efficient Near-Field to Far-Field Transformation on Strategic Scanning Geometries
S. Costanzo,G. Di Massa, November 2005
Direct far-field transformation is developed from bi-polar near-field samples. As compared to conventional interpolation and expansion methods, a significant reduction in the computation time is obtained by the efficient use of the Fast Fourier Transform and the related shift theorem. Numerical simulations on array of Huyghens sources are considered as validations.
Comparison of Gains Determined from the Extrapolation and Pattern Integration Methods
M. Francis,J. Guerrieri, K. MacReynolds, November 2005
Abstract. Scientists at the National Institute of Standards and Technology (NIST) have measured the gain of several antennas using two different methods. The first method is the three-antenna extrapolation method developed at NIST in the early 1970s. The second method is the far-field pattern integration method. We compare gain results and gain uncertainties for several antennas using these two methods.
Measurements of the CloudSat Collimating Antenna Assembly Experiences at 94 GHz on Two Antenna Ranges
J. Harrell,A. Prata, C. Lee-Yow, C. Stubenrauch, L.R. Amaro, R. Beckon, T.A. Cariveau, November 2005
This paper presents measurements of the CloudSat Collimating Antenna (CA) as fabricated for the 94.05 GHz CloudSat radar, which is to be used to measure moisture profiles in the atmosphere. The CloudSat CA is a 3 reflector system consisting of the 3 "final" (relative to the transmitted energy) reflecting surfaces of the CloudSat instrument. This assembly was fed by a horn designed to approximate the illumination from a Quasi-Optical Transmission Line (QOTL). This same horn was employed as a "standard" for measurement of the CA gain via substitution, and its patterns were also measured (this substitution represents a departure from the standard insertion loss technique in the near field range). The CloudSat CA presented a substantial measurement challenge because of the frequency and the electrical size of the aperture is approximately 600 wavelengths in diameter, with a nominal beamwidth of 0.11 degrees. In addition, very high accuracy was needed to characterize the lower sidelobe levels of this antenna. The CA measurements were performed on a 3122-ft outdoor range (this distance was 41% of the far field requirement), which were immediately followed by measurements in an indoor cylindrical Near Field (NF) range. The instrumentation challenges, electrical, mechanical, and environmental are described. Comparison of the outdoor vs. indoor pattern data is presented, as well as the effect of the application of tie-scans to the near field data.
A High Performance Combined NF-FF Antenna Test Facility
U. Shemer,C.T. Tong, November 2005
DSO National Laboratories (DSO) has commissioned a state-of-the-art combined near-field and far-field antenna test facility in 2004. This facility supports highly accurate measurement of a wide range of antenna types over 1–18 GHz. The overall system accuracy allows for future extensions to 40GHz and higher. The 11.0m x 5.5m x 4.0m (L x W x H) shielded facility houses the anechoic chamber and the control room. As the proffered location for this indoor facility is on top of an existing complex instead of the ground floor, antenna pick­up is facilitated by a specialized loading platform accompanied by a heavy-duty state of the art fully automated 2.0m x 3.0m (W x H) sliding door, as well as an overhead crane that spans the entire chamber width. Absorber layout comprises 8-inch, 12-inch, 18-inch and 24-inch pyramidal absorbers. The positioning system is a heavy-duty high precision 3.6m x 2.9m (W x H) T-type planar scanner and AUT positioner. The AUT positioner system is configured as roll over upper slide over azimuth over lower slide system. This positioning system configuration allows for planar, cylindrical and spherical near-field measurements. A rapidly rotating roll positioner is mounted on a specialized alignment fixture behind the scanner to facilitate far-field measurements. Instrumentation is based on an Agilent PNA E8362B. Software is based on the MiDAS 4.0 package. A Real-Time Controller (RTC), accompanied by an 8-port RF switch, facilitates multi-port antenna measurements, with the possibility of interfacing to an active antenna.
