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The principle of time-gated antenna measurements is differentiation of signals by time-of-arrival. On a far-field antenna range, all reflected stray signals arrive later in time than the direct path signal. A pulse-modulated waveform from the source antenna can be gated at the receiving antenna-under-test to produce a response to the wanted signal only.
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.
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].
Data are presented on the normal reflectivity of several commercially available microwave absorbing materials at frequencies of 8-18, 35 and 95 GHz. The materials tested were mostly of the carbon-loaded urethane foam type with non-pyramidal surfaces, ranging in thickness from 1/4" to 1". All testing was performed in the Millimetre-wave Scattering Range of the Boeing Military Airplane Co (BMAC) of Seattle, WA. * Current affiliation; this work not performed at NBS.
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.
Motorola's Government Electronics Group (GEG) located in Scottsdale, Arizona has recently completed construction of an indoor Antenna/RCS Test Facility. Motorola achieved quality construction of this new facility by utilizing local building contractors working under Motorola supervision through concept study, design, and construction phases. Motorola achieved quality chambers without turn-key costs. Three anechoic chambers and one shielded computer room were fabricated. The chambers sizes vary from 20'W x 16'H x 41'L to 36'W x 36'H x 72'L. All chambers were evaluated using techniques described by MIL-STD-285 (Attenuation measurements for enclosures, electromagnetic shielding, for electronic test purposes, Method of) and indicated shielding effectiveness, before absorber installation of -60 to -70 db at 400 MHz and -80 db from 1-18 GHz. Shielding effectiveness increased to -80 dB at 400 MHz and to greater than -115 dB from 1-18 GHz after absorber installation. In addition, the building contains eight individual security areas meeting government standards for security as prescribed in the Defense Intelligence Agency Manual (DIAM 50-3).
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.
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.
This paper describes methods commonly used by anechoic chamber manufacturers to characterize chamber performance. Test procedures depend first on the purpose of the test; second on the purpose of the anechoic chamber and third on the amount of information required. Most anechoic chambers are built for a specific use. In order to prove its design, the test will be done accordingly. In most anechoic chambers one measures the reflectivity level because this is a measure for the accuracy on future measurements when the chamber is in operation. Anechoic chambers can vary from Antenna Pattern Test Chambers to Radar Cross Section Test Chambers, Electronic Warfare Simulation Chambers and Electro Magnetic Compatibility Test Chambers. Each type of chamber will have its specific evaluation technique. Some techniques can be done by the chamber user himself. Other methods need some special equipment that will or can only be used for that particular test method. Some customers want to do their own calibration on a regular basis. They can purchase this special equipment from the chamber manufacturer, if necessary. More complicated methods make use of computer controlled equipment. The data required can be taken in the chamber. This can be done relatively fast. All sorts of information about the chamber characteristics can be obtained in a later stage in a different format by use of the right software. This paper gives possible evaluation methods for different types of anechoic chambers. Detailed information about each method can be obtained from Emerson & Cuming.
This paper presents both radar cross section and polarization scattering matrix measurements on microwave radar navigation targets. The polarization measurements are performed using a unique two-channel facility which allows for measuring the circularly polarized scattering matrix elements at X-band. For the same targets conventional RCS measurements are performed using an automated system comprising a network analyzer (HP-8510) and a desk top computer system (HP-236 or 310). This system allows wide frequency range measurements. Details of these measurement techniques, and results will be presented.
This paper describes an automated, frequency-step, pulsed/CW Radar Cross Section (RCS) measurement system using the HP 8510 network analyzer. The system has been built using the concepts developed at Lincoln Laboratory (1) and is being utilized in an operational capacity. The unique features of this system are the use of (a) a dual-probe antenna for the transmission and reception of RF signals, and (b) a pulse system for separating the target-scattered signals from the incident and background signals. The single antenna configuration provides a true monostatic backscatter measurement. A polarization control circuit makes RCS measurements for all combinations of transmit/receive polarizations possible (linear and/or circular). The pulse system uses pin-diode switches capable of generating a 7-ns pulse width and a repetition rate up to 8 MHz. The pulse system effectively eliminates unwanted signals at ranges other than the target range. Therefore, the full dynamic range of the receiver can be used for the measurement of the target.
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.
Narrow band RCS measurements are usually presented as RCS versus target aspect angle in either a rectangular or polar format. Wide band measurements are not normally analyzed in the frequency domain. The normal procedure is to perform either a one or two-dimensional Fourier transform of wide band data or obtain high resolution information on the location of scattering sources. In this paper, we investigate the possible uses of the wide band data directly. In particular, we show that a natural coordinate system for analysis of these data is a polar format with frequency taking on the polar distance parameter and aspect angle taking on the polar angle parameter. This format is not coincidentally, an intermediate step in the production of fully focused two-dimensional radar images. The polar format frequency domain plots are shown to be effective at categorizing the nature of the physical scattering. This is especially true when combined with image domain filtering to isolate scattering regions of interest. In addition, it can be useful in determining anomalies in the radar measurement system performance, and in assisting the analyst to explain unexpected image domain results.
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.
A pulsed, coherent radar system was used in the inverse synthetic aperture radar mode to obtain 1-way high resolution images of simple antennas. These high resolution images display the amplitude and phase distribution of the received wave. The images were then edited and reconstructed using System Planning Corporation's Image algorithms contained in the SPC RPS software package. The 2-D (range vs. cross range) image data is very useful for detecting defects in antennas and can also 0be applied to modification of illumination conditions such as wavefront sphericity (phase taper) and/or amplitude variabilities (taper, ripple). This technique offers an alternate approach to near field/far field transformation. The technique involves rotation of the antenna under test at a controlled, uniform rate. The antenna port is connected to the radar receiver and the radar transmitter attached to an illuminating antenna. The radar transmits a step chirp wave form. The received signal is recorded to tape and processed off-line on the SPC Image Reduction Facility. A calibration technique was developed using simple wide bandwidth horn antennas. The downrange and cross range resolution of these 1-way ISAR antenna images is half as large as with 2-way radar ISAR for the same bandwidth and angular integration interval. Image data will be shown on reflector-type antennas to illustrate the technique.
Microwave holographic metrology is considered to be a key technique for achieving improved performance from large reflector antennas, especially at the shorter wavelengths. An important benefit of microwave holography is that the mathematically transformed data yields precise information on panel alignments on a local scale [1-5]. Since the usage of the holographic technique requires both the amplitude and phase data of the measured far-field patterns, one must carefully assess the impact of systematic and random errors that could corrupt the data due to a variety of measurement error sources.