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T.L. Wilkey (Georgia Institute of Technology),E.H. Atkinson (Georgia Institute of Technology),
H.P. Cotten (Georgia Institute of Technology),
J.F. Kirksey (Georgia Institute of Technology),
J.M. Hudgens (Georgia Institute of Technology),
O.D. Asbell (Georgia Institute of Technology), November 1989
The Georgia Tech Research Institute has designed and installed a large outdoor compact range for the U.S. Army Electronic Proving Ground at Ft. Huachuca, Arizona. This range will primarily be used to obtain performance data for antennas installed on full-size tanks, aircraft and other vehicles to characterize antenna/vehicle interactions.
This paper describes the vehicle positioner that is being used with the compact range. Design considerations have resulted in a challenging positioner design. Some of the features of the positioner include: * positioning of large vehicles weighing up to 70 tons approximately 42.5 feet above ground * using a hydraulic servo system to drive the positioner * minimizing RF reflections by using ogive shaped shells on the positioner legs and tilting the legs forward
S. Brumley (Denmar, Inc.),R.G. Immell (Motorola Govt. Elect. Group), November 1989
The requirement to measure lower radar cross-section (RCS) levels within anechoic chambers has demonstrated the need to further analyze the performance of microwave absorbers. The interactions of the feed system, compact range reflector, target mount, and target/test body with the microwave absorber greatly effect both the measurement accuracy and ambient noise level within the anechoic chamber. Better absorber characterization and understanding leads to improved chamber performance analysis and chamber design modeling. Past absorber studies have evaluated the backscatter performance of most absorber types, however, bistatic performance characterizations have been limited.
This paper will discuss a method of obtaining bistatic absorber data which offers the advantages of time gating and synthetic aperture imaging to improve measurement isolation and accuracy. The approach involves illuminating a large absorber test wall about several incidence angles with the plane wave generated by a compact range. A receive antenna is then moved about the test wall and bistatic scattering is observed. The technique provides improved measurement results over methods utilizing NRL arch type systems. Bistatic absorber data has been collected and analyzed over angles from normal to near grazing incidence.
Test results will be demonstrated with different absorber shapes, sizes, orientations, and material transitions from wedge to pyramidal. Various bistatic conditions will be analyzed for both polarizations over a number of frequencies.
T.S. Watson (Texas Instruments Incorporated), November 1989
Circularly polarized radar cross-section (RCS) measurements place stringent requirements on an RCS range. Indoor compact ranges without the problems of ground reflections have the potential of making accurate circular polarization (CP) measurements. A simple method for CP RCS measurements is described using broadband meander-line polarizers over the compact range feed horns. Axial ratio and differential phase measurements were performed to evaluate the polarizer fabrication accuracy. Basic scattering shapes were measured to test the performance of the CP measurement system. Comparison of CP measurements with analytical predictions demonstrated the success and limitations of the technique.
K. Miller (Scientific-Atlanta, Inc.),W.G. Swarner (Scientific-Atlanta, Inc.), November 1989
Compact range facilities designed for RCS measurements have exhibited a performance-limiting effect commonly referred to as "feed ringing". "Feed ringing" is a phenomenon in which energy is stored in or about the RF feed structure and is sustained for a sufficient period off time after the source is turned off such that its presence contaminates the true target return. This effect has placed severe constraints on the design of the RF feed for the compact range, particularly in regard to its operating bandwidth. This paper presents the design of a lossless, waveguide type RF feed suitable for compact range application with a demonstrated useful bandwidth approaching a full octave.
A. Lai (The Ohio State University ElectroScience Laboratory),E.H. Newman (The Ohio State University ElectroScience Laboratory),
W.D. Burnside (The Ohio State University ElectroScience Laboratory), November 1989
Due to the limited size of a compact range, an antenna with low sidelobes, broad bandwidth, broad beam, small physical signature, low scattering level and reasonably high power handling are required.
Historically, slot line antennas are circuit board type antennas noted for their thin cross-section, low cost of fabrication, scalability and high package density in array applications. A broadband version, fed by a microstrip line (and therefore easily connected to microstrip transceiver circuits etched on the same circuit board) is described in this paper.