A Simple Probe Calibration Method of a New Compact Spherical Near-Field Measurement System for Antennas from 1 GHz to 10 GHz
M. Hirose,K. Komiyama, S. Kurokawa, November 2005
ABSTRACT We have developed a new compact spherical near-field measurement system using a photonic sensor as a probe and successfully measured the 3D antenna patterns of a double-ridged horn antenna from 1 GHz to 10 GHz. This system consists of a compact spherical scanner and a photonic sensor that is used for the probe of the spherical near-field measurements. In our system, only one probe can be used for the wide frequency range measurements and the probe compensation is not needed in the measurements. For the system, we propose a simple calibration method using a double-ridged horn antenna for our system. We calibrate the system by measuring the double-ridged horn antenna on the reasonable assumption that the antenna efficiency is 100 %. Comparing the absolute gain obtained by the proposed calibration method with the one decided by using three-antenna method at far-field range, we show that the agreement is good within 1 dB over the whole frequency range.
An Apparent Discrepancy Between Impedance Mismatch Factors for Near-Field and Far-Field Measurements
D. Hess, November 2005
In making accurate measurements of antenna gain one must correct for the impedance mismatches between (1) the signal generator and transmitting antenna, (2) between the receiving power sensor and the receiving antenna and (3) between the signal generator and receiving power sensor. This is true for both far-field gain measurements and near-field gain measurements. It has recently come to our attention that there is a lack of clarity as to the form the mismatch factor should take when correcting near-field measured data. We show that a different form of impedance mismatch factor is to be used with the voltage domain equations of near-field than has been used with the power domain Friis transmission equation.
Spherical Near-Field Arch Range Upgrade
j. Aubin,A. Kipple, C. Arnold, J. Puri, November 2005
An upgrade to the large 75 foot radius spherical arch range at the U.S. Army Electronic Proving Ground at Ft. Huachuca, AZ has presented a complex design challenge in order to accommodate multiple test requirements, including both far-field and near-field measurements, as well as antenna under test (AUT) mode switching, over a wide frequency range. The range features a 60 foot diameter turntable (capable of supporting 80 tons) for azimuth positioning of large vehicles. The large arch/turntable positioning system combination presents a number of design issues in the implementation of a high performance, wideband RF subsystem. In addition, a significant requirement for this range is to allow either the probe mounted on the arch or the AUT mounted on the vehicle to transmit. The RF subsystem design utilizes the Agilent PNA in conjunction with the Agilent 85310 Distributed Downconverter system. Location of all the primary RF components are key issues in achieving sufficient transmit power, LO power, and receive sensitivity. Moreover, the selection and placement of the long RF cable runs has a significant impact on system level performance, and required thorough investigation. A unique utilization of available synthesizers provides a compact physical configuration and also provides an increase in speed over other multiple source configurations. This paper examines the design considerations for the RF subsystem and the configuration for achieving both near-field and far-field measurements for the case of the AUT transmitting as well as receiving.
A Linear Measurement System for Large Array Antennas
J.L. Besada,C. Martinez, F. Martin, M. Calvo, M. Sierra-Castaner, November 2005
A system for measuring large linear arrays of antennas has been developed, fabricated and tested. The system consists on a 12 meters structure where the antenna under test (a L band array of dipoles in this case) is positioned. The measurement probe (another dipole) moves on a linear slide and stops in front of each element of the array to acquire the electric field. All the system is installed on an semi-anechoic chamber, that can be lifted (with two synchronized stepped motors). This semi-anechoic chamber covers the top and side parts of the structure. The bottom part consists on a metallic reflector, that controls the reflections from each antenna element. Once the data is acquired, the data are processed to obtain the far field patterns and parameters of the antenna array (element amplitude and phase, beam width, side level, beam pointing …) All the results are presented in a windows environment, and all the system is integrated in a friendly user interface.
