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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.
The aperture opening design of the subreflector chamber for a dual-chamber Gregorian compact range system is presented in this paper. The subreflector is a serrated edge ellipsoidal reflector. The performance of the subreflector chamber and absorber aperture opening has been evaluated in terms of pattern measurements and by cross-range diagnostic techniques. The results of this evaluation have been used to further improve the design of the aperture opening of the subreflector chamber.
A plane wave spectrum method of analysis is employed to examine a hybrid approach to compact range reflector design. In order to reduce edge diffraction, an illumination taper is used in conjunction with a serrated reflector. The optimum illumination taper is determined for several serrated reflector geometries. Maximum quiet zone is the optimality criterion. The aperture illumination functions considered are -symmetric, cosinudoidal in amplitude, and uniform in phase. The reflectors considered are characterized by a circularly periodic aperture boundary. The analysis is restricted to the low frequencies at which diffraction effects are most prominent.
A new approach has been developed to achieve an octave bandwidth, reduced size feed fot compact range reflectors. It can provide highly isolate, orthogonal polarizations with a minimal size, suitable for operation at frequencies down to 500 MHz and below. Its construction is relatively simple, with only a few specific dimensions. The beam-width is compatible with compact range reflector feed requirements. The method uses crossed dipoles over a small circular ground plane, with a rim to equalize the E- and H- plane patterns. Parasitic elements are employed to extend the bandwidth with matching provided via a section built into the feed line. The design was optimized using the Numerical Electromagnetics Code (NEC) computer program.
To improve measurements at lower signal levels and/or reduce the size of the compact range chamber, absorber with much better scattering performance is required. This high performance absorber can be realized by introducing multiple layers to obtain a better impedance transition from air to the absorber. The inhomogeneity leads to the use of the Moment Method. However, the truncated ends of a finite absorber panel produce a scattering so strong that the edge and valley diffractions from a typical wall of absorber cannot be recovered. Thus, an approach to solve and infinite wall of identical wedges has been developed for the TM case using the Periodic Moment Method (PMM). In this paper, PMM will be briefly discussed. Then, some interesting designs will be presented, including ordinary wedge absorber with different dopings, wedge widths and wedge heights, wedges with curves surfaces, and multi-layer wedge absorber designs.
With the need for shielding anechoic chambers on the rise, the costs associated with shielding the facility is also on the rise. Often a welded or modular 100 dB type of construction is utilized due to the need for an RF “quiet” environment, coupled with a variety of shielding specifications due to program classification levels. But is this overkill? Can the security and ambient concerns be more cost effectively addressed? What are the latest products on the market that can meet the changing needs of the security community? This paper will address a new RF shielded system that will meet both the upcoming regulations for low level TEMPEST security as well as the need to keep the shielding costs down. The system consists of a nonwoven fiber which is applied like wallpaper. It will consistently give 50 dB performance and actually improves as he frequency goes higher. Architectural details and the cost tradeoffs will be displayed and discussed.
This paper addresses the problem of absorber scattering into the target zone of a compact range. An approximate UTD lossy dielectric corner diffraction coefficient is found, and is used to calculate the bistatic scattering from the tip of an absorber pyramid. Scattering into the target zone of a compact range from the pyramidal absorber lining the room is then investigated, for both rolled edge and serrated edge reflectors, and is compared to the levels of the direct reflector diffractions. To build confidence in these absorber scattering predictions, calculations are compared with measurements of the bistatic absorber scattering in a compact range.
Comparative measurements have been made in a compact range to determine the performance improvements that can be achieved when adding a hardware gate to a CW-based measurement system. Starting with conventional stepped frequency CW measurements made in the time domain mode, high resolution downrange data was collected to determine the background levels of the compact range. This was followed by comparative measurements under the same conditions adding a narrow pulsed hardware gate to reject inter-horn coupling and high returns from the compact reflector. A second mode of comparison was examined by collecting aspect data with a specific range gate fixed about the target. Software gated measurements required more points to insure alias free operation, while the hardware gated measurements allowed fewer points which reduced measurement time without sacrificing any accuracy. Finally, imaging measurements were made with both software and hardware gating to compare the measurement time and accuracy
This paper reports on a study undertaken to assess the effects of range amplitude tapers on the measurement of low and ultra-low sidelobe levels and gain. It has been shown that low test zone phase tapers are required for the measurement of low and ultra-low sidelobe levels. A few papers have addressed the effect of amplitude errors but not for the measurement of low sidelobe levels. These papers have concluded that amplitude errors have much less effect than phase errors. This paper addresses antenna measurement ranges such as compact ranges where phase taper has been significantly reduced, but amplitude errors remain. The amplitude taper on some modern compact range configurations has not only, not significantly improved, it has often taken on a more complicated “double hump” shape. The effects of these modern amplitude tapers are demonstrated.