Test models with different shapes and using different dielectric materials were built and tested. The measured VSWR, radiation and scattering patterns of the various antenna designs are presented.
I.J. Gupta (The Ohio State University ElectroScience Laboratory),W.D. Burnside (The Ohio State University ElectroScience Laboratory), November 1989
The effects of metallic tapes which are used to cover gaps in a compact range reflector are studied in this paper. To study these effects, the normalized tape scattered fields are computed in the target zone. A method of moments technique is used to compute the tape scattered fields. It is shown that the tape scattered fields are directly proportional to the thickness and width of the tape and are inversely proportional to the square root of the distance from the tape. Using the computed results, an empirical formula for the tape scattered field is developed. One can use the empirical formula to compute the highest frequency for which a given size tape can be used.
J. Cantrell (Harris Corporation),C.J. Koepsell (Harris Corporation), November 1989
After having delivered a model 1630 and a model 1640 compact range plus a number of smaller 1606 and 1603 ranges, Harris has improved their product to meet the demanding needs for operating frequencies of 35 GHz and higher. In characterizing the two large ranges, it was discovered that the surface accuracy as originally optimized would not support the highest operating frequency. Achieving the required surface accuracy required additional surface measurement data in combination with RF contour plots and was very time consuming. From those lessons learned, several features have been incorporated into the next generation of compact ranges that make more accurate reflector surfaces easily achievable. The features include optimally located adjustment mechanisms, additional targets on each panel, software for best fitting the panel surface to minimize steps, techniques for eliminating panel steps in place, and gravity bias setting of panels.
A. Jain (Hughes Aircraft Company),I.R. Patel (Hughes Aircraft Company), November 1988
In practical ISAR applications the quality of the image obtained depends upon the distortions in the wavefront illuminating the target, effects introduced by the radar-target path, the accuracy of the angle and frequency steps used in obtaining the data, vibration, and multiple reflections from neighboring objects. Results of analysis, simulation and data obtained in an RCS compact range are presented to quantify the relationships of the image degradation introduced by these effects.
D.W. Hess (Scientific-Atlanta, Inc.),K. Miller (Scientific-Atlanta, Inc.), November 1988
In this presentation we consider the features and performance of a large serrated-edge compact range reflector. This is a straightforward innovation from earlier compact range reflectors. The virtual vertex reflector is a paraboloidal surface truncated to exclude the vertex. This layout provides the advantages of better use of reflector surface area, reduced feed blockage, and reduced feed backscatter. The design is made economical by the use of serrations.
M.L. Foster (Harris Corporation GCSD), November 1988
A simple computer program which performs a direct calculation of the Discrete Fourier Transform (DFT) was written to generate the plane wave spectrum of a compact range from sampled field probe data. This program was used to analyze idealized quiet zone fields, computer predicted quiet zone fields and measured field probe data. Data from this analysis is presented along with suggestions for the correct interpretation of the results.
K. Miller (Scientific-Atlanta, Inc.),R.W. Kreutel (Scientific-Atlanta, Inc.), November 1988
The use of serrated edge treatment in the design of a compact range collimating reflector is one method of mitigating the effects of edge diffraction on quiet zone performance. In this note a physical optics analysis is applied to the serrated reflector. The computational procedure is described and several results are presented. In particular, computed results are presented for the S-A Model 5755 compact range reflector and compared with experiment.
G.M. Briand (Harris Corporation GCSD), November 1988
The design, fabrication, and testing of a high directivity, constant beamwidth feed horn is presented in this paper. The subject feed horn is designed to illuminate a shaped reflector compact range operating from 140 to 170 GHz. Design considerations related to pattern control and VSWR are discussed. Fabrication challenges are also considered. Primary pattern test results are presented and compared to predictions. Integration (into the reflector system) considerations are reviewed and quiet zone performance is discussed.
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.
H.F. Schluper (March Microwave Systems, B.V.), November 1988
In the last few years, the interest in Radar Cross Section (RCS) measurements has increased rapidly. The development of high-performance Compact Ranges (CR) has made possible measurements on large targets down to very low RCS levels (below -70 dBsm).
