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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.
A wide variety of precise, automatic instrumentation systems is currently available for RCS testing. These systems, either commercially available as integrated units or assembled from laboratory test instruments, can automatically measure the RCS of a target over fine frequency increments spanning wide bandwidths. When the frequency responses are measured for discrete increments of target rotation, the resulting two-dimensional (frequency-angle) data arrays can be processed to obtain two-dimensional RCS images.
Near-field testing of a very low sidelobe, L-band, 32-element, linear phased array antenna was conducted. The purpose was to evaluate testing and calibration techniques which may be applicable to a much larger, space borne phased array antenna. Very low sidelobe performance in a relatively small array was achieved by use of high precision transmit/receive modules. These modules employ 12-bit voltage controlled attenuators and phase shifters operating at an intermediate frequency (IF) rather than at RF. Three array calibration techniques are discussed. One technique calibrates the array by means of a movable near-field probe. Another method is based on mutual coupling measurements. The last technique uses a fixed near-field source. The first two calibration methods yield substantially the same results. Module insertion attenuation and phase can be set to 0.02 dB and 0.2 degrees, respectively. Near-field measurement derived antenna patterns were used to demonstrate better than -20 dBi sidelobe performance for the phased array. Application of increasing Taylor array tapers showed the limitations of the measurement systems to be below the -35 dBi sidelobe level. The effects of array ground plane distortion and other array degradations are illustrated.
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.
A planar near-field scanner is described. It has an effective scan plane of more than 5 by 12 meter. The scanner will be used for the measurement of the Synthetic Aperture Radar (SAR) antenna of the European Remote Sensing satellite ERS-1. The requirements are discussed and the results of the first mechanical verification measurements are presented.
A major problem in direct broadcasting satellite (DBS) communication systems is the interference caused by transmission from adjacent satellite whose signals inadvertently enter a ground station receiver and interference with a desired signal. However, the interference may be as much as 25 dB below the desired signal level. In fact, the interference may be below the thermal noise in the channel. Although weak, these signals because of their coherent nature and their similarity to the desired signal, do cause objectionable interference and must be suppressed. The interference is of the nature of "ghosts" in a television signal.
At the National Bureau of Standards (NBS) we have examined the out-of-band response of array antennas from both a theoretical and experimental point of view. Theory shows that the out-of-band response of an antenna depends primarily on two factors: the antenna's input impedance, and its directivity. Experiment shows that, for most practical purposes, the out-of-band response of an antenna can be estimated from a measurement of the antenna's input reflection coefficient alone. If the reflection coefficient is low, the antenna response will be good; if the antenna coefficient is high, the antenna response will be poor.
The evaluation of adaptive antenna systems expands the scope of conventional antenna testing. In addition to the conventional antenna parameters, the evaluation of an adaptive antenna system measures the effectiveness with which the adaptive antenna reduces interference. Adaptive antenna testing is conducted on a system level rather than the component-level tests of conventional antennas. The expanded scope of adaptive antenna testing requires more general test facilities, instrumentation, and test procedures. The additional requirements for adaptive antenna testing will be discussed.
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.
A technique has been developed to characterize the illuminating signal present within an antenna test zone. Information of angular multi-path distribution as well as relative signal amplitudes of various paths can be ascertained by transforming phase and amplitude data measured at numerous intervals across the lineal aperture probe apparatus. An experiment was carried out to test the technique using a ten-foot linear aperture probe installed to probe an antenna test zone located at one end of a one-half mile range. During the experiment several measurements were carried out at two different locations within the far-field antenna range and at two different frequency bands. This paper discusses the results of the experiment as well as the practical aspects of this technique.
Near-field antenna measurements have many advantages, but also some limitations, which can be mainly attributed to the need for costly facilities or severe environmental effects. Although anechoic chambers are widely employed, absorbing material is very expensive and the whole construction becomes a considerable project, especially if it is required to accommodate various size antennas over wide frequency ranges.
A spherical backward transform technique has been developed and applied to the determination of radome anomalies from near-field measurements. This paper reports on this technique and presents measured data for a missile radome.
An antenna radiation pattern measurement technique which allows near real time pattern capture is presented. This technique uses relatively simple field probles and detectors to cover a reasonably broad operating band. The captured pattern data is digitized with a resolution of 1.0 degree and has an angular range of 150 degrees. Many captured patterns or snap-shots could be recorded during a given time interval and later viewed for diagnostic evaluations where rapid changes in the pattern are expected.
This paper describes a novel technique for acquisition of far-field antenna patterns from a very low side-lobe antenna. The low side-lobe requirement imposes stringent multipath restrictions on the measurement range and to accommodate this requirement a vertical range configuration is employed rather than the more conventional range which is parallel to the earth's surface. To assure accurate measurement of side-lobe levels, multipath levels were specified at minus seventy dB (-70 dB) relative to the direct-path, peak-of-the-beam level. In this novel range configuration, an Antenna Under Test (AUT) is oriented to face skyward and operated in a receive mode with E-Field illumination provided from an airborne source. An optical tracker provides data of airborne source location and time-division multiplexing of both frequency and antenna beam position enable optimization of data acquisition efficiency. Post-acquisition processing provides de-interleaving of the desired beam(s)/frequency(s). This paper will present a discussion of the problems encountered and the techniques employed to overcome them in the design of this range. A description of the range will also be presented.
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.
March Microwave Systems B.V. is manufacturer of the dual cylindrical reflector Compact Antenna Test Range (CATR) that was designed by Vokurka [1] (see Fig. 1). The analysis of the test-zone fields of such a design is necessary to be able to optimize the geometry.
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.