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Far Field
Polarization extraction of circularly polarized antennas
F. Colomb,J. Gentle, J. Swanstrom, P. Klock, P. Mayes, November 1994
A technique is presented for obtaining the radiation patterns and the antenna gain of elliptically polarized antennas from two vector measurements of the far-field. The two measurements correspond to different polarizations which can be obtained by rotating one of the antennas around its boresight axis. The discussion emphasizes a particularly interesting case, for which accurate radiation patterns and gain of the antenna under test (AUT) can be obtained without prior knowledge of the polarization of the second antenna. The radiation pattern of a nearly circularly polarized (CP) antenna is conveniently represented by the CP co-polarized and cross-polarized components. The axial ratio and any other quantities commonly used to specify the antenna polarization can also be obtained since the pair of initial vector measurements completely characterize the polarization of the AUT. The technique is illustrated by measurements of a CP patch antenna.
Automated production test facility for a MMW radar system
W.S. Arceneaux, November 1994
Martin Marietta has developed a new, automated facility for high-volume production testing of the Longbow millimeter wave missile. Two dedicated far field anechoic chambers were designed, both automated to support component test and analysis in the production environment. One standard far field chamber is used to perform the complete characterization of the antenna and rac1orne; it allows very accurate measurements of power sidelobes, monopulse errors, and cross­ polarization isolation. The completed radar missile sensor group is evaluated in the second far field chamber, which can reach higher-level parameters of the antenna, transceiver, and gimbal. This paper describes chamber and test station capabilities; time reduction benefits; and the novel, new assembly technique which allows for future portability of these chambers with limited downtime.
3-D low frequency radar target imaging
M.J. Gerry,E. Walton, November 1995
The imaging of radar targets is typically accom­ plished by measuring the radar cross section (RCS) of the target as a function of frequency and az­ imuth angle. We measure a third dimension of the RCS by tilting the target and collecting data for conical cuts of the RCS pattern. This third dimension of data provides the ability to estimate the three-dimensional location of scattering centers on the target. Three algorithms are developed in order to process the three-dimensional RCS data.
Study of DFT windows for radar imaging
P.S.P. Wei, November 1995
New windows which allow the user to select the level of sidelobe suppression near the DFT resolution limit are reported. By a parametric study, we identify the truncated Lorentzian and Gaussian functions as better choices compared with the popular Hann windows.
Near-field/far-field transformation
E. Lebreton,J.R. Levrel, November 1995
RCS data measured under near-field conditions is corrected to the far-field. The algorithm uses the HUYGEN's principle approach. The processing technique is describes and validates using anechoic chamber data and simulations taken on flat plate target at a distance from the radar R << 2D2/A, where D is the target cross range extend and A the wavelength. Good agreement with the theoretically predicted far-field RCS patterns is obtained.
Near-field/far-field transformation
E. Lebreton,J.R. Levrel, November 1995
RCS data measured under near-field conditions is corrected to the far-field. The algorithm uses the HUYGEN's principle approach. The processing technique is describes and validates using anechoic chamber data and simulations taken on flat plate target at a distance from the radar R << 2D2/A, where D is the target cross range extend and A the wavelength. Good agreement with the theoretically predicted far-field RCS patterns is obtained.
Comparison of polar, thinned-polar, and linear spiral sampling using the UCLA bi-polar planar near-field measurement system, A
L.I. Williams,R.G. Yaccarino, Y. Rahmat-Samii, November 1995
The UCLA hi-polar planar near-field scanner has a novel implemen tation which results in a polar sampling grid. The scanner was used to perform measuremen t comparisons using three sampl in g arrangements: polar, thinned-polar, and linear-spiral sampling. The data acquired using each was processed to the far-field for both simulated and measured near-field data. Excellent agreement was observed.
Multi-purpose large compact range for antenna, spacecraft payload, and RCS measurements, A
J.R. Jones,C.L. Allen, E. Hart, J-L. Cano, Garcia-Muller., November 1995
Compact ranges have found wide application for antenna measurements, RCS measurements, and, most recently, for spacecraft payload measurements. Each of these ap­ plications requires certain special features of the range optics, positioning systems, electronics, and software. The system design of a compact range measurement sys­ tem for making all these types of measurements presents a number of challenges. This paper will discuss the system aspects of the design of a multi-purpose compact range facility. Items of inter­ est include the RF electronics design, the positioning sys­ tem design, the optimization of the reflector and feeds and the specialized software design.
