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The development of the Harris 1640 compact range required significant technical advances in developing a method of constructing a 70 foot reflector to a 0.005 inch RMS operational surface accuracy. A panelized approach is believed to be the only practical way to achieve this level of accuracy. Four technology areas had to be developed, adapted to this use, or have their current limits extended. A method was required for reducing the RF shaping data to individual panel contours. The reflector has no axis of symmetry thus each panel has a unique contour and the description of each contour requires complex mathematical interpolation. A new fabrication technique was needed to produce 0.002 inch RMS panels. Positioning and initially aligning the panels would require the adaptation of multiple theodolite techniques. The final setting of the panels would then require the use of a photogrammetric measurement system, the most accurate method available.
When Harris first undertook to produce large compact ranges made up of multiple panels, a significant capability to be developed was the production of precision panels (< 0.002 inch RMS). Sandwich construction was chosen as the fabrication technique due to its excellent stiffness to weight ratio and the ability to support the reflective skin over its entire surface. A number of studies were then conducted to determine the optimum skin thickness, honeycomb core thickness and the number of adjustment points. An adjustable bonding fixture was also designed to accommodate the shaped reflector characteristic of each panel having a different shape. The results of those studies provided a fabrication process that has yielded 0.001 inch RMS panels. The process and the monitoring guidelines have yielded 224 acceptable panels of 225 fabricated to date.
For over a decade the National Bureau of Standards (NBS) has used the planar near-field method to accurately determine the gain, polarization and patterns of antennas either transmitting or receiving cw signals. Some of these calibrated antennas have also been measured at other facilities to determine and/or verify the accuracies obtainable with their ranges. The facilities involved have included near-field ranges, far-field ranges, and compact ranges. Recently, NBS has calibrated an antenna to be used to evaluate both a near-field range and a compact range. These ranges are to be used to measure an electronically-steerable antenna which transmits only pulsed-cw signals. The antenna calibrated by NBS was chosen to be similar in physical size and frequency of operation to the array and was also calibrated with the antenna transmitting pulsed-cw. This calibration included determining the effects of using different power levels at the mixer, the accuracy of the receiver in making the amplitude and phase measurements, and the effective dynamic range of the receiver. Comparisons were made with calibration results obtained for the antenna transmitting cw and for the antenna receiving cw. The parameters compared include gain, sidelobe and cross polarization levels. The measurements are described and some results are presented.
Naval Aviation Depot has completed final testing requirements for its unique antenna/radome test ranges. This paper provides an overview of the Navy's one of the kind test facility. Justification for our compact range facility was based on the requirement of a functional range during adverse weather conditions. Prior to 1986 all testing of antennas was completed at our outdoor range (Figure 1). This mode of operation was inefficient during periods of rain, snow or high wind conditions. These conditions are normally present during the winter months along the east coast (Dec-Mar). Testing antennas under these conditions could possibly lead to inaccurate data and damage to the antenna under test. Another requirement was to reduce the turn around time for repair and testing of radomes. This new test range will provide a significant reduction in the Navy's repair cycle, since no time is lost in certifying repaired radomes.
A method for the calculation of the errors induced through target-wall-target interactions is presented. Both near-field and far-field situations are considered. Far-field calculations are performed both with Fraunhoffer diffraction theory and target antenna analogies. Absorber is considered as both a specular and a diffuse scatterer. The equations developed permit trade studies of chamber size versus performance to be made.
The direct far field pattern measurement of an aperture antenna becomes more difficult as the size of the aperture increases. Recent measurements on reflector antennas with 2D2/? =1500’ at The Ohio State University ElectroScience Laboratory have demonstrated the usefulness of the compact range in obtaining the complete far field pattern of antennas with large far field distances.
Techniques developed for the design of shaped, off-set reflector antennas have been applied to the design of compact ranges. Shaped optics which map an axially symmetric feed pattern into an elliptical aperture distribution have been designed. Some of the major design considerations for this type of system are examined in this paper. The design has been verified both analytically and experimentally.
