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Measurements in the compact range system are susceptible to errors. Some of these errors are caused by chamber stray signals illuminating the target such as sidewall, backwall, ceiling and floor scattering. One of the major source of these stray signals is the feed spillover or the feed/subreflector spillover in a dual reflector system where the feed/subreflector is not isolated from the main chamber. Due to the limited chamber size, some of these errors cannot be eliminated by either hardware gating or software processing. An alternative approach to reduce these errors is by use of a feed cover such that the spillover field is highly attenuated before it can reach the target or chamber clutter sources. The feasibility of using feed cover in a compact range system to reduce the feed spillover has been studied in this paper. The effectiveness and problems associated with using a feed cover have been investigated in terms of numerical simulation and experimental measurements.
A panelized 56 by 50 foot compact range reflector with a wrap-around rolled edge treatment was installed in an anechoic chamber. Good quiet zone performance required that the as-built surface precisely follow the theoretical cosine blended contour. Commercially available CAD/CAM software served as the design platform for development of the overall system layout, rolled edge panel designs and the CNC milling machine source code for contour machining the rolled edge panels. Formed aluminum and machined composite panel fabrication techniques are described, and resulting aggregate surface accuracies as good as 1.0mil rms are presented. The use of multiple triangulating theodolites, photogrammetric measurements with peak accuracies of 0.5 mils, and custom bestfitting software used in surface alignment are described.
A code based n Geometric Optics, but applicable to diffuse surface scattering, it is evaluated for prediction of downrange high range resolution (HRR) plots of signatures generated in a compact range. A description of the technique is given, including physical justification, underlying assumptions, and flexibility of implementation. Data collected at the Hughes Compact Range will be presented in support of the analysis. Usefulness of this code in generating tradeoffs for compact range designs is demonstrated. Variations in the performance of the compact ranges are shown as a function of various range design parameters, including horn performance, chamber length, and target/wall interaction. Results are analyzed and presented in space and time domains.
A compact antenna range has the potential capability of accurately testing antennas larger than the quiet zone specified by its manufacturer. This expansion of the quiet zone can be achieved by using an analytically derived “Gain Correction Factor (GCF)” for a specific antenna under test (AUT). This GCF should be added to the gain measured in the range. A validation testing of the GCF for a 90” antenna at 44.5 GHz was successfully conducted. The antenna gain, sidelobe and axial ratio were measured in a compact range with a 4’x4’ quiet zone and in a larger range with a 12’ x 8’ quiet zone. The difference in gain was compared to the derived GCF and excellent agreement was achieved. The differences between first sidelobes and axial ratios were negligible.
In the present work, different configurations of reflector systems for indoor antenna and RCS measurements have been studied and compared. These include the Single Offset reflector, Dual Parabolic Cylinder configuration, Shaped Cassegrain, Front-fed Cassegrain and Dual Chamber Gregorian. The above comparison between the different systems is made in terms of: Configuration efficiency; Cross Polar level introduced by the reflector configuration; Scanning capability; ratio of the configuration equivalent focal length to main reflector aperture diameter and ratio of subreflector area to main reflector area; RCS background levels; phase errors due to reflectors surface roughness as a function of the frequency. In order to illustrate the above discussion, several examples of commercially available compact ranges (S.A., March, Harris) are examined, as well as some recently developed European facilities (MBB, ESTEC, RYMSA). As it will be shown, each configuration is best suited to satisfy different user requirements. For example Shaped Cassegrain/Gregorian configurations seem to be the most efficient for RCS measurements whereas the Front-fed Cassegrain quiet zone can be scanned with low degradation.
The new Compact Test Range at Dornier GmbH, operational since early 1990, is presented. The system is designed for both antenna and RCS measurements, for support of in-house projects as well as for third party measurement needs. Great emphasis has been on improving measurement through put to reduce effective measurement costs. The major system components are evaluated (anechoic chamber, compact range reflector system, RF instrumentation, positioner system, computer system and measurement software). System specifications, and where possible measured performance data are presented. Finally a typical antenna and RCS measurement are described to get an idea of possibilities together with required range time.
