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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.
Radar cross section (RCS) measurements were performed in the 0.1-1 GHz band in an anechoic chamber optimized for microwave frequencies. Selection of proper instrumentation, antennas, measurement techniques and processing software are discussed. Experimental results, showing the accuracy and sensibility of the system are presented.
Events such as the fire loss this year of a large anechoic chamber emphasize the need for consideration of the fire properties of microwave absorber and the toxicity of gases that are emitted during a fire. This report describes a small room-scale fire test, and other associated fire tests, carried out by the National Institute of Standards and Technology on polyurethane based pyramidal absorber. This work is intended to complement the measurements of microwave properties reported to AMTA in Monterey. The commercially supplied absorber had carbon (for absorption) and fire-retardant salts dispersed throughout the foam and its external surfaces were coated with fire resistant paint.
Texas Instruments Defense Systems and Electronics Group (TI-DSEG) has recently completed the development and documentation of our Flammability Test Procedure for Absorber Foams used in Anechoic Chambers. This Flammability Test is a major element of the TI-DSEG Anechoic Chamber Safety Policy. The Test Procedure was needed to help assure that absorber foam installed in our chambers consistently meets acceptable minimum fire-retardant specifications. Development of the procedure began with out interpretation of research documented in Naval Research Laboratory (NRL) Reports 7793 and 8093, which include testing for resistance to electrical stress, ease of ignition and flame propagation, and smoldering. Numerous iterations of the tests were conducted, using variations of the method. After the test was written, consultations with various vendors confirmed the producibility of absorber foam that will pass the Flammability Test and also meet or exceed electrical requirements.
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.
As the need for taking antenna measurements moves indoors, the antenna engineer must begin to work with a variety of constraints. Many of these constraints are directly related to facilities considerations. Often the current chamber size is not possible due to the available spaces within one’s organization. Perhaps a new building or an addition onto an existing building is possible, but more often than not the antenna engineer is faced with a number of sites that are “close” to his/her ideal model. But how does one valuate the tradeoffs? What are the ramifications of changing the performance characteristics, the size of the Quiet Zone, or the frequency of operation? What would happen to the size and price of a facility if the performance changed from an 8 foot Q.Z. down to a 2 GHz to a 6 foot Q.Z. down to 1 GHz? Is there a model to help sort out some of these issues. This paper will present a model to help the engineer sort out these issues in an organized manner.
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.
Among its different facilities, C.E.A. has an indoor range for radar cross section (RCS) measurements over a wide frequency range from 0,1 GHz to 18 GHz. The dimensions of this anechoic chamber, 45m x 13m x 12m and a quiet zone diameter of about 3m, make it one of the largest in Europe. It consists in a parabolic reflector for frequencies higher than 0,8 GHz and a system using inverse synthetic aperture radar (ISAR) techniques for lower frequencies associated with a short pulse coherent radar instrumentation equipment. In addition to performant instrumentation and illumination systems, the main features of this installation dedicated to measure stealth objects, are low residual clutter, discrete target supports, and powerful processing software. The technical solutions adopted are described.
Airborne or spaceborne radar systems often require adaptive suppression of interference and clutter. Before the deployment of this adaptive radar, tests must verify how well the system detects targets and suppresses clutter and jammer signals. This paper discusses a recently developed focused near-field testing technique that is suitable for implementation in an anechoic chamber. With this technique, phased-array near-field focusing provides far-field equivalent performance at a range distance of one aperture diameter from the adaptive antenna under test. The performance of a sidelobe-canceller adaptive phased array antenna operating in the presence of near-field clutter and jamming is theoretically investigated. Numerical simulations indicate that near-field and far-field testing can be equivalent.
A new spherical near field facility has been recently implemented at the Electromagnetic Propagation Area of INTA. The facility makes use of an existing big anechoic chamber (12 x 12 x 12 m.) and the near field/fair field transformation software developed by TICRA. This range has been calibrated by measuring an offset reflector antenna and comparing the results with those obtained in previous measurements of this antenna in other European testing facilities of different types. An experimental study has been carried out to check the dependence of the transformation software on the scanning parameters and different misalignments have been produced in order to determine the impact of the mechanical deformations on the accuracy of the system.
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.
This paper describes methodology for performing high resolution target radar cross section (RCS) diagnostic measurements with a new type of portable multifrequency radar. The Model 200 radar system is capable of operating at extremely short ranges, and does not require an anechoic chamber for performing highly sensitive radar cross section measurements. Measurements can be made in conventional low range resolution polar plot modes, in high-range-resolution (HRR) mode, in Inverse Synthetic Aperture (ISAR) mode, and in Synthetic Aperture (SAR) mode. The radar is described and the implications for present and future measurement technology are discussed.
