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AMTA Paper Archive

High resolution ISAR imagery for diagnostic RCS measurements
J.C. Davis (System Planning Corporation),E.V. Sager (System Planning Corporation), November 1985

Inverse synthetic aperture radar (ISAR) imaging is used to produce high cross-range and down-range resolution on objects undergoing a change of aspect angle relative to the radar. In this application, the ISAR technique was used on an outdoor ground-bounce radar cross-section (RCS) measurement range. The objective is to locate, identify, and quantify the scattering properties of the target-under-test (TUT). The TUT is mounted well above ground on a target pole and can rotate in azimuth and elevation. The TUT’s rotational motion about an axis perpendicular to the radar line of sight is used to produce the cross-range resolution. For range resolution, a high-bandwidth frequency stepped waveform is used. The data are processed entirely in the digital domain with an algorithm that consists of a procedure to remove the dispersive properties and amplitude variations of the complete end-to-end range response, followed by a two-dimensional, polar-to-rectangular resampling filter and a two-dimensional fast Fourier transform (FFT). The processor has achieved images with amplitude and distortion products that are below the system’s noise floor with up to 48 dB of processing gain. The radar imagery is presented to the RCS engineer on a high-resolution color graphics terminal with true-perspective color-coded RCS displays in logarithmic amplitude or linear phase scales. The design of the ISAR processing algorithm is described in this paper as are the results for both simulated and actual radar data.

Characteristics of bistatic scattering from a large absorber covered surface
B. DeWitt,E. Walton, November 1985

In any antenna or RCS measurement range, the walls, floor, and ceiling are covered with radar absorbing material (RAM) so that spurious scattering will be reduced. The bistatic scattering characteristics of these walls etc. are often not accurately known, however. This situation is exacerbated by the techniques often used to measure the scattering characteristics of the RAM used on the walls etc. The measurement techniques are typically “arch type” measurements, where the scattering from a section of absorber (often 3x3 feet) is compared to that scattered by a conducting plate of the same size. These type measurements are often corrupted by edge and corner diffraction terms and the results are often not very accurate.

The Sandia National Laboratories scatter facility
C.M. Luke (Scientific-Atlanta Inc.),B.C. Brock (Sandia National Laboratories), C. Smith (Scientific-Atlanta Inc.), M.C. Baggett (Scientific-Atlanta Inc.), R.D. Bentz (Sandia National Laboratories), November 1985

The two measurements PCAL / PMRC and PTARG / PMRT are ratioed and the PMRC / PMRT term accounts for changes in both power or phase since calibration, because the mid-range is of fixed RCS size and phase. Using this technique, Scientific-Atlanta has been able to hold calibrations to within 0.5 dB amplitude and 8 degrees phase for as long as 12 hours. This includes outdoor range effects.

Calibration techniques used in the Sandia National Laboratories scatter facility
M.C. Baggett (Scientific Atlanta),Billy C. Brock (Sandia National Laboratories) Charles M. Luke (Scientific Atlanta) Ronald D. Bentz (Sandia National Laboratories), November 1985

This paper briefly discusses the calibration techniques used in the Sandia National Laboratories Radar Cross-Section Test Range (SCATTER). We begin with a discussion of RCS calibration in general and progress to a description of how the range, electronics, and design requirements impacted and were impacted by system calibration. Discussions of calibration of the electronic signal path, the range reference used in the system, and target calibration in parallel and cross-polarization modes follow. We conclude with a discussion of ongoing efforts to improve calibration quality and operational efficiency. For an overview description of the SCATTER facility, the reader is referred to the article Sandia SCATTER Facility, also in this publication.

