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Recent results from RCS measurements on metal wires, rods and dielectric strings are presented. For a cylinder at broadside to the incident wave, theoretical from 3D formulas converted from 2D exact solutions are used for comparisons with the experiments. The lone-of-sight orientation dependence is described by the polarimetric scattering matrix. Several types of interference effects are analyzed. Of particular interest is finding the suitable objects for the cross-polarized calibrations over a wide frequency range. Details from a 36" wire of radius 0.01" for calibrations in the VHF range are described. While the wire is supported by fine fishing lines, mitigation of the unwanted string echoes is important.
In the last few years, a change has occurred in the RCS metrologist concerns for error analysis and the quantification of measurement uncertainty. The specific methods for range characterization and uncertainty estimation are the topics of many passionate technical discussions. While no single treatment can please everyone, most agree a measurement uncertainty program is critical to the understanding of measurement quality, the development of error reduction strategies, and to the planning of range improvement paths. We present the statistical case for the natural grouping of errors into multiplicative and additive classes. We will derive the two cases where one class dominates as presented by LaHaie [1], and then expand the analysis to include the general case of competing classes. We summarize the role and applicability of this method in estimating measurement quality and discuss how this procedure offers a logical and comprehensive error propagation solution to both top-down and bottom-up range characterization approaches.
Calibration standards for radar systems are being developed cooperatively by NIST and DoD scientists. Our goals are to develop standard procedures for polarimetric radar calibrations and to improve the uncertainty in the estimation of system parameters. Dihedrals are excellent polarimetric calibration artifacts, because (1) the consistency between dihedral scattering data and the mathematical model of scattering can be easily verified, and (2) symmetry properties of the dihedral data provide powerful diagnostics to reveal system problems. We apply Fourier analysis to polarimetric data from dihedrals over a full rotation about the line of sight to reduce the effects of noise and clutter, misalignment, and other unwanted signals. An extension of the analysis to satisfy nonlinear model constraints allows us to monitor data quality and to further improve the calibration. We obtain the system parameters from the Fourier coefficients of the data in a simple manner. We illustrate these concepts using polarimetric radar cross section calibration data obtained as part of a national interlaboratory comparison program.
Electromagnetic field measurement systems based on the Advanced Modulated Scattering Technique (A MST) permit fast and accurate diagnostic testing to be performed in the near-field (NF) or the far-field (FF) of antennas and scattering objects. A-MST probe arrays are particularly effective for rapid diagnostic testing applications where it is desired to obtain overall measurement duration reductions of 80% to 98% compared to conventional single-probe measurement times.
The accuracy of the probe antenna pettern used for the probe-corrected near-field measurements is critical for maintaining high accuracy results. The probe correction is applied differently in the three standard near-field techniques - planar, cylindrical, and spherical. This paper will review the differences in sensitivity to probe correction for the three techniques and discuss practical of probe correction models and measurements.
Nearfield Systems, Inc. (NSI) has delivered the world's largest vertical near-field measurement system. With a 30m by 16m scan area and a frequency range of 1GHz to 50GHz, the system consists of a robotic scanner, laser optical position correction, computer and microwave subsystems. The scanner and microwave equipment are installed in an anechoic chamber 40m in length by 24m in width by 25m in height. The robotic scanner controls the probe positioning for the 33m by 16m vertical scanner using X, Y, Z and polarization axes. The optical measurement package precisely determines the X and Y axes position, alignment errors along the X and Y axes, and Z-planarity over the XY scan plane.
Hughes Space & Communications Group uses near field measurement systems for satellite antenna qualification tests on many of its commercial satellites. Hughes contracted with Nearfield Systems Inc. for delivery of several large horizontal planar near-field scanners for these tests. A 40' x 22' system was commissioned in early 1997 and has since been used for numerous commercial satellite tests. Prior to satellite antenna range testing, this range was characterized for gain measurements, co-polarized and cross-polarized pattern measurements, and measurement repeatability at C-band frequencies. This paper will highlight some of the findings from the characterization effort for this particular test facility.
The four antenna subsystems on each of the twelve ICO satellites, includes two eight foot diameter S Band active arrays, driven by a digital signal processor (DSP). These phased arrays, each consisting of a triangular lattice arrangement of 127 radiating elements, must be tested for functionality and workmanship, before being integrated onto the spacecraft. With a two-month center to center delivery requirement, standard fabrication and test procedures had to be modified and automated in order to meet schedule without compromising the traditional conservative approach for performance verification. This discussion of the ICO S-Band test program includes descriptions of the nearfield testing, Field Aperture Probe tests, and other tests related to EMI problems (such as transmit to receive isolation and PIM) on the spacecraft, as well as a brief description of the PC-BFN, a rack of special test equipment designed to allow testing of the passive array without the satellite DSP. Emphasis is given to the design of tests compatible with a mass production environment.
Advances in the electromagnetic simulation of inflatable reflector antennas are described. Examples of proposed missions using inflatable antenna technology (IAT) are given. Techniques are presented to compensate for structural distortions using array feeds and subreflector shaping. It is possible to noticeably reduce the deteriorating effect of distortions using these compensation techniques. Additionally, the application of a technique based on focal plane diagnostics is summarized.
This paper presents a new method of separating and evaluating the effects of each residual reflection caused by antenna measurement environment by distance changing technique. The effects represent radiation patterns caused by residual reflections (hereafter, error patterns). The key processes of this new method are to suppress sidelobes of a Fourier spectrum applying a window as a function of the distance with the purpose of obtaining an accurate spectrum of reflections and to separate error patterns each other using a gating technique at each angle. Using this method applying the above two processes, we can evaluate the error pattern for each reflection source with accuracy. The validity of this method is confirmed by a computer simulation. This method is especially useful to detect the position of each reflection source in a case of evaluation for antenna test range.