RCS imaging is a powerful tool to determine the location of scattering sources on a target. The response of the target is measured as a function of the frequency and aspect angle. A two-dimensional Fourier transform then gives the reflection density as a function of down-range and cross-range. If the response is measured vs. azimuth and elevation, even a complete 3-D image is possible.
For high-resolution imaging (large bandwidth, wide aspect-angle span) a direct 2-dimensional Fourier transform gives rise to errors caused by the movement of the scatterers during the measurement. These errors can be corrected by applying a coordinate transformation to the measured data, prior to the Fourier transforms. This so called focused imaging allows further manipulation of measured data.
However, the measurement accuracy can be a limiting factor in application of these techniques. It will be shown that the Compact Range performance as well as positioning accuracy can cause serious errors in high-resolution imaging and thus in interpretation of processed data.
A. Dominek (The Ohio State University),I.J. Gupta (The Ohio State University),
W.D. Burnside (The Ohio State University), November 1988
Conventional radar imaging requires large amounts of data over large bandwidths and angular sectors to produce the location of the dominant scattering centers. A new approach is presented here which utilizes only two swept frequency scans at two different look angles for two-dimensional images or three swept frequency scans at three different look angles for three-dimensional images. Each swept frequency scan is the backscattered response of a target. A different plane wave illumination angle can be conveniently obtained by offsetting the feed horn from the focus of a compact range reflector without rotating the target. The two- and three-dimensional target information for the location of the dominant scattering centers is then obtained from the band limited impulse responses of these swept frequency scans.
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.
C.W. Sirles (Scientific-Atlanta, Inc.),W.L. Tuttle (Scientific-Atlanta, Inc.), November 1988
This paper describes recent advances in antenna measurement instrumentation for millimeter frequency applications. Application of a new, lightweight, programmable, ruggedized signal source at 40 and 60 GHz is outlined. An RF instrumentation system for millimeter frequency antenna range application is detailed. A millimeter-to-microwave converter is described which improves millimeter antenna range performance. System performance levels are predicted. Compact range configuration and operation at millimeter frequencies is detailed. Specific measurement examples are presented to demonstrate the measurement sensitivity which can be achieved.
A.L. Lindsay (Harris Corporation GCSD),S.G. Russell (Harris Corporation GCSD), November 1988
This paper reports the quiet zone characteristics of the Harris family of compact ranges. Field probe measurements of systems having quiet zones of 3, 6, and 40 feet are presented. The quiet zones were characterized using a two way measurement with a trihedral corner reflector target. One way CW field probe measurements with an open ended waveguide are also presented for the Model 1606 range. A Discrete Fourier Transform (DFT) is imbedded in the test set software and provides an angle domain signature of extraneous signals illuminating the quiet zone. The two way range transfer functions of the Model 1603 and 1606 ranges are verified using calibrated spherical targets with the HP-8510 network analyzer operating as a time domain reflectometer.
H.R. Phelan (Harris Corporation GCSD), November 1988
Harris Corporation is in the final stages of implementing the Model 1640 Compact Range for The Boeing Corporation. This paper provides an overview of the development, fabrication, installation, and test activities on this very significant advance in the compact range state-of-the-art. This range represents a significant increase in quiet zone size over prior art. It features the wide dynamic range, low noise floor, and high quality quiet zone that is achievable using the Harris-proprietary shaped range technique. Another feature of this range is the completely panelized construction technique. This allows the production of very large, very precise reflector systems. Primary technical features of the Model 1640 are a 40 foot quiet zone, a -70 dBsm noise floor, and a frequency range extending from VHF to millimeter-wave frequencies.
T-H. Lee (The Ohio State University ElectroScience Laboratory),W.D. Burnside (The Ohio State University ElectroScience Laboratory), November 1988
A near field focus procedure for image processing using near zone backscattered fields obtained in a compact range is presented in this paper. An array of defocussed feeds is used to illuminate a target in a compact range. The backscattered field received at each feed antenna with the target being stationary is compensated by a phase factor. These compensated signals are then summed coherently to obtain a cross range image of the target which indicates the location of the various scattering centers associated with the target. Numerical examples are presented to validate this technique.
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