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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.
A mini compact range system has been built for NASA, Langley Research Center. The performance of the system was evaluated at the Ohio State University by probing the fields along a vertical cut and a horizontal cut. The probe data showed that the target zone fields contain stray signals, which do not originate from the reflector surface. The probe data was imaged to locate the sources of the stray signal. Both conventional Fourier techniques as well as the MUSIC algorithm were used to image this data. The results of this study are discussed in this paper. It is shown that at the back end of the chamber, the absorber scattering can be quite significant. The aperture blockage due to the feed structure also contributes to stray signals in the target zone.
The extraneous signals that perturb antenna patterns can be found and identified by a method known as “longitudinal translation at selected points”. The method is usually applied to certain selected angular points on the antenna pattern. With this technique the composite pattern – consisting of the direct-path signal and the reflection signal – is measured at a series of translation distances along the axis of the antenna range. By utilizing both the amplitude and phase of the received signal, one can remove the signal that results from stray reflection and retain the desired direct path signal. The result is an improved and more accurate version of the pattern. In this presentation I review this technique as specifically applied to compact range antenna measurements, and apply it to several patterns, to demonstrate the method.
Comparisons of predicted and measured phase and amplitude data have been made for the Model 1640 compact range. The predicted data is generated using either a physical optics model for shaped offset reflectors with a feed model which takes the corrugation depth and spacing into account or a similar geometric optics model. Excellent agreement has been obtained including accurate prediction of the effects of axial feed movement.
This paper summarizes the performance of the new Harris 1612 Compact Range Standard Product. The Harris 1612 is a dual shaped reflector collimator system. Measured data, both amplitude and phase from the 12-foot diameter quiet zone is presented. The quiet zone was characterized using an automated two-way HP8510B based measurement system. The inherent system benefits for both antenna and RCS measurements are also discussed.
A new type of rolled-edge compact range reflector was designed and built by McDonnell Douglas Corporation. To minimize diffraction, the reflector contour was designed such that the surface radius of curvature and all its derivatives are continuous everywhere. This was accomplished by summing a parabolic function and two hyperbolic functions which have appreciable magnitude only in the edge-roll region. The bottom edge was treated using serrations.
Limitations of current compact range reflector systems are discussed with an emphasis on the diffracted energy and its effect on quiet zone size and quality. A new prime focus reflector design which minimizes the edge diffraction is presented. Computer predicted performance of this design is contrasted with measured field probe data.
This paper describes two signal processing techniques that have been used to overcome specific problems in a Lockheed Aeronautical Systems Corporation (LASC) indoor compact RCS measurement range. Both techniques are post processing techniques used to enhance the accuracy of RCS vs. Aspect measurements. These two techniques can speed up measurement time, increase measurement accuracy, and increase target sizes on a compact range.
Analysis and measurement activities to quantify compact range feed/subreflector time domain response are described in this paper. Reflection properties of various components are quantified and their interaction studied. Results indicate that although the feed/subreflector interaction is a factor, reverberation is dominated by instrumentation interaction particularly in the case of small compact ranges.
The construction of a large reflecting surface is invariably a compromise between the technical requirements and what is economically achievable. During the past three years, the compact range team at Harris has learned a great deal about this process. While aligning and testing the Harris Model 1630/1640 Compact Ranges, we have gone through a long learning experience. This paper presents some of the results of that experience.
A microwave holographic analysis system is shown to have successfully resolved the surface deformations on an 8' symmetric Cassegrain reflector antenna known to have significant surface deformation problems. The technique is based on the Fourier transform relationship between the aperture field of an antenna and its radiated far-zone field. A signal processing technique dubbed "pattern simulation and subtraction" is discussed that increases the resolution in the transformed aperture domain by removing unwanted signals from the aperture distribution. Measurements taken on the Cassegrain reflector at 11 GHz in the OSU-ESL Compact Range provided excellent amplitude and phase stable data to be processed by the holographic analysis system. Surface deformation profiles generated by this system were then compared to an optical measurement of the main reflector surface. Excellent agreement was obtained with a worst case deviation in the adjusted profiles being 0.05 ?.
An overview of results are presented for far field pattern, antenna gain and antenna temperature measurements of reflector antennas in several frequency bands. The pattern and gain measurements were taken in the compact range at The Ohio State University. The dynamic range available, which gives the ability to take a full 360 degree pattern, and the relatively high speed at which data is collected, are major advantages for pattern and gain measurements in the compact range. In a series of related measurements an 8-foot diameter Cassegrain reflector was used for antenna temperature measurements under clear weather conditions in an outdoor environment.
Compact ranges are special facilities, requiring a huge anechoic chamber and a large RF reflector to test a full size aircraft. These facilities are expensive and fixed structures, consequently they remain essentially research and design tools. However, as more and more aircraft are being made from composite materials, manufactures with high production volumes may be justified in having a compact range for purposes of quality control. The RF characteristics of these aircraft will change during their useful life cycle. The high cost of compact ranges will deprive most service and maintenance centers from owning one of these unique facilities, and force them to compromise the RF specifications of those aircraft in service. There is a definite need for a low cost and portable compact range. We present the design concept for such a range, whose reflector is divided into several identical pieces while the measurement is done sequentially. The edge effects of the portable reflector will be discussed.
In the last few years, the interest in millimeter wave systems, like radars, seekers and radiometers has increased rapidly. Though the size of narrow-beamwidth antennas in the 60-200 GHz range is limited to some 20 inches, an accurate far-field antenna test range would need to be very long. The achievement of precision antenna pattern measurements with a 70' or even longer transmission length requires the use of some power that is hardly available and expensive. A cost-effective and more accurate solution is to use a lab-sized compact range that presents several advantages over the classical so-called far-field anechoic chamber: - Small anechoic enclosure (2.5 x 1.2 x 1.2 meters) meaning low cost structure and very low investissement in absorbing material. No special air-conditioning is needed. This enclosure can be installed in the antenna laboratory or office. Due to the small size of the test range and antennas under test, installation, handling and operation are very easy. For spaceborne applications, where clean environment is requested, a small chamber is easier to keep free of dust than a large one. - The compact range is of the single, front fed, paraboloid reflector type, with serrated edges. The size and shape of the reflector and serrations have been determined by scaling a large compact range of ESD design, with several units of different size in operation. The focal length of 0.8 meter only accounts in the transmission path losses and the standard very low power millimeterwave signal generators are usable to perform precision measurements. The largest dimension of the reflector is 1 meter and this small size allows the use of an accurate machining process, leading to a very high surface accuracy at a reasonable cost. The aluminum alloy foundry used for the reflector is highly temperature stable. - Feeds are standard products, available from several millimeter wave components manufacturers. They are corrugated horns, with low sidelobes, constant and broad beamwidth over the full waveguide band and symmetrical patterns in E and H planes. - The compact range reflector, feeds and test positioner are installed on a single granite slab for mechanical and thermal stability, to avoid defocusing of the compact range. - A micro-positioner or a precision X Y phase probe can be installed at the center of the quiet zone. Due to their small size, these devices can be very accurate and stable. Due to the compactness of this test range, all the test instrumentation can be installed under the rigid floor of the enclosure and the length of the lossy RF (waveguide) connections never exceeds 1 meter.