The Concept of Compact Test Range has been recently much used for antenna testing facilities, its main characteristic of having far-field conditions in a small and closed place, for a very large frequency band, makes it very attractive. Antenna manufacturers are building them up when the millimetric waves and the spacecraft flight model antennas become part of their activities. The change of the point of view of the antenna characteristics – now, parameters like Gain and Radiation Patterns are replaced by EIRP, Flux Density or Coverage- modifies the classical test philosophy. It makes different the Test Procedures which, in addition, have to take into account the cleanliness and the quality control required for handling flight models, as well. The Compact Payload Test Range (CPTR) in ESTEC shows up a PWZ of 7 x 5 x 5 metres for a frequency range from 1.5 to 40 GHz.; it has been created for testing whole Spacecraft Payloads in space required cleanliness area. The particular properties of the CPTR as such as shielded room, feed scanning, multiaxis test positioner, etc. are used to improve its test possibilities.
Recently, super resolution techniques have been applied to image spurious signals in compact range measurement systems. These techniques include parametric modeling of the probe data as well as eigen-space based methods. In these techniques, in incident signals on the probed aperture are assumed to be planar, which may or may not be true. In general, if the separation between a signal source and the probed aperture is more than , where D is the size of the probed aperture, one can assume that the signal incident on the probed aperture is nearly planar. It is shown that this is not necessarily true for super resolution techniques. The signal level also affects the minimum distance requirements. The stronger the signal, the farther its source should be from the probed aperture to achieve the optimum performance.
Accurate scattering and antenna measurements require excellent plane wave purity in the target zone; however all measurement systems are contaminated by various stray signals which result in measurement errors. In this paper, a technique of evaluating the stray signal sources in a compact range using a diagonal plat plate as a test target is presented. The scattering cross section of the diagonal flat plate as a function of frequency and angle of rotation is first measured. Then the time domain response for each projection angle is processed to obtain a two dimensional ISAR image of the plate as well as the stray signals. From the stray signal images, the location and relative strength of the stray signals can be determined. Experimental results from the OSU/ESL Compact Range Facility are presented to demonstrate this stray signal imaging technique.
Much research has been done recently on the interpretation of measured field probe data in order to locate and quantify error sources present in the quiet zone of a compact range. This paper examines an alternative method of analyzing those data by applying spherical phase offsets to focus the field probe data to near-field distances. This method is applied to simulated field probe data for a large compact range. The technique yield the correct [x,y,z] coordinates of multiple scattering sources deliberately introduced into the simulated data.
Based on the complexity of the scattering mechanisms associated with a real-world target, it is obvious that measurement diagnostic tools are extremely helpful. On technique that has found great success in this regard is the conventional ISAR or down range/cross range image. However, the results are basically two-dimensional, which limits the usefulness of the data in that most real-world targets have significant three-dimensional features. A very efficient class of 3D image algorithms has been developed which are based on various time domain look angles relative to the target [1]. It has been shown that one can use multiple feed antennas in a compact range to collect this data and then process it directly to obtain a 3D image of the target. This can be done very rapidly, say every 10 seconds, using an approximate solution, or in 10 minutes using a 3D ISAR approach. The system design and techniques used to implement this system are presented in this paper.
This paper demonstrates the performance of the McDonnell Douglas Technologies Incorporated (MDTI) Compact Range A. This HP8510B network analyzer based system utilizes a R-card treated prime focus main reflector in a tandem with broadband 2-18 GHz feeds. A six foot quiet zone can be maintained over the 2-18 GHz bandwidth with no feed or hardware changes, allowing targets to be measured over the full bandwidth in one continuous sweep. Measured data will be presented demonstrating performance features such as quiet zone quality, dynamic range, sensitivity, and image resolution.