Some useful RCS test bodies
L., Jr. Peters (The Ohio State University ElectroScience Laboratory),A. Dominek (The Ohio State University ElectroScience Laboratory), W.D. Burnside (The Ohio State University ElectroScience Laboratory), R. Wood (NASA Langley Research Center), November 1985

Versatile test bodies are extremely useful for RCS measurement facilities for many reasons, some of which are listed below: 1) evaluate the performance achievable for a given measurement facility 2) measure the RCS of components normally mounted on a ground plane, and 3) terminate a target pedestal in order to measure its cross-section since most pedestals are designed to attach directly to a target. In order to perform all of these functions a versatile test body should have flat sections to mount components efficiently, it should have a known smooth cross-section with angle of incidence from very low values to large ones, it should not use absorber that could attenuate the signal meant to illuminate the component pieces being tested, etc. Several such test bodies have been studied, some of which will be described.

Computer-aided design of anechoic chambers
S. Mishra (National Research Council), November 1985

Review of a computational technique used in the design of anechoic chambers is presented. Details of an interactive computer program to predict fields inside anechoic chambers are discussed. Use of the program in (a) computing change in field distribution due to reflections from the walls of the chamber and (b) optimizing cost/performance ratio of the chambers is illusterated.

Antennas for optimum illumination of anechoic chambers
R. Flam (FLAM & RUSSELL, INC.),J.P. MacGahan (FLAM & RUSSELL, INC.), November 1985

A great deal of effort has gone into the optimum design of anechoic chambers over the years, however little attention is generally given to the choice of the source antenna used to illuminate these chambers. Typically, any antenna which operates at the desired frequency and that happens to be available in the antenna laboratory is commandeered for us as a source.

A Wideband low-sidelobe source antenna for a VHF antenna range
H.E. King (The Aerospace Corporation),J.L. Wong (The Aerospace Corporation), November 1985

The RF characteristics of a four-element diagonal array configured to yield low sidelobes, dual circular polarization with low axial ratio and high front-to-back ratio are described. The array was designed for use as source antenna in a VHF test range, where the test antenna is nearly omnidirectional and ground multipath effects are a major problem. To achieve broadband performance, crossed open-sleeve dipoles were used as array elements. The array is capable of operation over a 1.66:1 band with a VSWR of <2:1. Experimental studies were made by means of scale model antennas in the 240 to 400 MHz band. The axial ratio is <1 dB, and the sidelobe/backlobe levels vary from –25 dB to –30 dB over the measurement frequency range.

A New antenna test facility at General Electric Space Systems Division in Valley Forge, PA.
R.J. Meier (General Electric Co.), November 1985

This paper describes the new antenna test facility now in operation at General Electric Space Systems Division in Valley Forge, PA. The antenna test facility is located in a new building 155’ x 74’ x 53’ high. It consists of a shielded anechoic room 60’ x 56’ x 35’ high which contains both a Compact Range and a Spherical Near Field Range, instrumented over the Frequency Range 1-100 GHz to perform automatic and manual measurements of antenna characteristics. In addition it provides for a 700’ boresight range accessible through large doors with an RF trans-parent window. A 3-axis positioner can accommodate antenna apertures up to 20’. The facility is used for both, testing of antenna systems and testing of entire spacecraft for electromagnetic compatibility and interference.

Far field pattern correction for short antenna ranges
G.E. Evans (Westinghouse Electric Corporation), November 1985

Antennas are designed to operate with planar phase fronts, but are usually tested on finite length ranges that produce curved phase fronts. The result is a pattern error near the main beam. For conventional antennas the accepted range length requirement is R>2D2/? which produced a spherical phase error of 22.5 at the perimeter of a diameter D at wavelength ?. This, in turn, causes a 35 dB shoulder. For ultra low sidelobe antennas (ULSA) even longer ranges have been suggested. Such range sizes may be unavailable as well as undesirable, since the larger the range the more difficult it is to eliminate reflections.