Compact range systems have been widely used for high quality RCS measurements. However the taper and cross-polarization effects can lead to significant measurement errors especially as the target approaches the border of the target zone. The taper error is mainly caused by the feed’s finite beamwidth, and the cross-polarization error by the feed’s cross-polarized radiation and the offset configuration of the reflector. A method to correct these errors is presented. In order to perform taper and cross-polarization error corrections, one has to be able to predict the target zone fields and determine the locations and complex strengths of the various scattering centers associated with the target. The correction can then be done by compensating for the taper and cross-polarization effects for each localized scattering center. Several measurements have been taken, corrected and then compared with the theoretically expected results to validate this technique.
Amplitude taper removing by software implementation has been made beyond the quiet zone region of a compact range reflector where the phase variation is still small. To remove amplitude taper effect in RCS measurement, actual amplitude taper of the range s first obtained by theoretically calculating the field distribution from the given range geometry and confirming with field measurement result. The processed target RCS contour is later implemented with the actual amplitude distribution around the region where the target is located. It is found that with the software implementation of amplitude taper removing the effective quiet zone of the compact range has been able to extend up to the size of the reflector diameter.
As the advances in silicon technology continue to redefine the realm of “practical” for scientists and engineers, traditional techniques for acquiring measurements and processing the exceedingly large data sets generated must be constantly improved, and often times discarded as new concepts replace them. The new class of SuperWorkstations available today provides a convenient means to not only maximize the performance of the compact range instrumentation, but also suggests entirely new techniques and algorithms in data acquisition, storage, processing and interpretation. In considering these advances available through SuperWorkstations, benefits in the area of measurement data acquisition and local storage are detailed, recent improvements in magnetic and visual storage techniques and their application to data archiving are considered, new and unique techniques for scattering center identification in near real time are presented, and finally a discussion of tomorrow’s computer technology and the further impact on the compact range completes the study. This paper examines the efforts currently underway to exploit one such superworkstation, the Tektronix XD88, in the compact range at the ElectroScience Laboratory. In the effort to effectively utilize the superworkstation, many disciplines are coupled together (hardware, software, graphics, video presentation, among others) to augment each other. It is this multidiscipline coupling that will serve to expand the realm and utility of SuperWorkstations in the compact range, and the goal of this brief introduction is to present some aspects of these varied areas to the reader, hopefully motivating the reader to consider further extensions of SuperWorkstations.
This paper deals with design and evaluation of Compact Range Antenna and RCS measurement systems. Reflector subsystem and feeders design as well as quiet zone evaluation and system performance qualification are considered. Acquisition, process and presentation software to control the whole system has been developed and successfully implemented. Two systems have been designed and are now at implementation stage. A Gregorian concept Compact Range is now been constructed at RYMSA (Spain). This facility has been fully designed by IRSA and will be operative by the end of 1990. Compact Payload Test Range (CPTR) at ESTEC (ESA) is now been tested. System Instrumentation and PAMAS (Payload and Antenna Measurement and Analysis Software) have been developed.
The quiet zone performance of the Harris 1606 compact Range Collimator has been reported in the literature for 2 through 35 GHz 1,2. This paper discussed our achievements in the past year with the 1606 at 95 GHz. We will summarize the improvements in our fabrication and alignment methods that have yielded excellent performance at these frequencies using an intermediate size multi-panel main reflector. Quiet zone performance data will be presented from recent measurements on the Millitech Corporation’s Millimeter Wave Antenna Test Range in South Deerfield, MA and from the Harris 1606 Capital test equipment range.