An Improved Version of the Circular Near-Field to Far-Field Transformation (CNFFFT)
I. LaHaie,C. Coleman, S. Rice, November 2005
For many years now, GDAIS has described the devel­opment, characterization, and performance of an image-based circular near field-to-far field transformation (CNFFFT) for predicting far-field radar cross-section (RCS) from near-field measurements collected on a cir­cular path around the target. In this paper, we present an improved version of the algorithm that avoids a sta­tionary phase approximation inherent in earlier ver­sions of the technique. The improvement is realized by modifying the range-domain weighting used to imple­ment the frequency derivative in the existing method. A similar modification was presented in the context of lin­ear near-field measurements in an earlier AMTA paper. Numerical simulations are presented that demonstrate the improvement afforded by the technique in predict­ing far-field RCS patterns from near-field data collected using typical bandwidths and standoff distances. An additional benefit of the revised algorithm is that it readily admits a formulation that includes antenna pat­tern compensation, as described in a companion paper.
An Improved Version of the Circular Near-Field to Far-Field Transformation (CNFFFT)
I. LaHaie,C. Coleman, S. Rice, November 2005
For many years now, GDAIS has described the devel­opment, characterization, and performance of an image-based circular near field-to-far field transformation (CNFFFT) for predicting far-field radar cross-section (RCS) from near-field measurements collected on a cir­cular path around the target. In this paper, we present an improved version of the algorithm that avoids a sta­tionary phase approximation inherent in earlier ver­sions of the technique. The improvement is realized by modifying the range-domain weighting used to imple­ment the frequency derivative in the existing method. A similar modification was presented in the context of lin­ear near-field measurements in an earlier AMTA paper. Numerical simulations are presented that demonstrate the improvement afforded by the technique in predict­ing far-field RCS patterns from near-field data collected using typical bandwidths and standoff distances. An additional benefit of the revised algorithm is that it readily admits a formulation that includes antenna pat­tern compensation, as described in a companion paper.
Antenna Pattern Correction for the Circular Near Field-to-Far Field Transformation (CNFFFT)
I. LaHaie,C. Coleman, S. Rice, November 2005
In previous work [1], we presented an antenna pattern compensation technique for linearly-scanned near field measurements. In this paper, we present a similar tech­nique to mitigate the errors from uncompensated azi­muthal antenna pattern effects in circular near-field monostatic radar measurements. The antenna pattern co mpensation is implemented as part of an improved algorithm for transforming the near-field measurements to the far-field RCS. A description of this improved circular near field-to-far field transformation CNFFFT technique for isotropic antennas is presented in a com­panion paper [2]. In this paper, we formulate the near-field signal model in the presence of an azimuthal an­tenna pattern under the same scattering approximation used in the isotropic CNFFFT. Using this model, we derive a modified version of the CNFFFT that includes antenna pattern compensation. Numerical simulations are presented that demonstrate the ability of the tech­nique to remove antenna pattern errors and improve the accuracy of the far field RCS patterns and sector statistics.
Antenna Pattern Correction for the Circular Near Field-to-Far Field Transformation (CNFFFT)
I. LaHaie,C. Coleman, S. Rice, November 2005
In previous work [1], we presented an antenna pattern compensation technique for linearly-scanned near field measurements. In this paper, we present a similar tech­nique to mitigate the errors from uncompensated azi­muthal antenna pattern effects in circular near-field monostatic radar measurements. The antenna pattern co mpensation is implemented as part of an improved algorithm for transforming the near-field measurements to the far-field RCS. A description of this improved circular near field-to-far field transformation CNFFFT technique for isotropic antennas is presented in a com­panion paper [2]. In this paper, we formulate the near-field signal model in the presence of an azimuthal an­tenna pattern under the same scattering approximation used in the isotropic CNFFFT. Using this model, we derive a modified version of the CNFFFT that includes antenna pattern compensation. Numerical simulations are presented that demonstrate the ability of the tech­nique to remove antenna pattern errors and improve the accuracy of the far field RCS patterns and sector statistics.


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