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Absorber
Automated wideband, phase coherent polarimetric radar cross section measurements
T.K. Pollack (Teledyne Micronetics), November 1984
This paper describes the equipment, mechanics and methods of one of the outdoor ranges at Teledyne Micronetics. A computer controlled microwave transceiver uses pulsed CW over a frequency range of 2-18 GHz to measure the amplitude, phase and polarization of the signal reflected off the target. The range geometry, calibration and analysis techniques are used to optimize measurement accuracy and characterize the target as a set of subscatterers.
A Broadband RCS measurement system
R. P. Flam,R.E. Hartman, November 1984
The fast fourier transform capabilities of the Hewlett-Packard 8510 Network Analyzer provide the basis for an RCS measurement system covering the 50 MHz to 26 GHz frequency range. When used in the broadband mode, fine range resolution is achieved. Vector subtraction and gating capabilities permit the acquisition of accurate data in the presence of strong range reflections. Combining this instrument with a high speed data collection and analysis system yields a powerful RCS measurement capability.
The New ANSI RF Radiation Exposure Standard: Its Background and Impact
D.E. Hudson (Lockheed Aircraft Service Company), November 1984
This presentation will focus on the recently revised ANSI C95 RF Radiation Exposure Standard. Some of the research background for the new standard will be given, and its impact will be explained. Instrumentation guidelines for measuring potentially hazardous fields will be presented. The possible damaging effects of non-ionizing RF radiation is receiving increased attention in the public eye, and it behooves the practicing antenna engineer to be aware of the potential dangers to health and safety from exposure of RF energy.
Applications of new 16-bit SD-2000 digital synchro monitors
G.E. Bowie (Lockheed-California Company),M.B. Petri (Petri Associates), November 1984
Progress is reported on use of synchro to digital converter modules. The particular modules applied are 16 bit SDC-361 units, manufactured by ILC Data Device Corporation. Two converters are included in each pf five Model SD-2000 synchro monitors designed and fabricated by Petri Associates and acquired by the Lockheed-California Company for the antenna test facility of the Kelly Johnson R&D Center at Rye Canyon. Applications depended upon learning how Type 23TX6 synchro transmitter pairs in the model towers and elevation-over-azimuth positioners at the facility can be electrically zeroed to match the 16 bit resolution of SDC-361 synchro to digital converters.
Fourth generation indoor range
K.S. Kelleher, November 1984
The measurement of microwave antennas indoors began with the advent of commercial absorbing material. The use of absorbers can be traced back to a 2 gHz material developed by the Dutch in the Thirties. During the Forties, considerable progress was made on absorbing materials, but even after World War II, security considerations limited the application. Some materials found use as indoor shields for antenna tests, but limited bandwidth limited the utility of these materials. When a broad band absorber was developed the antenna experts did not believe that this material would be made commercially because they presumed a limited market.
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.
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.
Application of digital filtering (FFT) techniques to the measurement of absorber and anechoic chamber properties
J.C. Hungerford (Emerson & Cuming, Inc.),C.M. Robinson (Emerson & Cuming, Inc.), November 1986
Three measurements commonly used in the absorber industry include absorber testing in NRL arches, testing absorber in waveguides, and testing performance of anechoic chambers. These measurements are closely related. All are looking for the size of one E field vector in the presence of several other E fields of variable amplitudes and phases. The information is extracted from the behavior of the sum as a function of some physical position change or frequency change. Computer controlled, synthesized sources and receivers have had two effects on the way these measurements may be taken and interpreted. First, the data are now available as a series of numbers in a computer instead of a series of lines on a piece of paper. Precise and elegant processing is available to extract the information from the data. Secondly, since frequency changes are made rapidly with this type of instrumentation, and precise position changes are made slowly, the data may be taken for many frequencies at each physical position, this makes it possible to extract additional information from the observed data changes as a joint function of frequency and position. These changes are spread throughout the block of data for signal amplitude vs position and frequency.
Conformal test coupler for measurements through antennas mounted on fuselages
A.D. Ergene (General Dynamics Convair Division), November 1986
Theory, design, and test results of a conformal test coupler that can be mounted on the exterior of a vehicle for direct on site measurements of a fuselage mounted L-band antenna are presented. When there is a requirement to test vehicle instrumentation for radiated power, signal format, etc., a desired method is to couple the test equipment directly to the dedicated antenna on the vehicle. Cavity test couplers have been traditionally employed for direct measurements at the antenna under test. However, a low-profile conformal cavity has poor performance when there is no match between the energy radiated by the antenna and the received energy in the cavity. To suppress unwanted resonances and a high Standing Wave Ratio, such mismatched cavities are loaded heavily with absorber material inside, and in operation exhibit high sensitivity to surface contact and high insertion loss, yielding nonrepeatable measurements. The coupler presented here is a nonresonant cavity that supports a TEM mode compatible with the radiation from the vehicle antenna and avoids spurious resonance spikes. It exhibits extremely low insertion loss and is not sensitive to mounting misalignment. A circumferential microstrip radiator with multiple feed points and a matching network on the back side of the same substrate is wrapped around the inside of a top-hat cylindrical aluminum container. The particular test cavity was designed for the vertically polarized L-band IFF antenna on the cruise missile; however, the same principle makes testing of other fuselage-mounted antennas easier and more reliable.
