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The design of wide-band, multi-layer radar absorbing materials involves the solution of what is essentially an N-dimensional optimization problem. Genetic algorithms appear to offer significant advantages over conventional optimization techniques for this type of problem due to their robustness and independence of performance function derivatives. To illustrate their use, the paper considers the optimum design of wideband, multi-layer, Jaumann radar absorbers for normal and oblique incidence.
One critical issue in designing absorber for an anechoic chamber is the bistatic scattering performance of the absorber and its effect to the quiet zone field quality. The bistatic scattered fields from the absorber side walls, floor and ceiling of the range result in undesired stray signals which can cause significant measurement errors. Consequently, it is very important to analyze the performance of the absorber from the overall system point of view; i.e., the performance of the absorber in the range environment. This paper will address this issue and present calculated results of absorber wall performance for a compact range with a blended rolled edge reflector.
Absorber material us used in antenna measurements to reduce multiple reflections and multipath effects. However, in some cases the effect of the absorber can still have an uncertainty larger than the desired uncertainty of the measurement. For accurate antenna gain measurements, using the planar, cylindrical and spherical near-field methods and the extrapolation technique, insertion-loss measurements should be accurate to within + 0.03dB. To satisfy this requirement it is important to minimize the multiple reflections between the probe and antenna under test. If the multiple reflections are too large, the insertion loss becomes very position sensitive and uncertainties on the order of 1 dB can occur. It is imperative that absorber be used to cover all metal surfaces of the antenna mounts. Uncertainties can also occur if the absorber is not used carefully. The effects on antenna fain data measured with and without absorber will be shown. Measurement results showing the effect of the placement of the absorber on the antenna under test will also be presented. This will include absorber distance from the antenna's aperture, the rotation of absorber about the antenna's coordinate system, and the use of different types of absorber.
For an anechoic chamber design, one normally spec ifies the field quality throughout the quiet zone in terms of the ripple level requirement. The ripple in the quiet zone field is caused by the interfer ence of various stray signals with the desired plane wave. The stray signals in an anechoic chamber can come from absorber or other parts of cham ber. However, from a range performance point of view, it is more important to know the ef fects of stray signals on the measurement accu racy of an antenna radiation or target scattering pattern. Consequently, it is very critical to eval uate how the chamber stray signals will affect a given measurement. This paper addresses this is sue by simulating pattern measurements of a phase scanned array in a compact range and discuss the effects of various stray signals associated with the scattering from absorber walls and feed spillover.
A method is presented to qualify a compact range measurement system for low sidelobe antenna measurements. The method uses the target zone fields (probe data) of the compact range. Using the method, one can identify the angular regions around which the measurement errors can be significant. The sidelobe levels which can be measured around these angular regions with less than a 3 dB error are also defined.
In this paper, a new error correction technique is introduced to improve the accuracy and efficiency of the traditional NRL Arch method. The use of this integrated technique allows one to correct the error terms in the traditional NRL arch setup so that a broadband evaluation of the performance of the absorber product can be performed with much better accuracy and efficiency. This technique also allows one to conduct large bistatic angle evaluation of absorbers without the cross talk and other error signal interferences. Design guidelines for a broadband NRL test arch are provided so as to successfully implement this improved NRL Arch method for a broadband evaluations of anechoic absorbers. Sample test results from Ray Proof's broadband test arch (0.5-6 GHz) are also presented.
With the recent use of dual-polarized transmission and reception on communications links, the capability to perform accurate polarization measurements is an important requirement of test-range systems. Satellite antennas are commonly measured in the clean, protected environment of compact and near-field ranges, and a circularly polarized feed/field probe is a primary factor in establishing their polarization properties. The feeds also provide excellent source-horn systems for tapered anechoic chambers, where their circular symmetry and decoupling of the fields from the absorber walls improve the often troublesome polarization characteristics of tapered chambers. Circularly polarized feeds are generally composed of four primary waveguide components: the orthomode transducer, quarter-wave polarizer, scalar ring horn, and circular waveguide step transformer. Linearly polarized feeds omit the quarter-wave polarizer. This paper discusses the design and performance of high-polarization-purity source feeds for evaluating the polarization properties of antennas under test. Circularly polarized feeds have been constructed which operate over 10- to 20-percent bandwidths from 1.5 to 70 GHz. Gain values are generally in the area of 12 to 18 dBi, with cross-polarization isolation in excess of 40 dB. Representative measured data are presented.
The utility of array feeds for compact range reflector antenna applications is discussed. The goal is to feed a circular-aperture, offset parabolic reflector such that the central illumination is uniform and the rim illumination is zero. The illumination taper results in significant reduction of edge-diffracted fields without the use of reflector edge treatment. A methodology for designing an array feed requiring only two real excitation coefficients is outlined. A simple and cost effective array implementation is presented. The array beam forming network is realized as a radial transmission line which is excited at the center from a coaxial transmission line, and terminated at the perimeter with absorber and conductive tape. Energy is probe-coupled from the radial line to balun-fed dipole array elements. The required element amplitude excitation is obtained by adjusting the probe insertion depth, and the required element phase excitation is supplied by the traveling radial wave. Construction and test of an X-band array are summarized. The measured array patterns display a flat-topped beam with a deep null at angles corresponding to the reflector rim.