Performance verification of an adaptive array requires direct, real-time sampling of the antenna pattern. For a space-qualified array, measurements on a far-field range are impractical. A compact range offers a protected environment, but lacks a sufficiently wide field of view. Conventional near-field measurements can provide antenna patterns only indirectly. This paper shows how far-field antenna patterns can be obtained in a relatively small anechoic chamber by focusing a phased array in the near-field. The focusing technique is based on matching the nulls of far-field and near-field antenna patterns, and is applicable to conformal or nonuniform phased arrays containing active radiating elements with independent amplitude and phase control. The focusing technique was experimentally verified using a 32-element, linear, L-band array. Conventionally measured far-field and near-field patterns were compared with focused near-field patterns. Very good agreement in sidelobe levels and beamwidths was achieved.
The MUSIC (Multiple Signal Characterization) algorithm uses an eigenvector decomposition of measured data to classify signals in the presence of noise. It has been used for the angular classification of multiple radar signal emitters and ISAR imaging. Interest has grown in stray signal analysis in anechoic chambers. This paper will discuss the modification and use of the MUSIC algorithm for the decomposition of field probe data to angular spectrum. A brief discussion of the MUSIC algorithm theory will be presented. Modifications required for use in compact range angular spectrum analysis will be discussed in detail. Requirements on field probe measurements will be presented as well as their effects on the implementation of the algorithm. Both one way and two way measurements are considered for their relationship to the array manifold. Finally, some experimental validation generated on the Boeing range will be presented.
The Vulnerability Assessment Laboratory (VAL) anechoic chamber at White Sands Missile Range, New Mexico was reconfigured and refurbished during the last part of 1988. This paper will be a facility description of the state-of-the-art Special Electromagnetic Interference (SEMI) investigation facility. Electromagnetic susceptibility and vulnerability investigations of US and, in some cases, foreign weapon systems are conducted by the EW experts in the Technology and Advanced Concepts (TAC) Division of VAL. EMI investigations have recently been completed on both the UH-50A BLACKHAWK and AH-64A Apache helicopters in the chamber. The paper will cover the facility's three anechoic chambers, shielded RF instrumentation bay, computer facilities for EM coupling analyses, and the myriad of antenna, antenna pattern measurement, amplifier, electronic, and support instrumentation equipment for the chambers. A radar cross section measurement and an off-line RCS data processing station are also included in the facility.
Ray Proof has recently completed the construction of a shielded anechoic chamber in the Air Force Anechoic Facility at Edwards Air Force Base in California. Measuring 250 feet by 264 feet x 70 feet high, it is believed to be the largest anechoic chamber in the world. The facility will be used for EW testing of full-scale aircraft such as the B-1 B and B2 and will be operated for the Air Force by Rockwell International, the prime contractor for the project. This paper discusses parameters, statistics, and design features. The shielding was designed and quality controlled during construction in order to meet the NSA 65-6 specification, modified to extend to 18GHz. Layout of pyramidal anechoic material, varying from 12 inches to 24 inches in thickness with 36 inch around lighting fixtures, was designed to meet a return loss specification of 72 dB at 500 MHz, and up to better than 100 dB in the 3-18 GHz region. The chamber features a sliding pocket door 200 feet long and 66 feet high. To meet the stringent NSA 65-6 requirement, a threefold inflatable-bladder/ fingerstock seal was used around the door. The other feature of the chamber is an 80 foot turntable with a separately shielded control room suspended beneath. The table can rotate a 250,000 pound load through plus-or-minus 190 degrees, positioning to an accuracy of plus-or-minus 0.1 degree. A number of innovative procedures such as locating a portable factory to manufacture the absorber near the construction site enabled Ray Proof to complete and test the chamber ahead of schedule.
The reflectivity levels of the three anechoic chambers of TKK and VTT were measured at 110 GHz. The sidewall reflections were measured by the free space voltage standing wave ratio method. Typical values measured with 20 dB pyramidal horns were below -60 dB at azimuth angles less than 20 degrees and about -50 dB at angles larger than 50 degrees. When a waveguide end was used as the transmitting antenna, the reflectivity level was nearly 10 dB higher. The backwall reflections could be measured directly because the reflected field was much larger than the direct field. The maximum backwall reflection varied in the three chambers from -33 dB to -36 dB.
The hardware and software developments undertaken to upgrade two far-field measurement facilities - a 12-m anechoic chamber and a 35-m outside range - are described. A method (termed quasi-far-field, QFF) for deriving antenna far-field patterns from a single plane scan at a distance less than the traditional distance of 2D2/? is described. The QFF technique involves pattern sample and subsequent pattern transform and reconstruction, from the easement distance to the far-field distance. A discussion of the limitations inherent in the QFF transform, including range length, is given. Experimental results for measurements made on circular-aperture antennas with both symmetric and asymmetric illumination, and on antennas with elliptical apertures, are described.