Lockheed’s Advanced Development Company (LADC), located in Burbank, California, has been evaluating the capability of indoor anechoic chambers to measure VHF/UHF RCS. Two chambers were available for evaluation. A 155 feet long, 50 feet high by 50 feet wide tapered horn chamber and a compact range having dimensions of 97 feet long, 64 feet high by 64 feet wide, featuring a 46 feet wide collimator. For comparison purposes, a common instrumentation radar was used in each chamber. This radar was based on a network analyzer using a Lockheed designed pulse-gate unit to increase transmit/receive isolation. Various antenna feed system were tried in both chambers to ascertain their characteristics. Theoretical and experimental data on system performance will be presented emphasizing practical implementation and inherent limitations.
Compact range, millimeter wave reflector, serrated edges.
In 1984 MBB has started the design and development of a compensated dual reflector Compact Antenna Test Range (CATR) with a quiet zone of 5.5x5.0m (w x h). Since early 1989 this test facility is fully operational and qualified for a frequency range from 2-204 GHz.
Adaptive antenna systems will expand the test requirements for conventional antenna testing. The specific example of adaptive uplink antennas for satellite communications illustrates this required expansion. Test facilities will require additional capabilities to generate both desired and interference test signals with differing arrival directions. A novel extension of compact range technology is described for testing spaceborne designs. Instrumentation likewise will require further development for testing wide bandwidth adaptive cancellation designs used with spread spectrum modems.
Traditional range requirements are evaluated for spherical and compact range measurement systems. Based on the chamber cost to meet these requirements, it is shown that a compact range is more appropriate for targets as small as 3 (wavelengths). The commercially-available compact range systems are, however, normally restricted to 10 or larger targets. This is due to the excessive diffraction levels associated with it presently-available reflectors. It is shown that one can overcome this limitation by using a blended rolled edge reflector. For example, a 9 x 9 blended rolled edge reflector can be used to measure 3 targets at its lowest frequency of operation. As the frequency of operation increases, the test zone of the reflector approaches one half of its linear dimension.
The use of shaped reflector compact range collimators for application to indoor bistatic RCS measurements is discussed including electromagnetic performance and structural design issues. Room sizing and layout are presented for an assumed measurement system configuration. Coupling paths between the two collimators and associated time delays are reviewed for the assumed configuration and a range of bistatic angles. Collimator/chamber interaction issues are discussed. The mechanical design of the moveable collimator in a bistatic range is similar to the design of large steerable antenna structures. The same analytical tools and techniques are applied directly to the panelized reflector system, resulting in a design that will accommodate small deflections between the individual panels without permanent deformation. These conditions are not unlike the requirements for the Harris 40 foot quiet zone compact ranges to withstand Zone 4 earthquakes. The forces resulting from moving the collimator and the unevenness of the track are the input conditions to the finite element model. A real-time characterization of the collimator is provided by a laser measurement system similar to that used on the Harris compact range field probe.
In dual chamber compact range measurement systems, a Gregorian subreflector is used to illuminate the main reflector. Since the subreflector is finite in size, there will be diffracted fields from its edges which degrade the incident field on the main reflector and subsequently lead to undesired stray fields in the target zone. Some treatment of the subreflector edges is therefore required. One way to reduce the subreflector edge diffraction is to use a serrated edge subreflector. In this paper, the performance of a dual chamber compact range system with a serrated edge Gregorian subreflector is discussed. It is shown that by using the serrated edge subreflector, one can reduce the ripple in the target zone due to the subreflector edge diffraction from 3 dB to 0.5 dB. One can further reduce the ripple by separating the two chambers by an absorber fence with a small coupling aperture.
Increased productivity and higher resolution imaging capabilities are becoming of greater concern for RCS ranges. The ideal measurement scenario involves taking data on all desired frequencies for a target combination in a single rotation. This could involve one or more frequencies in several bands, imaging data on more than one band or very high resolution imaging data covering several bands. Placing several feeds in a cluster at the focal point of an offset fed com-pact range can provide these capabilities. The effects of feed clustering such as beam tilt are discussed along with cluster sizes that provide little if any degradation in compact range performance. Experimental data is shown that gives an indication of the quality of data that may be obtained. The concepts are also applicable for outdoor ranges that have an array of antennas offset from range boresight.