Broadband reflectivity and scatter evaluation of RF absorbers
A.R. Howland (The Howland Company, Inc.),T.J. Lyon (The Howland Company, Inc.), November 1986
This paper describes specially constructed instrumentation and positioning systems used in evaluating RF absorber, discusses measurement techniques, and presents data and conclusions from current programs. The selected absorbers which were evaluated are typical of those used in anechoic chambers and terminated ranges for antenna, radome and RCS testing.
Methods for the calculation of errors due to wall effects in an RCS measurement compact range
T.P. Delfeld (Boeing Military Airplane Company), November 1987
A method for the calculation of the errors induced through target-wall-target interactions is presented. Both near-field and far-field situations are considered. Far-field calculations are performed both with Fraunhoffer diffraction theory and target antenna analogies. Absorber is considered as both a specular and a diffuse scatterer. The equations developed permit trade studies of chamber size versus performance to be made.
Evaluation of anechoic chamber absorbers for improved chamber designs and RCS performance
S. Brumley (Motorola Govt. Elect. Group),D. Droste (Motorola Govt. Elect. Group), November 1987
This paper discusses an anechoic chamber absorber evaluation which was conducted for the purpose of improving anechoic chamber and compact range performance through better absorber characterization. This study shows that performance of conventional absorber materials is dependent on selection of the material's shape, size and orientation with respect to the incident energy direction. This, demonstrates the importance of better characterization of the material. Nonhomogeneities in the material composition and physical structure were also found to significantly modify performance; in some cases even improving it. Also shown, is the need for improved evaluation techniques and procedures over conventionally used methods. An evaluation procedure using modern imaging techniques is presented. Several measured results for various absorber types and sizes are presented which show the usefulness of the evaluation technique and demonstrate relative performance characteristics for these materials. Measured reflectivity data on various absorber types, which consistently show better performance than levels specified by the vendors, are also presented.
Cost Effective, High Performance Anechoic Chamber Design
R.G. Immell (Motorola Government Electronics Group), November 1987
Motorola's Government Electronics Group (GEG) located in Scottsdale, Arizona has recently completed construction of an indoor Antenna/RCS Test Facility. Motorola achieved quality construction of this new facility by utilizing local building contractors working under Motorola supervision through concept study, design, and construction phases. Motorola achieved quality chambers without turn-key costs. Three anechoic chambers and one shielded computer room were fabricated. The chambers sizes vary from 20'W x 16'H x 41'L to 36'W x 36'H x 72'L. All chambers were evaluated using techniques described by MIL-STD-285 (Attenuation measurements for enclosures, electromagnetic shielding, for electronic test purposes, Method of) and indicated shielding effectiveness, before absorber installation of -60 to -70 db at 400 MHz and -80 db from 1-18 GHz. Shielding effectiveness increased to -80 dB at 400 MHz and to greater than -115 dB from 1-18 GHz after absorber installation. In addition, the building contains eight individual security areas meeting government standards for security as prescribed in the Defense Intelligence Agency Manual (DIAM 50-3).
A Low Cost Spherical Near-Field Range Facility
J.R. Jones (Scientific-Atlanta, Inc.),C.E. Green (Scientific-Atlanta, Inc.), D.W. Hess (Scientific-Atlanta, Inc.), K.H. Teegardin (Scientific-Atlanta, Inc.), November 1987
In any type of electromagnetic measurements, the ideas of "precision and accuracy" and "low cost" tend to be mutually exclusive. At Scientific-Atlanta, for instance, production testing of antenna products is conducted in low cost miniature "anechoic chambers" which are fabricated in-house. These "chambers" are actually medium-sized to large (64-200 cubic feet) rectangular boxes with absorber attached to their walls. They are usually equipped with single axis positioners at one or both ends, and their usefulness is limited to the measurement of axial ratio on low gain small antennas.
