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This paper discusses an anechoic chamber absorber evaluation which was conducted for the purpose of improving anechoic chamber and compact range performance through better absorber characterization. This study shows that performance of conventional absorber materials is dependent on selection of the material's shape, size and orientation with respect to the incident energy direction. This, demonstrates the importance of better characterization of the material. Nonhomogeneities in the material composition and physical structure were also found to significantly modify performance; in some cases even improving it. Also shown, is the need for improved evaluation techniques and procedures over conventionally used methods. An evaluation procedure using modern imaging techniques is presented. Several measured results for various absorber types and sizes are presented which show the usefulness of the evaluation technique and demonstrate relative performance characteristics for these materials. Measured reflectivity data on various absorber types, which consistently show better performance than levels specified by the vendors, are also presented.
The indoor compact range has proven to be quite successful in measuring the radar cross section (RCS) of various targets. As the performance capabilities of the compact range have expanded, the use of larger, heavier, and more sophisticated targets has also expanded. Early target dimensions were limited by the size of the useful test area, as well as the capacities of the low RCS pedestal mount used. Today, our anechoic chamber has a large useful test area, thus the size and weight of targets dictate that a new method be employed in target handling and positioning, as well as target mounting to a low RCS pedestal. Work was recently completed here at the Ohio State University ElectroScience Laboratory to remodel our anechoic chamber to allow for the new generation of targets and the demands that they place on the anechoic chamber. This work included the addition of a one ton motorized underhung bridge crane to our anechoic chamber, the design and construction of an hydraulic assist to smoothly and precisely raise and lower the target for the final linkup of the support column and the receiving hole in the target, the design and installation of a one ton telescopic crane in the chamber annex to link with the main chamber crane, the design and installation of the necessary microwave treatments to minimize the impact of the remodeling on accurate RCS measurements, the development and installation of a sloping raised floor, the design and manufacture of a track guided rolling cart to shuttle operating personnel to and from the target area, the replacement of the existing radar absorbing material, the improvement of the ambient lighting in the chamber to facilitate film and video tape documentation, and the development of new target mounting schemes to ensure ease of handling as well as secure mounting for vector background subtraction.
There is renewed interest in the idea of determining the near-zone and far-zone bistatic RCS of complex targets from near-field data. This paper addresses the issue of efficient acquisition and processing of the requisite scattered near-field electric field data for determining the wide-angle bistatic RCS of electrically-large targets. Toward that end, several potential combinations of target illumination and near-field scanning techniques are considered in this paper. The techniques considered encompass mechanical and electronic scanning methods using single probes, linear probe arrays, and planar probe arrays to accomplish the near-field scanning, combined with either (a) compact range illumination or (b) "synthesized" plane wave illumination employing a single probe, a one-dimensional (1-D) probe array, or a two-dimensional (2-D) probe array. A general Spherical Angular Function (SAF) integral formulation of near-field bistatic coupling/scattering is presented, and an approximate "deconvolution" technique for electrically-large targets is described.
The recent activity and study of the compact range has been increasing the past few years. Both radar cross section (RCS) and antenna measurements have been conducted in the compact range. Important research and analytical investigation has also been done in the design and construction of the reflectors so characteristic of these types of ranges. Edge diffraction from the reflector has been studied and characterized by methods of geometrical optics, geometrical theory of diffraction, physical optics and physical theory of diffraction. Treatment of edge diffraction effects on the reflector have included serrations, rolled edges, and absorbing materials. The primary goal is to obtain as perfect a plane wave as possible in the enclosed chamber with reduction of edge diffraction from the reflector.
As the operating frequencies of compact range antennas increase, the accuracy of the field probes used to characterize their performance must also increase. Obtaining the required accuracy through mechanical design becomes more and more difficult as the size of the area to be probed increases. This paper describes the use of a laser measurement system to sense the probe's mechanical displacements thereby allowing corrections of compact range measurement. The relatively simple laser alignment system is well-suited for compact range probing in which accuracy is much more critical in the Z direction than the X-Y direction.
The compact range reflector used these days for RCS and antenna measurements have rolled edges [1] to reduce the stray fields diffracted from the rim of the parabolic section. For optimum performance (small edge diffracted fields), blended rolled edges [2] are used. A blended rolled edge ensures that the radius of curvature of the surface is continuous at the junction between the paraboloid and the rolled edge. By selecting an appropriate blending function, one can make the first and higher derivatives of the radius of curvature continuous at the junction [3] which in turn results in a weaker diffracted field. However, the resulting reflector may be too large to be practical. Also, the minimum radius of curvature of the reflector surface in the lit region may become less than one fourth of the wavelength at the lowest operating frequency, which is not desirable. Thus, the choice of blending function and rolled edge parameters is quite important in the design of compact range reflector antennas. In this paper, a procedure to design blended rolled edges for such applications is discussed. The design procedure leads to a rolled edge that minimizes the edge diffracted fields while satisfying certain constraints regarding the reflector size and minimum operating frequency of the system. Some design examples are included.