Lockheed has recently completed the construction of a Large Compact Range (LCR) for antenna and RCS measurements. The dimensions of the facility are 60' (h) x 100' (w) x 120' (l) with a 20' x 20' cylindrical quiet zone and operational capabilities from 0.1 to 18.0 GHz. The requirement to measure low RCS levels in a room which is smaller that the desired has resulted in a unique system design. Elements of this design include a feed pit, a feed hood, and a rolled edge reflector; special absorber layouts to minimize background scattering, a high performance instrumentation radar, fast ring down feed antennas, and a unique string suspension and positioning system. This paper presents the various sub-systems that make up the LCR along with chamber validation methods and preliminary performance data. The subsystems listed in this paper are LCR's: Reflector, radar system, feed antennas, feed positioner, absorber, target handling equipment, and string positioning system. Initial design requirements are listed for some sub-systems along with range characterization data such as un-subtracted clutter levels, background subtraction performance, and theory vs. measured data for some simple conical shapes.
In this paper a novel technique for suppressing edge effects which can corrupt reflectivity measurements of large absorbers, is presented. In consists in mounting a collar of small absorbers around the test sample of the large absorbers to be evaluated. It is shown that the edge effect return is by far the most dominant return during the reflectivity measurements of large absorbers whereas the inherent reflectivity levels of these absorbers can be very low. It is claimed that the so-called superior performance of small absorbers at very high frequencies as compared to large absorbers is probably not a reality but a misinterpreted measurement result due to edge effects.
Near field range design includes the placement of RF absorber in the test area. Absorber placement depends highly on the antennas being tested. A common approach is to design an expensive low-reflection chamber around the near-field scanner. The chamber and the additional floor space can sometimes cost more than the near-field scanning system itself. Another approach seeks to identify multi-path reflection to minimize cost by optimally placing absorberto meet specific antenna test requirements. The results is a lower cost range using less floor space. This paper describes a technique of evaluating near-field range multi-path.
A plasma-absorber system consists of a membrane that confines a collusional gas at atmospheric pressure and an ionization source. The ionization source generates a dense plasma that tenuous near the confinement membrane. An electromagnetic wave propagating through this plasma is attenuated. The mechanism for absorption is momentum transfer among electrons, driven by an incident wave, and a gas. Because the momentum-transfer collision rate, v, at atmospheric pressure can be as high as 870 x 10^9 s-1, the 3-dB bandwidth for absorption (~v/20) is approximately 40 GHz. The plasma thickness between the source and confinement membrane is approximately one wavelength at the lowest frequency. The magnitude of absorption depends on this thickness, the maximum electron number density, and the electron density gradient. A smooth gradient reduces reflections. This paper discusses a theoretical model that predicts general absorption and reflectivity phenomena. Experiments have quantified 40-dB performance at VHF using a 4-mil polyethylene vessel, and at X-band using a 2-mil Mylar inflatable support system. Applications to precision RCS measurements include reduction of backwall reflections and target interaction with the ground plane, and a shutter for reference targets.
For the last few years, the Periodic Moment Method (PMM) has been used to analyze the scattered fields from an infinite absorber wall. Using this approach the absorbe4r can have different periodicities in the x and y directions, as well as arbitrary shapes and any dielectic (sic) distribution. This makes this analysis method very general such that it can treat any conventional wedge or pyramidal designs. Plus, it has been used to develop new ones, which is the subject of this paper. Traditionally there have been chamber uses for both wedge and pyramidal absorber (sic). In a normal RCS range, one uses pyramidal material in forward sector around the feed, wedge absorber through the target zone and pyramids on the backwall. Using this approach, one takes advantage of the basic features of the two types of absorber. To improve wedge material, one is interested in reducing its normal incidence reflection coefficient because the long straight edge is a rather large scatterer. Through the use of the PMM analysis, curved and serrated wedge absorber designs have been developed and tested. Both show significant improvement relative to conventional material. As for the pyramidal model, one would hope to improve its size requirements especially for lower frequencies. Recall that two wavelengths at 100 MHz is twenty feet. By placing twenty foot material throughout a chamber, one greatly restricts the size of the room. Again, the PMM analysis has been used to develop a new curved pyramid design which can perform as well as a conventional pyramid twice its size. Thus, one could use curved pyramids that are ten feet at 100 MHz and achieve the same performance as the commercially available twenty-foot material.
