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Composites of the electrically conducting polymer polypyrrole with paper, cotton cloth and polyester fabrics have been evaluated for use in radar absorbing structures. Reflectively measurements on the composites in the range 8-18 GHz and transmission line modelling have revealed impedance characteristics with a common transition region. Relationships between substrate material, polymer loading and electrical performance have been explored. Polarization characteristics have also been measured. The electrical model has been successful in predicting the performance of Salisbury screen and Jaumann multi-layer designs of RAM.
The plane wave quality of a compact range (CR) is usually specified in terms of the crosspolar level and the magnitude and phase ripple in the test zone. The way these deviations from the ideal plane wave affect the measurement of different antenna types can be treated by the application of the reciprocity principle between the transmitting and receiving antenna in a measurement set-up. By the application of the sampling theorem, it is found that the measured antenna pattern can be expressed as a summation of the plane wave spectrum components of the field at the test zone weighted by the true radiation pattern of the antenna under test (AUT) evaluated at the CR plane wave directions in the rotated coordinate system of the AUT. The inverse procedure can be used to extract the CR plane wave information (and therefore the CR field at the test zone by means of the Fourier series) from the measurement of a standard antenna with a known radiation pattern.
Time-frequency distributions (TFD) describe a signal in terms of its joint time and frequency content. In this paper, it will be shown that TFDs are particularly useful for the analysis of frequency-dispersive electromagnetic scattering. A TFD based on the wavelet transform (WT) of the scattering data is presented. As an example, measured scattering from a waveguide cavity is considered. It is shown that the wavelet TFD can provide good time resolution for specular/point scattering features, and good frequency resolution for resonant features. Application to the scattering data from the KC-135 aircraft in flight shows that the WT is capable of detecting the resonant modes of the engine outlets of the aircraft.
To gain greater insight into the design of surface ships with reduced radar cross-section characteristics, a structure resembling a ship deckhouse was physically modeled and measured. The structure was represented as a truncated pyramid. Four scaled pyramids were fabricated, all identical except for the radii of the four vertical (slanted) edges. The pyramids were measured at the University of Massachusetts, Lowell Research Foundation, submillimeter laser compact range. Measurements were made a scaled X-band using a laser-based system that operates at 585 GHz with the pyramids scaled at a ratio of 1:58.5. These shaper were measured at 0.75 degrees depression angles on a smooth metal ground plane at both HH and VV polarizations. The goal of this study was to determine if small changes in the radius of the curvature of the slanted edges could significantly affect the radar cross-section of the pyramid. In this paper the results of measurements of the pyramids will be presented. The data are compared with computer code predictions and the differences are discussed.
This paper contrasts indoor and outdoor implementation of efforts during upgrades of VHR RCS measurement capabilities. Sites studied are two McDonnell Douglas Technologies Incorporated, Range Measurements Services facilities. Indoor. Radar Measurement Center (San Diego, CA) is a large compact range. Equipment-Harris Corporation Model 1630 Collimator System, Scientific Atlanta Model 2090 radar. Outdoor. Microwave test facility (Victorville, CA), large ground plane facility. Equipment-Steerable dipole feed dish, System Planning Corp, Mark III radar.
Northrop Corporation's Business and Advanced Systems Development Group has recently completed a very successful Radar Cross Section (RCS) measurements program on the USAF/Northrop B-2 bomber. One of the capabilities spawned from the program is a measurements radar system, comprised largely of off the shelf hardware, which provides high resolution whole body two-dimensional RCS images of large targets on the ground in the near field. Its high gain antennas allow operation in a space limited area and utilizes Synthetic Aperture Radar (SAR) data collection techniques. The system, though designed for use at VHF, has been expanded to operate from 100-2000 MHz in three bands. The hardware, associated signal processing, its applications and limitations are discussed.
A 585 GHz compact range has been developed for obtaining full scale RCS measurements on scale model targets. The transceiver consists of two CW submillimeter-wave gas lasers along with two colled-InSb heterodyne mixers. Coherent detection has been implemented to maximize sensitivity and allow for a vector measurement capability. In addition, the target can be rapidly translated in range to generate a doppler modulation which is used to reject background signals during low-RCS measurements. Although most scaling has evolved to develop non-metallic materials with scaled dielectric properties as well as validation and test program, RCS measurements are made on scaled simple and complex shapes and compared with full-scale measurements and computer predictions. A description of the 585 GHz compact range along with measurement examples are presented in this paper.