Time gating of antenna measurements II
D.W. Hess (Scientific-Atlanta, Inc.),V. Farr (Scientific-Atlanta, Inc.), November 1988
Currently many new compact range facilities are being constructed for making antenna pattern measurements indoors. Limited suppression of stray signals ~ due to range layout, confined surroundings and residual absorbing material reflectivity ~ represents a limitation on the accuracy of the measurements made in these facilities. Time-gating of the compact range signal appears to be a very attractive technique to reduce unwanted reflections. The authors have carried out an experimental investigation of time gating in a compact range. It is demonstrated that time-gating can improve the uniformity of the aperture field by removing the feed backlobe radiation; and, it is demonstrated that time-gating can remove the effects on a pattern of certain room reflections and of feed backlobes. When compared to conventional methods of reducing reflections based on placement of absorber, time gating appears equivalent. It does not appear however that time gating improves the conventional methods, except for measuring wide beamwidth antennas.
Measurements of compact range and room target interactions using a large flat reflecting plate
J.D. Young (The Ohio State University),A. Moghaddar (The Ohio State University), C. Clerici (The Ohio State University), November 1988
Results of an experimental study of the interactions between a scattering target and the absorber-coated walls and ceiling of the OSU Compact Range Anechoic Room are reported. A 6 ft. square flat metal reflector was mounted in the quiet zone and oriented at selected angles non-orthogonal to the range symmetry axis. In theory, this target (when non-orthogonal) has a relatively low backscattering signature, and a strong planar bistatic scattering beam which can be pointed at several regions and absorber types in the room. By processing, the bistatic iteration terms can be separated form the plate backscatter, and frequency domain spectra and/or transient response signatures of the different mechanisms produced. Th paper will present calibration information on the actual performance of the bistatic scattering beam of the plate, and measurements of both backscattering and bistatic scattering of the absorber-coated walls in the ESL chamber. Suggested guidelines for use of this as a standard anechoic room diagnostic test will be discussed.
Improving the performance of anechoic absorbers
S. Brumley (Motorola, Govt. Elect. Group), November 1988
This paper presents a simple and straightforward technique which significantly improves the performance of some anechoic absorbing materials. The method is easily applied to existing absorbers and chambers and does not change the basic design of the material. The technique involves the proper placement of additional absorbing materials between the shaped structures of the absorber to reduce major scattering contributions. These scattering mechanisms are demonstrated in the paper with measured evaluation data for various absorber types and sizes. The effectiveness of the technique has been best realized for pyramidal shaped absorbers 24 inches and longer and for normal plane-wave incidence. Improvements in the absorber's reflectivity of up to 30 dB have been demonstrated. An example illustrating the method for the reduction of the backwall RCS level of a compact-range chamber is presented.
A High performance RF absorber material optimized for millimeter frequencies
J.J. McSheehy (Spectrum Materials Inc.), November 1988
A free-space RF absorber material (RAM) has been developed and optimized for frequencies above 30 GHz. It is particularly suited for use on equipment and fixtures for RCS, antenna, radiometric, and quasi-optical testing. The material has unique geometry which yields enhanced RF performance when compared with conventional wedge or pyramidal absorbers. Mechanically, the material is elastic, resists damage from flexing or repeated contact and is non-flammable and non-toxic. It offers advantages in size, durability, and mechanical uniformity over previously available products. Data describing RF and mechanical performance are presented.
Analysis and measurements of horns in absorber-lined tunnels
G.E. Stewart (The Aerospace Corporation),R.B. Dybdal (The Aerospace Corporation), November 1988
The utility of absorber-lined tunnels to control the sidelobe levels of horns has previously been demonstrated. The use of such a tunnel gives the designer the option of designing a broadband feed, for example, and later tailoring the sidelobe level to meet a given specification. In this paper, a technique for calculating the radiation characteristics of a horn in an absorber-lined tunnel will be presented. The analysis is based on an absorbing phase screen approximation which has been used by one of the authors in analyzing the diffraction of signals around rocket plumes. Propagation through the tunnel is treated as if the wave travels through a sequence of layers in which the absorption depends on the transverse coordinates. The absorbing phase screen model will be developed, and then applied to the analysis of a Narda standard gain horn in a square tunnel which is lined with wedge absorbing material. For the determination of E and H-plane pattern cuts, a two dimensional model can be utilized. In order to determine the radiation pattern over the full range of theta and phi as is required for illuminating a reflector, a three dimensional model is needed. All calculations were implemented in Fortran on an IBM personal computer.
Advanced RF chamber absorber using resistive gradient techniques
R.N. Hubbard (Brunswick Corporation, Defense Division), November 1989
A new method for producing chamber absorber (patent applied for) is described. The absorber makes use of a resistive gradient through the bulk foam in contrast to standard pyramidal and wedge absorbers that use a geometric shaping of bulk uniform resistive material such as foam loaded with carbon. The absorber is a lightweight open cell reticulated foam, with a metallic material coated onto the fibers of the foam, with varying thickness and varying material properties used to produce the electrical gradient.


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