Design of a multipurpose antenna and RCS range at the Georgia Tech Research Institute
C.P. Burns (Georgia Tech Research Institute),N.C. Currie (Georgia Tech Research Institute), N.T. Alexander (Georgia Tech Research Institute), November 1985

The design of a multipurpose Antenna/RCS range at GTRI is described. A novel approach to design of the far-field antenna range utilizes the bottom 40-foot section of a 130-foot windmill tower. The top 90-foot section is used as the main support for a slant RCS measurement range offering a maximum depression angle of 32º. A 100-tom capacity turntable, capable of rotating an M1 Tank, is located 150 feet from the 90-foot tower. The rigidity and stability of the tower should allow accurate phase measurement at 95 GHz for wind speeds up to 10 mph. In addition, a 500-foot scale-model range uses the ground plane effect to enhance target signal-to-noise and is designed to be useful at frequencies up to 18 GHz. Initially, the radar instrumentation to be utilized with the ranges includes several modular instrumentation systems and associated digital data acquisition equipment at frequency bands including C, X, Ku, Ka, and 95 GHz. The properties of these systems, which include coherence, frequency agility, and dual polarization, are discussed.

Indoor impedance measurements using a time-domain filter
D.A. Katko (Rockwell International Corporation),M.R. Matthew (Rockwell International Corporation), November 1985

This paper examines the development tests performed at Rockwell International in Anaheim, CA on VHF meandering monopole and dipole antennas which are part of the Global Positioning System satellite. The development tests included numerous impedance measurements of individual antennas configured first in their operational positions on a full-scale mockup of the GPS satellite spacecraft and second while mounted on an indoor ground plane. The initial measurements of antennas positioned on the mockup required the mockup to be located in an exceptionally large, obstruction-free environment because of the low operating frequencies (large wavelengths) of the antennas under test, and in our case a suitable environment was an empty parking lot approximately one-half mile away from the necessary test equipment. This situation necessitated frequent transportation of fragile test equipment to and from the test site which was both impractical and time-consuming. To avoid this situation when production units are to be tested later this year, a ten-foot diameter ground plane was constructed in order to perform the antenna parameter measurements indoors, which presents a very reflective environment. To minimize and theoretically eliminate the effects of these reflections on our measurements, the time-domain gating (time filter) feature of the HP 8510 Network Analyzer was utilized at the indoor test site. The gating function removes any time-domain responses outside of the gate span, the span in this case being the radius of the ground plane. When the time-domain response is Fourier-transformed back to the frequency domain, the effect of the unwanted (gated) responses is eliminated in impedance measurements. While the gated, ground plane parameter measurements will not yield the same values as those measured on the mockup, they can be used to establish an impedance, VSWR, or return loss standard from a known “good” antenna against which production antennas can be compared to determine electrical failures.

ANA antenna impedance measurements using finite-length non-precision transmission lines
C. Smith (University of Mississippi), November 1985

A method for calibrating an automated network analyzer for antenna impedance measurements through a long interconnecting transmission line is developed. The transmission line is non-precision and of nominal characteristic impedance, loss, and dispersion

The Measurement of both complex permittivity and permeability of absorptive materials
S. Tashiro (Hewlett-Packard Company), November 1985

Measurement of complex permittivity (er) and permeability (µr), both vector quantities of absorptive materials, has gained increasing importance with expanding use of the RF and microwave spectrum, particularly in communications and electromagnetic countermeasure applications. In addition, the network analyzer has seen increasing use in non-destructive measurements to determine the chemical composition of a sample dielectric material. The method described here is suited for the measurement of complex permittivity and permeability of ansorptive materials. These measurements have been made for years using numerous methods. A conventional technique involves a two-step process using a slotted line or network analyzer. First, the sample is backed up by a short circuit and the input impedance is measured. Next, the short circuit is moved ¼ ? from the sample to simulate an open circuit termination (where ? is the incident signal wavelength), and a second measurement is made. The results of these two measurements are used to solve simultaneous equations for er and µr. This procedure is repeated for each frequency of interest. Uncertainties in the measurement include test set-up frequency response, mismatch, and directivity errors, as well as the uncertainty in the physical position of the short circuit.