This paper reports on work conducted to improve the test zone performance of an offset-fed point-source compact range by shaping the edge serrations of the paraboloidal range reflector.
Current demands for accurate low-level radar cross section (RCS) measurements require anechoic chambers and compact ranges to have extremely low background scattering levels. Such demands place difficult requirements on the entire chamber and warrant the need to predict and mathematically model chamber performance. Accurate modeling, prior to chamber construction, also aids in chamber performance optimization through improved chamber designs.
A way has been found to utilize the reflector return in a compact range as a source of continuous drift compensation. This is performed by translating receive polarizations 45 degrees with respect to the transmit polarizations to ensure returns in co- and cross-polarizations. An added benefit is the simplicity of alignment for the polarization calibration standard.
Harris Corporation is in the final stages of implementing the Model 1640 compact range for the Boeing Corporation. This paper provides an overview of the development, fabrication, and test activities on this very significant advance in the compact range state-of-the-art. This range represents a significant increase in the quiet zone size over past available equipment. It features the wide dynamic range, low noise floor, and high quality quiet zone that is achievable using the Harris-Proprietary shaped compact range technique. In this technique, a dual reflector system is used so that quiet zone characteristics may be completely optimized. Another feature of this range is the completely panelized construction techniques. This allows the production of a very large, very precise reflector system. When completed in the winter of 1987-1988, the Model 1640 will represent a new dimension in compact range technology. Primary technical features are a 40 foot quiet zone, a -70 dBsm noise floor, and a frequency range extending from VHF to millimeter-wave frequencies.
Reducing ripple in the aperture field of the parabolic reflector is one of the main considerations in the design of a compact range, since it determines the "usable" target zone for RCS and antenna measurements. The usable target zone is typically defined as the aperture region where the ripple is less than 0.1 dB [1]. Studies [2,3] have shown that edge diffractions and therefore ripple can be significantly reduced by using blended rolled edges such as in Figure 1. For low aperture field ripple, it is assumed that the junction between the parabolic surface and the blended rolled edge is smooth. In practice, however, the rolled edges may be machined separately and then fitted to the main reflector. If this is done, small wedge angle errors (Figure 2) or step discontinuities (Figure 3) may be mechanically introduced at the junctions. Typically, angle deviations of plus-or-minus 0.5 degrees and steps of plus-or-minus 0.005 inches may be expected. If the parabola and part of the rolled edge is machined as a unit, diffraction due to discontinuities in the mechanical junction between this surface and the rest of the rolled edge can have less effect on ripple in the aperture field. Now, the questions to be answered are: * How much of the target zone is lost due to discontinuities at the edge of the parabola? * How much of the rolled edge need to be machined with the parabola to prevent mechanical discontinuities from decreasing the usable target zone? * What range of discontinuities can be tolerated? In this paper, these questions are answered for a 12 foot radius semi-circular compact range reflector with cosine-blended rolled edges.
A key element in the performance of the Harris compact range is that the mathematical shaping of the main and subreflector maximizes the percentage of the total radiated energy collimated in the quiet zone. This extra measure of performance doesn't come without an impact on other areas of the design. Specifically, the use of non-geometric shapes means that for large reflectors, where the surface must be segmented for fabrication accuracy, the shape of each segment is unique. Thus, the traditional method of forming each reflector segment, or panel, on a hard surface tool, or bonding fixture, becomes prohibitively expensive for large systems that consist of over a hundred panels in the two reflectors. The development of an adjustable bonding fixture that can be accurately set to the mathematically defined shape for each panel has made the Harris approach to compact ranges achievable. The use of high accuracy coordinate axis measuring machines to refine and verify the surface of each panel has then made the approach producible. The measurement machines have critical axis accuracies of .0005 inch that provide the capability for verifying .001 inch RMS panel accuracies.
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].