AMTA Paper Archive
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Automated millimeter wave evaluation system for advanced materials and frequency selective surfaces
An automated instrumentation system has been configured for the purpose of evaluating advanced composites, radar absorbing materials, and frequency selective surfaces (FSS) in free space. Electrical test frequencies are divided into three bands that range from 18 to 60 GHz for any linear polarization. Software has been incorporated to calculate dielectric properties from the measured transmission and reflection characteristics. Using the HP9836 computer, software was written to automate and integrate the Anorad 3253 positioner with the HP8510 network analyzer. This system allows for the input of up to five incident angles at vertical, horizontal, and cross polarization. The measured transmission loss (amplitude and phase) at multiple incident angles is then plotted for comparison. This paper gives a complete description of the system configuration, calibration techniques, and samples of output data. Material properties are computed and compared to specified and theoretical values. Measured results of an FSS structure are compared to its predicted response.
Electromagnetic surface roughness for composite materials
Present day manufacturing techniques often employ composite materials in the fabrication of many structures. Graphite is one common material used to form structurally strong fibers for use in a resin binder. The material characteristics of graphite composites naturally differ from those of metallic materials. An interesting characteristic is the smoothness or roughness of composite materials as examined from an electromagnetic viewpoint. Radar backscatter measurements of several different planar panels were performed near grazing incidence to compare their scattering characteristics against a smooth metallic surface. These results show the "electrical" smoothness of the surfaces in terms of fabrication and material dependencies.
Detection of conductivity gaps and material imperfections using surface radar diagnostics
Low RCS signatures require verification of test body conductivity and material performance. A miniaturized radar system with a unique horn antenna was designed for the detection of conductivity gaps and material imperfections in radar absorbing material. The antenna system has a small aperture and low VSWR permitting direct placement against a surface for localization of electromagnetic phenomena. Test results indicate that test body construction gaps and material imperfections are readily detectable using the test system in either a handheld or robotic-type configuration. Preliminary results also indicate delaminations, conductive panel penetration, and structural component steps will be detectable.
VHF/UHF RCS measurements in indoor microwave facility
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.
Radar cross section measurements in a reflecting chamber
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.
RATSCAT integrated radar measurement system
This paper presents an overview of the integrated Radar Measurement System (IRMS) installed at the Air Force Radar Target Measurement Facility (RATSCAT) for AFSC/6585 TG/RX Holloman AFB, New Mexico.
TRACSAR High-Performance Diagnostic Radar System
This paper will describe the production version of the Model 200 TRACKSAR radar, which provides high-resolution imaging in downrange and crossrange using wideband waveforms and both synthetic aperture radar (SAR) and inverse synthetic aperture radar (ISAR) processing. Several other novel features of the system and technical aspects of performing such measurements will be addressed, and sample data outputs will be presented.
Electrical and mechanical characteristics of line for string supported models
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.
An HP-8510-based 45-GHz instrumentation radar for ISAR image and glint studies
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.
Laser tracker for radar calibration sphere position measurement
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.
Productivity improvements for a gated-CW radar
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.
Monostatic and bistatic polarimetric radar cross section measurements on canonical targets
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.
Multipath effects on radar cross section measurements
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.
Correction/calibration of wide-band RCS radar data containing I/Q error
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.
Short term stability performance of pulsed instrumentation radars using TWTAS
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.
Coherent RF measurements with a long reference path
In coherent measurements, one measures the interference of signals from test and reference paths. These techniques are widely used in RF image measurements of antennas and radar cross sections. The success of a coherent measurement depends highly upon the stability of the path length difference between test and reference signals as well as the quality of the reference signal. The stability and quality are hampered when the experiment has to be conducted with a long reference path length, particularly at outdoor ranges. A new measurement scheme, based on the scattering process initiated by two coherent beams, will be presented here are has the advantage over others in reducing the problems associated with the path length difference instability.
The Test and set-up of spaceborne, active SAR antennas
This paper presents an overview of the test and set-up requirements of active Synthetic Aperture Radar (SAR) antennas. The specific antennas under consideration are those that are intended to be used in the next generation of spaceborne SAR C-Band satellites. These antennas are typically 1m to 2m wide and 10m to 20m long, possessing between 3000 to 12000 radiating elements. The paper considers each unit of the active antenna in turn and identifies which tests are to be carried out where. In considering the test of the whole antenna some initial result of focussing techniques, to allow the antenna to be tested in real time at reduced distance, are presented.
Wideband polarimetric determination of antenna radiation and scattering characteristics by RCS-measurements
A novel and very powerful measurement technique is presented which allows the determination of antenna radiation and scattering by radar-cross-section (RCS-_ measurements. The antenna under test is treated as a loaded scatterer using a polarization dependent network model that allows a complete antenna description in terms of scattered, radiated and absorbed waves. A load variation principle is used to determine the network model parameters and all commonly used antenna parameters like gain, antenna polarization, axial ratio, polarization decoupling, input impedance and also structural scattering can be derived from the backscatter measurement without using any additional standard antenna. With the antenna network description it is furthermore possible to examine the antenna behavior for arbitrary excitation or loading on their waveguide or radiation port.
An Overview of parameters determining productivity and sensitivity in RCS measurement facilities
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.
Hughes Aircraft Company's new RCS measurement facility
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.
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