Discrete implementation of an image-based algorithm for extrapolation of radar cross-section (RCS) from near-field measurements
I. LaHaie,E. LeBaron, November 1995
ERIM is currently investigating several near-field to far-field transfonnations (NFFFfs) for predicting the far-field RCS of targets from monostatic near-field measurements. Each of the techniques uses approximate­ tions and/or supporting information to overcome the need for the bistatic near-field data which is required to rigorously transfonn a target's scattered field from the near zone to the far zone. Our focus has been on spheri­ cal near-field scanning, since this type of collection geometry is most compatible with existing RCS ranges. One particular NFFFT is based on the reflectivity approximation commonly used in ISAR imaging to model the target scattering. This image-based NFFFT is the most computationally efficient technique under con­ sideration, because, despite its theoretical underpinnings, it does not explicitly require image fonnation as part of its implementation. This paper presents an efficient discrete implementation of the image-based NFFFT, along with numerically-simulated examples of its perfonnance. The advantages and limitations of the technique will be discussed. A simplified version which applies to high aspect ratio (length-to-height) targets and requires only a single great circle (waterline) data in the near field is also summarized.
Economic solution to multiple antenna range operation at Katholieke Universiteit Leuven, Belgium, An
F. Dethier,A. Geva, G. Vandenbosch, S. Snir, Z. Gandelman, November 1995
The ESAT-TELEMIC division at Katholieke Universiteit Leuven (KUL) has three antenna ranges: an indoor Far-Field range, an indoor planar Near-Field range and an outdoor Far-Field range. The positioning equipment is of a variety of manufacturers. The division launched an effort to modernize the range complex and add automatic measurement capabilities, while still retaining control of all three ranges from one control console and using one positioner controller, one angle readout and a single receiver to save costs. The system upgrade included some electrical refurbish­ ment of the positioning equipment and the replacement of all the old control and data recording equipment with Orbit Positioner Controller/Programmer, Power Control Unit and combined Near-Field and Far-Field software. Control of all three sites is achieved using a special Orbit Junction Box. With the new configuration all three ranges can be operated in fully automatic mode, one range at a time. The software package controls both Near-Field and Far­ Field measurements using compatible data formats and human interfaces.
Discrete implementation of an image-based algorithm for extrapolation of radar cross-section (RCS) from near-field measurements
I. LaHaie,E. LeBaron, November 1995
ERIM is currently investigating several near-field to far-field transfonnations (NFFFfs) for predicting the far-field RCS of targets from monostatic near-field measurements. Each of the techniques uses approximate­ tions and/or supporting information to overcome the need for the bistatic near-field data which is required to rigorously transfonn a target's scattered field from the near zone to the far zone. Our focus has been on spheri­ cal near-field scanning, since this type of collection geometry is most compatible with existing RCS ranges. One particular NFFFT is based on the reflectivity approximation commonly used in ISAR imaging to model the target scattering. This image-based NFFFT is the most computationally efficient technique under con­ sideration, because, despite its theoretical underpinnings, it does not explicitly require image fonnation as part of its implementation. This paper presents an efficient discrete implementation of the image-based NFFFT, along with numerically-simulated examples of its perfonnance. The advantages and limitations of the technique will be discussed. A simplified version which applies to high aspect ratio (length-to-height) targets and requires only a single great circle (waterline) data in the near field is also summarized.
Near-field to far-field transformation of RCS measurements
D. Mensa,K. Vaccaro, November 1995
The RCS of extended objects measured in the near field is subject to errors induced by the spherical nature of the incident and scattered wavefields. A number of techniques have been applied to estimate far-field responses from results of monostatic near-field measurements. While the results indicate successful transformations for linear scatterers, the lack of a sound theoretical basis brings into question the appli­ cability to general objects. The paper explores the theoretical basis of the far-field transformation of RCS data and the consequence of the limited data obtained from monostatic measure­ments. The limitations of approaches reported to date [1-4] are explored from conceptual and physical con­ siderations with the goal of establishing reasonable expectations for practical methods. Examples using simulated and measured near-field data are presented to illustrate successes and failures of the algorithms in transforming results to far-field RCS.
Enhancement of efficiency and accuracy of near-field measurement
G. Seguin,T. Pavlasek, November 1995
This paper examines the possibility of increasing the speed of Near-Field measurement of an Antenna, by reducing the number of measurement points and by determining the degree of truncation permissible while maintaining a prescribed degree of precision of the reconstructed far-field. The Near-Field of a planar radiating array is analysed in depth. A formulation and a procedure to correct the spectral domain of the field are established. It is shown that correction in the spectral domain can improve the accuracy of the Far-Field while using the same amount of Near-Field data. The technique has a good potential to be applied to Near­ Field data of large radiating Antennas leading to new information about the accuracy and speed of measurement achievable.