A computer simulation for prediction of equivalent system temperature is presented. The figure-of-merit for a receiving system is given by the ratio of the receiving system gain by the equivalent system temperature (G/Ts). While the gain, G, can be well characterized by laboratory measurements, measurements of system temperature, Ts, taken in the laboratory do not correspond well to measurements in outdoors, due to a myriad of environmental factors. A computer methodology to statistically characterize the noise performance of a satellite earth station receiver in the operational environment was developed for the Department of Commerce National Institute for Standards and Technology, as of a Small Business Innovation Research contract. The result of Phase 2 of the SBIR is an implementation of this computer methodology called the Receiving System Analysis Tool. This paper describes the methodology, the RSAT simulation, and its application to SATCOM terminal analysis.
A simulation tool used during the design of near-field ranges for phased array antenna testing is presented. This tool allows the accurate determination of scanner size for testing phased array antennas under steered beam conditions. Estimates can be formed of measured antenna pointing accuracy, side lobe levels, polarization purity, and pattern performance for a chosen rectangular phased array of specified size and aperture distribution. This tool further allows for the accurate testing of software holographic capabilities.
Properly designed elevated antenna ranges, that are to be used on aircraft sized structures, at VHF and UHF frequencies, are prohibitively large. Conventional ground reflection ranges can measure only one frequency at a time because the source antenna height must be set for each frequency. This paper describes a broadband antenna ground reflection range that has been designed for the purpose of making antenna pattern measurements at arbitrary frequencies between 30 MHz and 400 MHz on aircraft sized vehicles. This design uses multiple transmit antenna elements with the complex weighted excitation determined by the use of genetic algorithms.
A new measurement technique for integrated antennas is presented, which allows measurement of the radiation characteristics using a wafer-prober structure. This technique provides a simple inexpensive method of assessing the radiation characteristics of an integrated antenna. Unique difficulties in measuring and modeling integrated antennas are discussed.
This paper is concerned with the measurement and analysis of a circularly polarized, flat plate patch array receiving antenna at 12.5 GHz. Input impedance and far field pattern measurements of the antenna over the frequency band from 10 to 15 GHz were performed. The small Compact Range (CR) facility of the Ohio State University Electro Science Laboratory OSU/ESL was used to measure the gain pattern. Gain pattern measurement of the antenna was done by using the gain comparison method. A broadband (2-18 GHz), constant phase pyramidal horn antenna was used as a reference. The data were analyzed to determine the radiation efficiency of the antenna.
Compact range reflector edge diffraction can be reduced by placing a well-designed R-card fence in front of the reflector edges. The impact of this fence can be expressed mainly in terms of its ability to attenuate transmitted energy through the R-card. Thus, the resistance of the R-card is synthesized to satisfy a chosen GO aperture taper. A Kaiser-Bessel taper produces an ideal taper transition and hence a large target zone at the lowest operating frequency. Since a proper design requires that the R-card be located near the curved reflector edge, multi flat R-card segments are designed and assembled around the periphery of the reflector. The R-cards then attenuate the transmitted edge diffracted field and direct the reflected signal away from the test zone onto the anechoic chamber walls, which results in a significant improvement in the uniformity of the test zone fields.
It is known that electromagnetic signalls can penetrate through non-metallic barriers such as building walls. A hand-held Sythetic Aperture Radar (SAR) unit capable of transmitting and receiving such signals is desirable in various military and civilian applications. Theoretical and experimental issues associated with through-wall Ultra Wide Band (UWB) SAR imaging of buildings are studied here. It may be inconvenient and impractical for a hand-held unit to collect data at uniformly spaced positions. A backprojection algorithm is developed for the case where spatial sampling is not uniform. In addition, a spherical wavefront (as opposed to a uniformly planar wave front) is assumed in the algorithm to account for the proximity of a radar unit relative to a target scene. Images of simulations using point targets and measurements of canonical targets such as a corner reflector and a cylinder are generated. Images of a standing human in free-field and through-wall are compared.
This contribution will show how active frequency selective surfaces (AFSS) loaded with PIN and varactor diodes can be used concurrently to dynamically control the reflectivity characteristics of a microwave absorbers. The PIN and varactor diodes are used respectively to effect abrupt and gradual changes in an absorber's reflectivity characteristic.
Ths paper demonstrates the plane wave, pattern subtraction method for performing range compensation of full-sphere antenna patterns measured on a fixed line-of-sight far-field range. The range field is measured on the surface of a sphere and a plane wave model of the range field illuminating the antenna under test (AUT) is determined. The range compensation algorithm uses information contained in both the plane wave model and the AUT pattern measurement to estimate the error pattern that is added to the measured AUT pattern by an extraneous source. This estimated error pattern is subtracted from the antenna pattern measurement to obtain a compensated pattern. The compensated pattern and estimated error pattern are improved iteratively. This paper demonstrates the technique using measured data. The AUT is measured in a far-field anechoic chamber that contains a secondary horn antenna located 20 degrees off-axis from the range antenna, which is used as an extraneous source. The AUT is a 474 element planar array operating at a frequency of 9.33 GHz.
For improvement of the measurement accuracy of compact range test facilities under the constraint of maintaining the realtime measurement capability, a versatile hardgating system has been developed at the Fachhochschule Munchen. With this measurement and diagnostic tool a flexible, computer controlled variation of the pulse widths down to some ns can be performed to obtain a high spatial resolution. Besides selective measurements of the quiet zone field with suppressed interferers it is also possible to select particular inte fering field contributions in order to determine their amplitude and direction of incidence. The paper describes the hardgating system and the measurement results obtained with low and high gain antennas in the compensated compact range test facility at the Fachhochschule Munchen.