Recently, needs have arisen for low axial ratio feed horns for prime focus fed compact ranges. The compact range environment necessitates a feed possessing low back lobes to minimize extraneous radiation. Circular polarization demands dual orthogonal linear polarizations with symmetrical radiation characteristics. An iris loaded square waveguide section was developed to produce a quadrature phase shift in one linear polarization versus the orthogonal polarization. This 90 degree phase shifter was incorporated into a corrugated horn to achieve a 1 dB axial ratio or less over a full waveguide band. Theoretical and experimental data will be presented for several of these horns. Extensions to lower axial ratios (less than .5 dB) using a double tuned circuit approach will also be presented.
The offset fed parabola is one type of reflector used in compact radar ranges. Cross-polarization problems have been noted when a parabola is used in near field applications. A good understanding of the near field cross-polarization effects was needed to evaluate this type of reflector for a compact range. We found that the polarization vector was rotated differently at each location in the "quiet zone." The polarization vector rotation is due to the parabolic curvature. In addition, a mathematical model was derived that compares well with the data. A theoretical study of how the RCS measurements of a wing are affected is presented.
Many experimental and analytic studies on travelling waves have been performed in relation to their radiation properties for antenna applications. One common structure that has supported a fast travelling wave is a slotted waveguide. Such structures can also support travelling waves from a scattering viewpoint. This aspect was verified by incorporating a trough in an almond test body to observe its scattering characteristics using aspect angle patterns, frequency spectra and transient signatures from compact range measurements at the ElectroScience Laboratory, OSU. The travelling wave behavior is also correlated to the calculated travelling wave propagation constant for this structure with good agreement.
The development of a high efficiency compact range has made it possible to consider alternative equipment for making radar cross section measurements. Historically, high power radars were required to make measurements on low efficiency, high clutter ranges. Their high power and narrow pulse capability was essential in making precision measurements. Such instrumentation is complex and expensive. There is, however, a relatively inexpensive approach which uses test equipment commonly found in the laboratory. It is centered around an HP8510 network analyzer and an RF switching network.
This paper presents the results of a study conducted to determine the effects of reflector surface errors on compact range performance. The study addressed only the reflector surface accuracy and not edge scattering, reflector illumination or reflector size. The study showed that low spatial frequency sinusoidal surface errors are significant contributors to amplitude ripple in the quiet zone field. Simple equations are presented for estimation of quiet zone amplitude ripple due to reflector surface errors. The study also presents measured surface error for two manufactures of reflector panels. The spectral (plane wave) components of the reflected field are displayed for a compact range reflector composed of a collection of these panels. *This work supported by the U. S. Army Electronic Proving Ground, Ft. Huachuca, AZ and the Joint Services Electronics program
Prime focus fed paraboloidal reflector compact ranges are used to provide plane wave illumination indoors at small range lengths for antenna and radar cross-section measurements. The "quiet zone", which is the region of space within which a uniform plane wave is created, has previously been limited to a small fraction of the reflector size. A typical quiet zone might be six feet by four feet for a ten foot radius reflector.
A compact range is a facility used for the measurement of antenna and target scattering parameters. It offers many advantages over other types of ranges, and consequently a lot of effort is being directed towards the improvement of compact range performance. This discussion focusses on the reduction of diffracted fields from the termination of the parabolic main reflector. *This work was supported in part by the National Aeronautics and Space Administration, Langley Research Center, Hampton, Virginia under Grant NSG 1613 with the Ohio State University Research Foundation.
In this paper a theoretical study is reported of the electromagnetic performance of the new Scientific-Atlanta compact range reflector system. The reflector consists of a 15-foot semicircular parabolic reflector with a 5-foot blended rolled edge added to the circular section and skirt mounted on the base. The performance of this system is examined in terms of its probed near field data at the center (36-feet) and back end (50-feet) of the target zone. The calculated results are for the three-dimensional reflector and include the skirt and blended rolled edge diffracted field as well as the aperture blockage scattering caused by the feed and associated feed/mount structure. The potential target zone size based on these parameters is presented as a function of frequency and desired ripple level requirement. *This work was supported by the National Aeronautic and Space Administration, Langley Research Center, Hampton, Virginia under Grant NSG-1613 with The Ohio State University Research Foundation.
