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R. Henderson (GE-Astrospace Division),M. Yaffe (GE-Astrospace Division), November 1990
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.
The intrinsic electromagnetic properties of bulk materials must be accurately measured to skillfully apply them in new designs. This paper surveys a variety of measurement methods based on RF/microwave network analyzers, and discussed the strengths and limitations of each. This includes recent enhancements to the popular “transmission-line-method”, plus a new open-ended coaxial probe for making permittivity measurements easier and more convenient.
S. Brumley (Denmar Inc.),G. Tanakaya (Hughes Aircraft Company), November 1990
The Hughes Aircraft Company conducted a study to characterize the backscattering performance of wedge shaped anechoic absorbers for use in treating the sidewall regions of RCS chambers. ISAR imaging techniques were utilized to obtain a diagnostic results at near-grazing incidence angles which were not possible with conventional testing methods. These techniques allowed for separation and identification of individual scattering sources from each of the evaluated samples. As a result, the backscattering from an entire wall of absorber can be simulated by evaluating only a few samples. Absorber performance data was collected over frequencies from 2 to 40 GHz.
Results from this study clearly show that differences in absorber fabrication methods have a significant impact on the performance of the materials. Various approaches for impregnating, loading, and cutting the absorber have also been evaluated. Gaps, formed during installation, at the joint between two pieces of material are shown to significantly degrade performance, whereas, offsets and glue lines are shown to have less of an effect, provided the absorbers are uniformly loaded.
C.F. Yang (Ohio State University),R.C. Rudduck (Ohio State University), W.D. Burnside (Ohio State University), November 1990
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.
D. Kremer (National Institute of Standards and Technology),A. Newell (National Institute of Standards and Technology),
F. Mayo-Wells (National Institute of Standards and Technology),
W.J. Parker (National Institute of Standards and Technology), November 1990
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.
J.I. Barnes (Texas Instruments Incorporated), November 1990
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.
P.J. Joseph (Air Force Institute of Technology),I.J. Gupta (The Ohio State University ElectroScience Laboratory),
R.J. Mariano (The Ohio State University ElectroScience Laboratory),
W.D. Burnside (The Ohio State University ElectroScience Laboratory), November 1990
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.
D.C. Bishop (Boeing Military Airplanes),J.B. Elbert (Boeing Military Airplanes), L.A. Snover (Boeing Military Airplanes), November 1990
Several electromagnetics laboratories are now using “string” or line to support their test bodies. There is no standard line material used and often this material is chosen fairly arbitrarily.
This paper compare electrical and mechanical characteristics for various types of line. The line types to be tested include Spectra 1000, Kevlar, nylon, Teflon, and wire rope. Each characteristic will be tested for 0.04” and 0.10” nominal diameters.
Radar cross-section tests will be run for each string, both in a vertical position and at an angle of approximately 15 degrees. Each measurement will be run with a frequency sweep from 2-18 GHz. Dielectric constants for each of the line types will also be compared. Mechanical attributes such as tensile strength, creep, and yield, if any, are compared for each of the various line types and sizes. Both vendor data and laboratory results will be presented.
The electrical and mechanical characteristics will then be used to discuss which line material is optimum for use during electromagnetic testing.
R. Dinger (Naval Weapons Center),D.J. Banks (Naval Weapons Center),
D.R. Gagnon (Naval Weapons Center),
E. Van Bronkhorst (Naval Weapons Center), November 1990
A 45 GHz instrumentation radar system unique in several respects has been developed for inverse synthetic aperture radar (ISAR) and tracking angle scintillation (glint) studies. The system, based on a Hewlett-Packard HP-8510B network analyzer, is fully polarimetric and operates on a 1000-m outdoor far-field range. An amplitude monopulse receiver provides a measure of the instantaneous apparent-center-of-scattering of the target. Successful glint and ISAR measurements have been made on targets as large as 8 m.
W.D. Sherman (Boeing Defense and Space Group),C.R. Pond (Boeing Defense and Space Group),
M.D. Voth (Boeing Defense and Space Group),
P.D. Texeira (Boeing Defense and Space Group), November 1990
A laser tracker using a computer controlled feedback loop has been designed and tested. The tracker follows a small retroreflector embedded in a radar calibration sphere. Angle encoders coupled to two orthogonal scanning mirrors give azimuth and elevation pointing angles to the target. Phase measurements of an intensity modulated laser beam give change in distance to the target, while absolute range is determined by knowing the initial 2p ambiguity interval of the target position. The crossrange accuracy of the system is limited by the scanning mirror encoders to =.063 inches rms at 105 feet (50 microradians). The downrange accuracy of the system is ˜.015 inches rms. This versatile system can be used for: a) contour measurements of models with the aid of a retroreflector moving over the surface, b) accurate determination of the coordinates of a single moving target, and c) determination of the orientation of a large extended target. Anticipated modifications of the system, with their potential precision measurement capabilities and applications, are discussed.
