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An RCS measurement system based on the HP 8510 and a Compact Range reflector system has the following limitations: high clutter levels limit the maximum transmit power and therefore the system's sensitivity, the maximum number of frequency points limit the maximum resolution and/or range length, and the proper separation of clutter and test target data requires taking data describing the entire range, even for a desired CW measurement, thus increasing measurement times significantly.
Characteristics of the Harris Model 1606 Compact Range are summarized and considered for applicability to RCS measurements. Measured characteristics of quiet zone performance (amplitude and phase distributions) and standard target RCS data are presented. Of particular interest is a comparison of predicted and measured radar cross section versus aspect angle of some familiar standard targets under various conditions.
During the development of the McDonnell Aircraft Corporation (MCAIR) compact range a low back scatter field probe was built to evaluate the range's performance using realistic signal levels. The probe was built using off-the-shelf electronics and a standard Hewlett Packard desk top computer system to drive the probe and record the data. The mechanical components were designed to be easily assembled and quickly mounted upon a low cross section pylon.
A new dual chamber concept using a Gregorian subreflector system is being proposed for compact range applications. This concept places the feed and subreflector in a small chamber adjacent to the measurement range which contains the main reflector and target. These two chambers are connected together by a small aperture opening which is located at the focus of the main reflector. This system can potentially provide improved taper, ripple, and polarization performance. Because it uses a subreflector, the main reflector focal length can be decreased without a loss in performance. This in turn reduces the minimum length requirement for the main chamber. The design of this type of system plus the test results that have been performed will be presented at the conference.
Harris Corporation has developed and introduced a miniature version of its shaped compact range called the Model 1603. This model is actually a scaled version of its very large compact ranges. The range features a three foot quiet zone in a very compact configuration, allowing the range to be set up in an anechoic chamber as small as a normal conference room. Performance features are equivalent to those achieved in large compact ranges by Harris, such as the Model 1640 with a forty foot quiet zone. Key features are very low quiet zone ripple, extremely low noise floor, and low cross polarization. This range can be used for the full gamut of precision RCS testing of small models or precision testing of antennas. It should also find wide application in production testing of these items. Harris can also provide turnkey compact range test systems based on the Model 1603 that use available radar instrumentation. Several of these miniature compact ranges have been delivered and are in use.
The Georgia Tech Research Institute is designing a large outdoor compact range for the U. S. Army Electronic Proving Ground at Ft. Huachuca, Arizona. This range will be used to measure patterns of antennas installed on aircraft and vehicles. The goal of full hemispherical coverage with vehicles weighing up to 140,000 pounds has resulted in a unique positioner design, described in this paper. The 5-foot diameter quiet zone is centered 42.5 feet above the ground. The positioner's azimuth over elevation geometry keeps even large systems inside the quiet zone through the full range of positioner motion. The turntable is driven in continuous azimuth rotation by a hydraulic motor. The tilt table is driven through its -1 degree to +91 degree elevation range by two hydraulic cylinders. The tower is designed to carry a 140,000 pound vehicle in a 100 MPH survival wind. The structure consists of two steel frames, joined at the top. Both are enclosed in sheet metal shells to minimize scattering into the quiet zone.
It is well known that the compact range can be and has been used very successfully for scattering measurements. Recently, the compact range at The Ohio State University ElectroScience Laboratory was used to measure the patterns of two 8-foot diameter reflector antennas and their microwave horn feeds. Very good measurements have been achieved. In the paper, the results of the horn antenna measurements are presented while the results of the reflector pattern measurements are discussed in another paper. [1].
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.