Measurements of the Electromagnetic Field in the quiet zone of Scientific-Atlanta's Model 5753 Compact Range are presented. The Model 5753 is believed to be the largest high frequency compact range yet built and measurements demonstrate a quiet zone exceeding 8 ft. high by 12 ft. wide. Both field probe measurements and pattern comparison measurements are presented in the operating frequency range of 1-94 GHz.
The geometry of a dual parabolic cylindrical reflector system and its projection on the plane of symmetry are shown in Fig. 1. It consists of a point source f and two parabolic cylindrical reflectors S1 and S2 with curvatures in two orthogonal planes and of focal lengths F1 and F2, respectively. Alpha is the angle between the generator of the sub-reflector and the main beam direction. It is considered positive if the generator of S1 rotates towards the main reflector and negative if it rotates in the other direction. The feed orientation is specified by the angle gamma which is the angle between the feed axis and the normal from the feed point f to S1. The feed angle is 2f , which is the angle subtended by the sub-reflector in the principal planes. The sub-reflector geometry is selected such that it subtends equal angles from the feed in two orthogonal planes. The main reflector geometry is selected to intercept all reflected rays from the sub-reflector. The projected aperture of the main reflector is rectangular in shape, the sides of which are denoted as A and B. The ratio between these aperture sides is given by [1]. The separation between the two reflectors may be increased by any value delta which results in reducing the aperture dimensions. The feed radiation pattern is assumed to be rotationally symmetric and its electric field distribution in the feed coordinates is represented by cos??. If the feed is vertically polarized in the asymmetric plane (along y-axis), the y and x-components of the aperture field are the co-polar and cross-polar components, respectively. The feed may also be horizontally polarized along the unit vector [sin (?+a) i + cos (?+a) k] in the symmetric plane. In this case the co-polar and cross-polar components of the aperture field are the opposite of the above case.
Recently there has been an increasing interest in the compact ranges for antenna measurements. Most of the early attempts used lenses, but recently reflectors have become more acceptable [1]. Both dual cylindrical parabolas and symmetric or offset paraboloidal reflectors have been used as compact ranges. In this paper, the performance of both systems is studied and some of their advantages and shortcomings are presented. For both systems the aperture field distributions, under varying conditions have been determined and compared.
The advent of improved compact ranges has promoted the development of a test body, named the almond, to facilitate the measurement of scattered fields from surface mounted structures. A test body should at least have the following three features: (1) provide a very small return itself over a large angular sector, (2) provide an uncorrupted and uniform field in the vicinity of the mounted structure and (3) have the capability to be connected to a low cross-section mount. The almond satisfies the first two requirements by shaping a smooth surface which is continuous in curvature except at its tip. The name almond is derived from its surface similarity to the almond nut. The surface shaping provides an angular sector where there is no specular component. Hence, only low level tip and creeping wave scattering mechanisms are present resulting in a large angular quiet zone. The third requirement is accomplished by properly mounting the almond to a low cross-section ogival pedestal. The mount entails a metal column between the almond and the pedestal covered with shaped absorbing foam. These contoured pieces hide the column and form a blended transition from the almond to the pedestal and yet allow an unobstructed rotation of the almond. Backscatter pattern and swept frequency measurements performed in our compact range illustrate the scattering performance of the almond as a test body. The almond body alone has a backscatter level of -55 dB/m(squared) in its quiet zone. Comparisons of measured hemisphere backscattered returns on the almond are made with those calculated of a hemisphere over an finite ground plane for both principal polarizations for a verification performance test. * This work was supported in part by the National Aeronautics and Space Administration Langley Research Center, Hampton, Virginia under Grant NSG 1613 with the Ohio State University Research Foundation.
In recent years many of the problems making RCS measurements on a compact range have been addressed [1,2,3]. Factors such as ripple and taper in the target zone have been analyzed and existance of lower level effects such as stray radiation in the chamber. This paper discusses this problem and the way it was addressed in the design of the Harris Model 1606 Compact Range shown in Figure 1, 2 and 3. This range was designed to operate from 2 to 18 GHz with a six foot quiet zone with extension of the frequency range to 95 GHz possible.
The compact range has been used for many years to measure directive antenna patterns. More recently, however, there has been increased interest to use the compact range for scattering measurements. In order to provide the proper field illumination for such measurements, the traditional designs must be improved in terms of the stray signals coming from the reflector termination. One attempt to improve the field quality in the measurement zone was to use a rolled edge structure added to the basic parabolic reflector. This improved the system performance but required excessively large structures to meet the system requirements. Thus, a novel blended surface was developed which satisfies the measurement requirements without adding large structures. This new design can provide ripple levels no larger that 1/10th of a dB across the target zone as will be shown in the oral presentation.