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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.
A 75 foot diameter offset paraboloidal outdoor compact range reflector was designed for operation up to 95 GHz and installed at Ft. Huachuca, Arizona. The need for high frequency operation required that a highly accurate reflector surface be maintained in the desert’s harsh thermal and wind environment. The use of thermal modeling to predict the temperature distribution in the structure, along with extensive finite element analysis to determine the structure’s distortions from thermal, wind and gravity loads were integral to the reflector design. Using the above tools, thermal isolation techniques were developed to minimize the harmful effects of the thermal environment on surface accuracy. A surface error budget based upon both calculations and measurements shows an overall rms error of 4.9 mils under optimal environmental conditions, degrading to only 6. Mils under the worst operating conditions.
A major objective in the design of an RCS measurement facility is to obtain the greatest possible productivity (overall measurement efficiency) while maintaining the accuracy and sensitivity necessary for low radar cross section targets. This paper will present parameters affecting the total throughput rates of an indoor facility including instrumentation, target handling, and band changes-one of the most time consuming activities in the measurement process. Sensitivity and accuracy issues to be discussed include radar capabilities, feeds and feed clustering, compact range, background levels, and diffraction control.
This system provides improved techniques for controlling positioning axes, and secure transmission of position data from remotely located control systems. Advancements in controls technology have allows more complex configurations for use in the manipulation of RCS targets in indoor ranges. This paper will discuss a unique system design that provides automated testing and positioning of RCS test bodies. The current system uses seven axes of motion, and allows for simultaneous motion as well as synchronous motion of any axis pairs in the system. These axes include Target azimuth and elevation, Pylon azimuth and elevation, Upper and Lower turntable azimuth, and carriage linear drives. In addition, the concepts of secure data transmission through the use of specialized fiber optics are addressed. Finally, a complex set of safety interlocks and man and machine protection is discussed. The entire system is currently implemented and running in the Boeing range.
The Hughes Aircraft Company recently completed the design, development, and construction of a new engineering facility that is dedicated to providing state-of-the-art Radar Cross Section Measurements. The facility is located at the Radar Systems Group in El Segundo, California and consists of two secure, tempest shielded anechoic chambers, a secure high bay work area, two large secure storage vaults, a secure tempest computer facility, a secure conference room, and the normal building support facilities. This RCS measurement test facility is the result of Hughes committing the time and money to study the problems which influence user friendly RCS measurement facility design decisions. Both anechoic chambers contain compact ranges and RCS measurement data collection systems. A description of the facility layout, instrumentation, target handling capability, and target access is presented.
Lockheed’s Advanced Development Company (LADC), located in Burbank, California, has recently completed construction of a state-of-the-art indoor Antenna/RCS test facility. This facility is housed in a dedicated 40,000 square foot building which is a maximum of 80 feet high. This building contains three anechoic chambers providing Antenna/RCS measurement capability from 100 Mhz to 100 Ghz. The largest chamber, with dimensions of 64 feet by 64 feet by 97 feet is configured as a compact range. This chamber utilizes the largest collimating reflector that Scientific-Atlanta has ever constructed. Primary test usage of this chamber is for RCS measurements in the frequency band of 700 Mhz to 100 Ghz. The second chamber is configured as a tapered horn test range. Its dimensions are 155 feet long with a 50 foot by 50 foot by 55 foot volume measurement zone. This chamber is utilized for RCS tests in the VHF, UHF, and L frequency bands and antenna tests from 100 MHz and up. The third chamber, with dimensions 14 foot by 14 foot by 56 foot, is a far field chamber designed to check out and evaluate small items up to 100 GHz. The entire facility has been designed to maximize efficiency, minimize the cost of operation, and produce outstanding quality data from Antenna/RCS measurements. A number of innovative techniques in model handling, model access, and model security were incorporated into the facility design. These features, as well as utilization of unique Lockheed designed and built pylons, allowed achievement of all these goals.
An HP 8510B based RCS measurement system is presented. It can be operated in CW, hardware gating, and fast-CW modes. A VAX-3800 computer and a MAP 4000 array processor are used to speed up the data analysis and a PS 390 graphic system is used to display graphic. Three ISAR techniques, i.e., DFT approximation, focusing image processing, and diffraction limited methods, are available in the analysis program to get the target image. With an amplitude taper removing technique, this system can measure large target whose size is almost up to the size of compact range reflector.
A near field synthetic aperture imaging technique using three main beam suppression methods is used to locate and quantify the sources of stray signals in a compact range. First, main beam cancellation by subtracting the complex average of the measured field for the overall probe aperture is used. Second, a software on-axis null is generated by preprocessing the data. Third, an antenna with a broadside null is used as the prober. It is shown that the software on-axis null enhances the resolution of the spurious scatterer images and is able to detect small spurious scattering centers, such as the surface discontinuity at the top of the reflector, which are otherwise undetectable. Probe data with two metallic tapes placed on the compact range reflector is used as another example to show the performance of the nulling technique.