Comparison of K-correction and Taylor-series correction for probe-position errors in planar near-field scanning, A
M.H. Francis, November 1995
We investigated two methods of probe-position error correction to determine how well the corrected results compare to the uncorrupted far field: the k-correction method and the Taylor-series method. For this investigation, we measured a 1.2 m dish at 4 GHz and a 1.2m by 0.9m phased array at 2.2 GHz. Measurements were made first without position errors and then with deliberate z-position errors. We perfonned probe­ position error correction using both methods and compared the results to the error-free far field. For errors up to A/4, the fifth-order implementation of the Taylor­ series correction was slightly better than the k-correction. For errors of ')..J2, the k-correction was better than the Taylor-series correction.
Method to transform measured Fresnel patterns to far-field based on a least-squares algorithm with probe correction, A
F. Las Heras,B. Galocha, J.L. Besada, November 1995
A method to transform Fresnel field data to far-field data with probe correction, based on a non linear least­ squares algorithm, is presented. The functional to be considered is the expression of the Fresnel field radiated by an array of isotropic sources located on the antenna aperture, and the complex excitations are the coefficients that minimize the rms error between the measured data and the functional values. The intermediate step of determining the complex excitations can be used as a diagnostic tool. Probe pattern correction has been included in the method, improving the performances of antenna measurement systems placed in small size anechoic chambers.
Near-field/far-field phase retrieval measurements of a prototype of the AMSU-B space-borne radiometer antenna at 94 GHz
C.A.E. Rizzo,A.P. Anderson, G. Junkin, November 1995
Far-field patterns obtained from planar near-field measurements of a prototype of the AMSU-B radiometer antenna by phase retrieval at 94 GHz are presented in this paper. Comparison with results from a compact range facility show good agreement within the main beam A modified algorithm takes into account any misalignments of the two intensity data sets so that the RMS near-field error metric comparing retrieved and measured values converges to < -30 dB. Phase retrieval is revealing itself as a useful technique to be applied to electrically large antennas at frequencies extending into the millimetre and sub­ millimetre bands.
Intelsat VIII antenna measurements
M. Boumans,J. Habersack, L. Jensen, November 1995
Daimler-Benz Aerospace AG (Dasa) in Munich, Germany designed, developed, build and tested most of the INTELSAT VIII antennas. RF test requirements and results are presented for the Hemi/Zone antennas. These tests cover the Beam Forming Networks (BFN), the feed array in the cylindrical near field facility at ambient temperature and in a temperature range from -61 to +85 deg centigrade, and finally the complete antenna sub­ system, without and with satellite mock-up, in the large Compensated Compact Range. Dasa and TICRA software was used to calculate the far field results from the measured BFN coefficients and from the feed array results measured in the near field facility. Also alignment aspects are considered.
Time domain near-field far-field transformation using optimal plane-polar sampling representation
O.M. Bucci (Universita di Napoli “Federico II”),G. D'Elia (Universita di Napoli “Federico II”), M.D. Migliore (Universita di Napoli “Federico II”), November 1996
A time domain near-field far-field transformation technique based on a non redundant plane-polar sampling representation of the field is presented. The method allows to obtain the far-field with a minimum number of samples and/or a reduction of the scanning area. Various computational schemes are presented.
Design and verification of an elevated far field antenna measurement facility at 30 MHz - 1000 MHz
D.P. Walsh (ORBIT-Flam & Russell Inc.),Douglas Kremer (Measurement Systems Inc.) Rolando Berrios (Naval Explosive Ordinance Disposal Technical Center) David Ports (Naval Explosive Ordinance Disposal Technical Center), November 1996
This paper will address the design and verification procedures for an Elevated Far Field antenna measurement facility at the Naval Explosive Ordinance Disposal Technical Center, Indian Head, MD for operation at 30 MHz – 1000 MHz.
Accurate determination of main beam position and beamwidth from near field measurements
M.H. Paquay (TNO Physics and Electronics Laboratory), November 1996
For narrow beam antennas or track antennas some parameters like main beam or null position and 3 dB beamwidth need to be determined with an accuracy of less than a mill or mrad. With Near Field measurements, the Far Field is normally calculated by FFT-processing. This does, however, not provide the required accuracy. Nevertheless, the measured Near Field data contains information about any Far Field point. An iterative approach is presented to determine the Far Field antenna characteristics with high accuracy.


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