An Automated antenna isolation measurement system for multiple antenna pairs on full scale aircraft
J.S. DeRosa (Rome Air Development Center), November 1985

Modern fighter aircraft carry dozens of transmitting and receiving antennas for purposes of electronic countermeasures (ECM), communications, threat warning, fire control, navigation, etc. As new antenna systems are added or changed, it is important to measure the intersystem and intrasystem antenna coupling (or isolation) to insure compatibility and effectiveness of onboard systems.

A High speed measurement receiver
E. Hjort (RADC),E.C. Nordell (RRC), R. Dygert (RRC), November 1985

The receiver discussed in this paper was developed for Rome Air Development Center (RADC) under Contract F30602-81-C-0261 for testing Electronic Counter Measure (ECM) antenna systems at the Stockbridge Test Annex. This receiver, under computer control, can record ECM responses to threat radar stimuli. The ECM testing required the receiver to have a 400 kHz frequency multiplex rate in the 2-18 GHz frequency band and a 20 MHz amplitude sampling rate capability. An 80 dB interference rejection provides an accurate recording of low level signals in a multiple emitter environment. Although designed for ECM antenna testing, this receiver can have multiple uses for general antenna tests.

Pulsed, computer-controllable receiver and exciter having wide instantaneous bandwidth for testing active-element phased arrays
P.N. Richardson (Texas Instruments Incorporated), November 1985

This paper describes a receiver and exciter built by Texas Instruments for automated testing of electronic-scan antennas. The equipment is suitable for both near-field and far-field testing, and is programmable through a General-Purpose Interface Bus (GPIB) conforming to IEEE Standard 488. A two-channel design is described, but the technology is equally applicable to receivers from one to three (or more) channels. The receiver outputs are digitized as 10-bit I and Q (In-phase and Quadrature) components.

Pulsed Transmission Used for Improved Antenna Pattern Measurements
W.D. Burnside (The Ohio State University ElectroScience Laboratory),M.C. Gilreath (NASA Langley Research Center), November 1985

Pulsed systems have been used for many years to eliminate unwanted clutter in RCS measurements, but have not been used much for antenna measurements, even though similar clutter problems are common to both. There are many reasons for this, such as cost, increased bandwidth requirements, lack of necessary hardware, etc. However, with the development of modern pin diode switches, one can construct a low cost pulsed measurement system that simply adds to existing CW equipment. Using the system design presented in this paper, one can eliminate unwanted clutter from antenna measurements simply by adjusting the transmit and receive pulse widths and the delay between them. For example, it can be used to range gate out the ground bounce for outdoor measurements or the backwall for an indoor facility so that one can accurately measure the backlobe of a high gain antenna. The pulsed system is presented along with several measured examples of its use.

A 1-40 GHz synthesized source for antenna range applications
M.L. Guenther (Scientific-Atlanta Inc.),J.B. Wilson (Scientific-Atlanta Inc.) Charles H. Currie (Scientific-Atlanta Inc.) Robert C. Hyers (Scientific-Atlanta Inc.) Vincent M. Franck (Scientific-Atlanta Inc.), November 1985

Increased interest in antenna development at millimeter-wave frequencies has contributed to a growing need for signal sources operating to 40 GHz and beyond. The desirable features of such sources include broad frequency coverage; accuracy, stability, and resolution afforded by frequency synthesis; the ability to switch frequencies rapidly; and physical attributes which lend themselves to efficient use in the automated antenna range environment. This paper describes how a recently developed synthesizer meets these requirements. Design approaches used, engineering trade-offs considered, and applications information are presented.

G/T measurement of highly directive antenna systems
G.M. Briand (Harris Corporation), November 1984

A technique for improving the accuracy of G/T measurements of highly directive antennas is introduced. The technique presents was developed to overcome uncertainties in ephemeral information, antenna positioning, system gain stability, and other random and nonrandom phenomena. The particular application discussed uses Casseiopeia-A as a noise source but the technique can be adapted for use with other extraterrestrial noise sources.







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