This paper will describe the requirement, design, implementation, and performance evaluation of MMWRCS measurement subsystems to be integrated with an existing RCS measurement system in the Sikorsky Compact Range in Bridgeport, CT. The subsystems will operate at V-band (58-62 GHz) and W-band (92-98 GHz). The requirements to test at V-band and W-band is driven by limitations of quiet zone physical volume. The Harris model 1606 reflector system produces a 6 foot diameter zone of virtual uniform amplitude and phase. Therefore scale models are fabricated for test. This translates to approximately 1/6 scale of contemporary Sikorsky Helicopter designs. Testing at 60 and 95 GHz will provide accurate simulated full scale RCS data at X and Ku-bands.
Lockheed has recently completed the construction of a Large Compact Range (LCR) for antenna and RCS measurements. The dimensions of the facility are 60' (h) x 100' (w) x 120' (l) with a 20' x 20' cylindrical quiet zone and operational capabilities from 0.1 to 18.0 GHz. The requirement to measure low RCS levels in a room which is smaller that the desired has resulted in a unique system design. Elements of this design include a feed pit, a feed hood, and a rolled edge reflector; special absorber layouts to minimize background scattering, a high performance instrumentation radar, fast ring down feed antennas, and a unique string suspension and positioning system. This paper presents the various sub-systems that make up the LCR along with chamber validation methods and preliminary performance data. The subsystems listed in this paper are LCR's: Reflector, radar system, feed antennas, feed positioner, absorber, target handling equipment, and string positioning system. Initial design requirements are listed for some sub-systems along with range characterization data such as un-subtracted clutter levels, background subtraction performance, and theory vs. measured data for some simple conical shapes.
This paper briefly reviews existing compact range performance characterization methods showing the limitations of each technique and the need for an accepted and well understood technique which provides efficient and accurate characterization of compact range measurement accuracy. A technique known as the transverse pattern comparison method is then described which has been practiced by the author and some range users for the past several years. The method is related to the well known longitudinal pattern comparison method, however, comparisons are conducted in the transverse planes where the required span of aperture displacement is much smaller and does not exceed the dimensions of the quiet zone. This method provides several advantages for characterizing compact range performance as well as enables range users to improve achievable measurement accuracies by eliminating the impact of extraneous signal errors in the quiet zone.
Traditional Compact Range Antenna (CRA) applications are related to Antenna Pattern and RCS measurements. For these purposes, as a rule, CRA are installed within or outside of an anechoic chamber as stationary equipment. However, for some modern applications, such as Electronic Warfare development, radar tracking system testing, indoor RF environment simulation and others, where dynamic and pointing properties of an AUT are to be tested, the mobile and multi-beam CRA is of great importance, since it provides the designer with powerful simulation and testing capabilities. Such a CRA has been designed, built and tested at ORBIT ADVANCED TECHNOLOGIES, LTD. The design trade-offs, CRA analysis, test set-up and results are discussed in the presented paper.
Prony's method has been found useful in extracting the time domain response over extended time using data samples of limited time span. This paper describes results of studies underway to apply Prony's method to extracting the RCS time and frequency response from limited measured and computed data. The technique has been applied to characterize the RCS response of structures with inherent multiple resonances, e.g., a dielectric cube constructed using high-permittivity dielectric material. Implications of the technique to gated antenna and RCS measurements are discussed.
A Ground to Air Imaging Radar system (GAIR) used to perform diagnostic imaging and total RCS measurements on low observable airborne targets has been developed by the Environmental Research Institute of Michigan (ERIM). In order to ensure accurate measurement of the scatterers contributing to a target's radar signature, proper calibration in imperative. The use of external calibrators to measure the end-to-end system transfer function is the ideal way to perform a system calibration. However, this is a more difficult and challenging task with a ground based radar viewing an airborne target, as opposed to a traditional airborne SAR which views an array of ground based trihedral corner reflectors. This paper will discuss the internal and external calibration methods used in performing an end-to-end system calibration of the GAIR. Primary emphasis is placed upon the external calibration of the GAIR and the three independent measurements utilized: a ground based corner reflector, a sphere drop, and an in-scene calibrator. The system calibration results demonstrate that the GAIR is an accurately calibrated radar system capable of providing calibrated images and total RCS data. Moreover, only the ground and internal measurements are required on a daily basis in order to maintain system calibration
A small compact range measurement facility has been installed at the Environmental Research Institute of Michigan (ERIM) for research aimed at improving RCS measurement and radar imaging techniques. This paper describes the facility, which is referred to as the Experimental Range Facility (ERF). The ERF has two instrumentation radars; a Flam & Russell FR959 gated CW radar and a Hughes MMS-300 pulsed radar. The radars are connected to a suite of workstations, which support a variety of internally and externally developed radar imaging and data exploitation software. The ERF is also equipped with sophisticated target positioning control and sensing equipment.