A.R. Lamb (Hughes Aircraft Company),H. Hgai (Hughes Aircraft Company),
J. Paul (Hughes Aircraft Company),
Y. Chu (Hughes Aircraft Company), November 1990
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
J.B. Wilson (Scientific-Atlanta, Inc.), November 1990
This paper will describe new developments in a gated-CW radar that has been designed to improve the productivity and sensitivity of RCS measurements.
Improvements in data acquisition speeds result from the design of a fast synthesizer, a data acquisition co-processor and a pulse modulator. Each of these new products have been specifically designed to take advantage of the high speed capabilities of Scientific-Atlanta’s Model 1795 Microwave Receiver. The RF sub-system has also been designed to permit continuous 2-18 GHz, full polarization data acquisitions. Critical RF components are now mounted at the feed in the chamber, improving the sensitivity and ringdown of the system.
Productivity in analysis activities has been improved by the use of a multi-tasking system controller which permits simultaneous use of the system for acquisitions, analysis and plotting.
J. Saget (Dassault Electronique),J. Garat (CEA/CESTA), November 1990
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.
S. Mishra (Canadian Space Agency),J. Mantz (Canadian Space Agency), November 1990
This paper describes results of extensive polarimetric Radar Cross Section (RCS) measurements on canonical targets. Amplitude and phase of both co- and cross- polar returns are measured for horizontally and vertically polarized transmit signals in order to determine the complete complex scattering matrix. Measurements have been carried out on a variety of targets. Results presented with this summary show data for a metallic and a dielectric disk. Details of measurement and calibration procedure, hardware, and software are also presented.
E. Walton (The Ohio State University ElectroScience Laboratory),L. Beard (The Ohio State University ElectroScience Laboratory), November 1990
This paper is concerned with the measurement of RCS in a room with conducting walls. The experimental measurement system uses a moving antenna to produce a scan of the target and the clutter. The scattered signal as a function of frequency and position is recorded. New field crossrange processing is then used to map the target zone.
Example images will be shown for both two-dimensional and one-dimensional scans. Images from point targets and distributed targets will be presented.
A.J. Stoyanov (Douglas Taylor Research Center),M.A. Sekellick (Douglas Taylor Research Center),
W.H. Schuette (Douglas Taylor Research Center),
Y.J. Stoyanov (Douglas Taylor Research Center), November 1990
Accurate radar measurement of complex marine targets from a shore-based radar are difficult to achieve because of the effects of a multipath environment. This paper summarizes multipath effects at low and very low grazing angles. The investigation of scattering from the sea surface and from marine targets is important for radars operating at low elevation angles over a sea surface because severe fading and distortions in measured target radar cross section can occur due to multipath propagation. It can be shown that the lobing structure due to the interference of the direct and reflected signal is still a problem for very low elevation angles even in high sea states, suggesting a limited usefulness for low elevation radar cross section measurement sites. Further it has been observed that in the microwave region at elevation angles smaller than 0.5 degrees, scattering centers located at certain heights above the sea surface may be masked. At higher elevation angles, however, multipath interference is reduced, thus giving a more stable basis for measurement and evaluation.
D.E. Pasquan (Texas Instruments Incorporated), November 1990
In-phase and quadrature (I/Q) aberrations in radar receiver data create problems in radars used for radar cross section (RCS) measurements. I/Q errors cause incorrect representations of the target under test. A method for correcting I/Q error and calibrating the measured amplitude to a scattering standard provides a means of obtaining a more accurate representation of the target under test.
The RCS measurement instrumentation addressed here uses a wide band receiver with a single quadrature mixer for conversion of radio frequency (RF) to base band (also referred to as video) frequency. In the one-step down conversion, distortions in the I/Q constellation occur, causing I/Q errors. This method quantifies the extent of the I/Q problem by estimating the actual I/Q error from a series of calibration measurements. An algorithm is presented which quantifies parameters of the I/Q distortion, then uses the distortion parameters to remove the I/Q aberrations from the target measurement.
O.M. Caldwell (Scientific-Atlanta, Inc.), November 1990
The effects of non-systematic receiver instrumentation errors on precision antenna measurements are investigated. A simple uncertainty model relating dynamic range to random perturbation effects on amplitude measurements is proposed. Examples of measurement uncertainty versus both input level and measurement speed are presented using data taken on modern measurement receivers. Dara are compared with the model to estimate measurement uncertainty at various pattern levels and acquisition speeds. Equivalent dynamic range specifications are deduced from the measures data.
J. Allison (Hughes Aircraft Company),J. Paul (Hughes Aircraft Company),
R. Santos (Hughes Aircraft Company), November 1990
Pulse-to-pulse amplitude and phase noise can affect the overall measurement accuracy of RCS instrumentation radars. Depending upon the measurement requirements, such noise can limit the overall performance whenever pulse-to-pulse repeatability is required in the signal processing. Radar systems using pulsed TWTAs are subject to high noise due to limitations in the performance of the TWTA modulators and power supplies. A characterization of this additive noise is important to understand the limitations in system performance. Measurements have been made on kilowatt power TWTAs at L and X band as well as 20 watt pulsed TWTAs at S, C, and X/Ku band at various duty cycles and PRFs.
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