Most of the present available pyramid absorber lined anechoic chambers do not meet the new stringent requirements (plus or minus 4 dB criterion of CISPR) for EMC measurements at lower frequencies, say below 100 MHz, due to poor absorber efficiency. In this paper the actual field configuration in those chambers at these critical frequencies is numerically computed for extracting frequency dependent correcting relations for EMC measurements. To this end a finite difference formulation in frequency domain is used. The absorbers are modeled as planar dielectric layers. Examples of computed field configurations are presented and compared with measurement values. The results show the frequency response of the electrical field configuration with respect to the position of device under test, the test antenna, as well as the effect of chamber asymmetries.
Recently, the theory and computer programs on the Periodic Moment Method (PMM) for scattering from both singly and doubly periodic arrays of lossy dielectric bodies have been developed. The purpose is to design microwave wedge and pyramid absorber for low reflectivity so that one can improve measurements and/or reduce the size of the anechoic chamber. With PMM, the reflection and transmission coefficients of periodically distributed bodies illuminated by a plane wave have been accurately calculated on the Cray Y-MP supercomputer at the Ohio Supercomputer Center. Through these studies, some wedge and pyramid absorber configurations have been designed, fabricated and tested in the OSU/ESL Anechoic Chamber. Very good agreement between calculations and measurements has been obtained. In the 1990 AMTA meeting, several wedge absorber designs and results for the TM case and normal incidence were presented. In this paper, the measured and calculated frequency responses of some experimental wedge designs, as well as an 8” and 18” commercial wedge and pyramid absorber panels will be reported for both TM and TE polarizations. Time domain responses will also be shown for both measurements and calculations.
It has been recently shown that an optimized blended rolled-edge compact range reflector can be successfully used to measure two or three foot targets at microwave and millimeter frequencies. In addition to the reflector design, one is faced with many other practical range design issues, such as absorber treatment, target mount and access, feed mount and access, etc. Each of these design aspects has been evaluated and an actual range has been constructed to illustrate the capability of such a system. The feed is mounted on a rotating side door for easy access. The target zone is approached from the rear of the chamber by rotating the backwall. These design concepts allow the range operator to quickly modify the measurement setup, yet still maintain extremely stable results. The simplicity of this design as well as its excellent measurement capability are presented.
Multiple mechanisms for the generation of extraneous signals exist in a compact range. These include edge diffraction, scattering from surface imperfections, direct feed radiation, and scattering from absorber or other objects in the range. The field quality in the quiet zone is the resultant of the direct signal and these multiple scattering mechanisms. Since the scattering mechanisms are independent, their effects are often modeled independently and statistically combined to yield an estimate of quiet zone field quality. This paper examines the statistics of multiple independent extraneous signals in a compact range. It is shown that the amplitude ripple produced by an extraneous signal computed as the root sum of the squares (RSS) of the individual extraneous signals does not correctly predict the final quiet zone amplitude ripple. Theoretical results for scattering from multiple thin gaps in the surface of a compact range are presented and statistical computer models are used to demonstrate the computation of the resultant compact range quiet zone.
In near field antenna measurements, knowledge of the the [sic] probe antenna’s pattern, polarization and gain are of vital interest. To calibrate a probe for near field measurements is a delicate task, especially if the probe is small, i.e. low gain. The near field probe and the parameters general to a probe calibration are presented. The delicate task of obtaining an accurate gain for small aperture antennas as well as the problem of transfering [sic] the calibration from the facility where the probe is calibrated to the facility where it is to be used are focussed [sic] upon For a small aperture, the pattern is that of the radiating aperture. The unwanted scattering may be removed by filtering in the spherical mode domain thus obtaining the true aperture radiation. The gain derived from this may however be of little use in reality since the aperture always needs some form of mounting. Such a mounting may be covered with absorber which may reflect and diffract and thus affect the gain value.
Martin Marietta designed and brought on-line an indoor far-field chamber used for radar cross section (RCS) evaluation. The range has conductive walls on all sides except for the pyramidal absorber covered back wall. The chamber was designed such that wall/floor/ceiling interactions occur with a distance (time) delay allowing for their isolation from the test region. Software gating techniques are used to remove these unwanted signals. This paper presents an analysis of the conductive chamber using Geometrical Optics (GO). The objective was to analyze and evaluate the plane wave quality in the chamber test region. The evaluation of the plane wave was performed using the angle transform technique. The measured results were compared to analytical results and measured antenna patterns.
In dual chamber compact range measurement systems, a Gregorian subreflector is used to illuminate the main reflector. Since the subreflector is finite in size, there will be diffracted fields from its edges which degrade the incident field on the main reflector and subsequently lead to undesired stray fields in the target zone. Some treatment of the subreflector edges is therefore required. One way to reduce the subreflector edge diffraction is to use a serrated edge subreflector. In this paper, the performance of a dual chamber compact range system with a serrated edge Gregorian subreflector is discussed. It is shown that by using the serrated edge subreflector, one can reduce the ripple in the target zone due to the subreflector edge diffraction from 3 dB to 0.5 dB. One can further reduce the ripple by separating the two chambers by an absorber fence with a small coupling aperture.