The U.S.Army Redstone Technical Test Center (RTTC), Test and Evaluation Command, has developed a comprehensive antenna metrology and Radar Cross Section (RCS) evaluation facility. This facility features the compact antenna test range technique for millimeter wave measurements and the near-field scanning technique for microwave measurements. This paper described RTTC's use of these measurement techniques, instrumentation with PC Windows based automation software, anechoic chambers, and types of tests performed. Planned future thrust areas are also discussed.
A complete description is given of the unique radar cross-section (RCS) measurement facility built at the Houston Advanced Research Center in The Woodlands, TX. The uniqueness of this chamber comes from its ability to independently move the transmit and receive antennas, which can each be moved to any position within their respective ranges of motion to a resolution of about 0.05 degrees. The transmit antenna is fixed in azimuth, but can be moved in elevation: the receive antenna is free to move in both azimuth and elevation. Additionally, the target can be rotated in azimuth by means of an azimuth positioner. Analysis has been performed to determine the impact of chamber effects on measurement accuracy. The most notable chamber effect comes from the two large aluminum truss structures, which are the mounting supports for the transmit and receive antennas. Fortunately, the scattering from these structures can be readily separated from the desired target return through the use of range (time) gating. Time domain results are presented showing the effects of these structures.
Numerous monostatic radar cross-section (RCS) calibration routines exist in the literature. Many of these routines have been implemented at the RCS measurement facility built at the Houston Advanced Research Center in The Woodlands, TX. Key monostatic results are presented to give an indication of the measurement accuracy achievable with this chamber. Unfortunately, bistatic calibration routines are not nearly as common in the literature. As with the monostatic routines, a number of bistatic routines have been implemented and typical results are presented. Additionally, descriptions are given for some of the reference targets along with their support structures that are used during calibration.
Lockheed Sanders, Inc., has constructed a state-of-the-art electromagnetic measurement system. Cost considerations dictated the use of existing facilities and space, We took advantage of the lessons learned from the Lockheed Advanced Development Company's (LADC) Rye Canyon, California Facility [1]. Lockheed Sanders, Inc. now has a complete indoor measurement capability from VHF to MMW. Lockheed Sanders, Inc. needed a facility capable of making measurements over a broad range of frequencies. The system consists of a tapered chamber and a compact range. The system consists of a tapered chamber and a compact range. The tapered chamber has a measurement area of 28' x 28' x 34'. This range is capable of antenna and RCS measurements from .1 to 2 GHz. The compact range is designed for 2 to 40 GHz. Using a Scientific Atlanta, Inc. reflector scaled from the Rye Canyon reflector, a 6' x 6' quiet zone is possible. Feeds consist of a feed cluster aligned for phase and limiting parallax and horn cross-talk. Both chambers use the Flam and Russell 959 measurement system. This paper will discuss the chambers and their operation. The paper will close with a demonstration with measurements on standard, complex targets.
In polarimetric RCS measurements, the cross-polarization levels which are required in the test zone, correspond closely to those which are realizable with most Compact Antenna Test Ranges (CATR). On the other hand, such a performance may not satisfy the accuracy requirements in cross-polarization measurements of high performance microwave antennas. These applications include spacecraft antennas, ground stations for satellite communications or microwave antennas for terrestrial applications, where two polarizations are used simultaneously.
There has been a great deal of interest in microstrip antennas and arrays in the past decade or so due to their low cost, light weight, and conformability. Most research to date on microstrip antennas has been focused on developing techniques for characterizing their radiation properties. However, interest in evaluating the scattering properties of such antennas is increasing. The RCS of three configurations of circular patch antennas have been measured versus frequency and are compared to Moment Method predictions; a single open-circuited element, a single element terminated in a 50 ohm load, and a 3 x 3 array of open-circuited elements. In most cases, the measurements and predictions are in good agreement.