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Polarization

A Method of making fast high accuracy polarization measurements
G.B. Melson (Scientific-Atlanta, Inc.),J.J. Anderson (Scientific-Atlanta, Inc.), November 1986

A method is presented for making fast multi-frequency high accuracy polarization measurements using a digital computer. This paper will provide a brief review of the IEEE standard polarization definitions, their applicability to the three antenna method, and finally a fast two antenna method. [1] The fast two antenna method uses a dual polarized orthomode sampling antenna along with a standard antenna whose polarization is known. The dual polarized sampling antenna is calibrated before the test data is acquired using the polarization standard in two different orientations 90 degrees apart. Once the calibration data is acquired the dual polarized orthomode antenna is used as a sampling antenna for the AUT. Since the sampling antenna is dual polarized the AUT polarization data can be obtained rapidly for many frequencies since neither antenna is required to rotate. This method has been used to acquire polarization data for over 500 frequencies in less than 20 seconds.

An Automated antenna measurement system using the HP8510
D.J. Markman (Flam & Russell, Inc.),R.E. Hartman (Flam & Russell, Inc.), November 1986

An automated antenna measurement system using the HP8510 is described. The system controls the HP8510, associated signal source, and antenna positioner, to provide a fully integrated, automated test facility. Automation speeds and enhances testing by implementing the following features: - Multiple frequency pattern measurements in a single cut of the pedestal. - Patterns with rotating linear polarization - Automatic pedestal control - Storage and presentation of fully documented test data. - Storage and recall of test routines These features complement the premier microwave receiver available today, the HP8510 which offers: - Continuous frequency coverage from .045 to 26.5 GHz - Unparalleled measurement accuracy - 80 dB dynamic range - Time domain gating These features are integrated through software developed using modern software management techniques to form a system which is state of the art in measurement performance, operator interface, and software life cycle supportability.

Near-feld testing of the 30 GHz TRW proof-of-concept multibeam antenna
R.R. Kunath (National Aeronautics and Space Administration),R.J. Zakrajsek (National Aeronautics and Space Administration), November 1986

Near-field testing was conducted on the 30 GHz TRW proof-of-concept (POC) Multibeam Antenna (MBA). The TRW POC MBA is a dual offset cassegrain reflector system using a 2.7 m main reflector. This configuration was selected to assess the ability to create both multiple fixed and scanned spot beams. The POC configuration investigated frequency reuse via spatial separation of beams, polarization selectivity and time division multiple access scanning at 30 GHz.

Measurement of EIRP and receive flux density in the near field
R.D. Ward (Hughes Aircraft Company),E.J. McFarlane (Hughes Aircraft Company), November 1986

Near field ranges have been used extensively to measure antenna parameters. These ranges have been shown to be very accurate for measuring absolute gain, polarization, and gain patterns. Most antennas are intended to be used with a receiver, a transmitter, or both. In many cases, it is important to characterize the antenna and active electronics as a system.

VHF/UHF short pulse RCS measurement system
J.F. Aubin (Flam & Russell, Inc.),R. Flam (Flam & Russell, Inc.), November 1986

Flam & Russell, Inc. has developed a short pulse radar cross section measurement system (Model 8101) which operates from VHF up to L band. This paper describes operation of the system, with emphasis given to the design considerations necessary to minimize susceptibility to a number of problems that have imposed serious limitations on achievable sensitivity at lower frequencies in pulsed RCS outdoor measurement systems. These problems have been, to a great extent, solved in the current system design. The system has been designed for use in outdoor range facilities with a variety of target sizes. A w ideband, high power transmitter is capable of producing pulses 50-350 nanoseconds wide at peak levels of up to several kilowatts. A phase coherent wide bandwidth receiver provides amplitude and phase information at video for sampling. A maximum of four independently located range gates may be selected and set with a resolution of one nanosecond. The data collection system features a three-tier processor structure for dedicated position data processing, target data processing, and system I/O and control, respectively. A real time display of RCS versus position coordinate is available to the operator, as well as a real time indication of the presence of radio frequency interference (RFI). A 60 foot reflector antenna equipped with a duo polarized feed provides full scattering matrix capability with 30 dB of polarization isolation and better than 50 dB of "ghost" suppression. Careful antenna structure and transmission line design has eliminated reverberation or "pulse ringing" problems. A radar "figure of merit" (ratio of peak transmitted power to receiver noise floor for the required pulse bandwidth) of better than 150 dB has been achieved.

A Useful test body
A. Dominek (The Ohio State University),H. Shamansky (The Ohio State University), R. Barger (NASA Langley Research Center), R. Wood (NASA Langley Research Center), November 1986

The advent of improved compact ranges has promoted the development of a test body, named the almond, to facilitate the measurement of scattered fields from surface mounted structures. A test body should at least have the following three features: (1) provide a very small return itself over a large angular sector, (2) provide an uncorrupted and uniform field in the vicinity of the mounted structure and (3) have the capability to be connected to a low cross-section mount. The almond satisfies the first two requirements by shaping a smooth surface which is continuous in curvature except at its tip. The name almond is derived from its surface similarity to the almond nut. The surface shaping provides an angular sector where there is no specular component. Hence, only low level tip and creeping wave scattering mechanisms are present resulting in a large angular quiet zone. The third requirement is accomplished by properly mounting the almond to a low cross-section ogival pedestal. The mount entails a metal column between the almond and the pedestal covered with shaped absorbing foam. These contoured pieces hide the column and form a blended transition from the almond to the pedestal and yet allow an unobstructed rotation of the almond. Backscatter pattern and swept frequency measurements performed in our compact range illustrate the scattering performance of the almond as a test body. The almond body alone has a backscatter level of -55 dB/m(squared) in its quiet zone. Comparisons of measured hemisphere backscattered returns on the almond are made with those calculated of a hemisphere over an finite ground plane for both principal polarizations for a verification performance test. * This work was supported in part by the National Aeronautics and Space Administration Langley Research Center, Hampton, Virginia under Grant NSG 1613 with the Ohio State University Research Foundation.

Performance of an optimum antenna range illuminating horn
J. Russum (Texas Instruments Inc.), November 1986

The paper describes a simple feed horn designed to illuminate an antenna test range used to measure broad bandwidth antenna patterns with rotating linear polarization. Principal requirements of the feed are equal E and H plane beamwidths with minimal sidelobes in all planes. These characteristics are required to avoid undesirable pattern modulation caused by varying specular scatter and unequal beamwidth vs rotation angle. A survey of pyramidal, conical, and diagonal feed horn patterns revealed that each configuration has high sidelobes in at least one plane making it undesirable for the intended application. Both the pyramidal and conical horns have high side lobes in the E plane. The diagonal horn has very good sidelobe characteristics in the principal planes, but has 13 to 16 dB sidelobes in the diagonal plane.

Automated three-antenna polarization measurements using digital signal processing
J.R. Jones (Scientific-Atlanta, Inc.),D.E. Hess (Scientific-Atlanta, Inc.), November 1985

In this paper we present a three-antenna measurement procedure which yields the polarization of an unknown antenna to an accuracy comparable to that of the improved method of Newell. The complete method is based on step-scan motion of the two polarization axes on which the antenna pairs are mounted. As a special case this step-scan procedure includes the usual single axis polarization pattern method of polarization measurement. This three antenna polarization measurement method can be readily automated and is carried out straightforwardly with the assistance of a minicomputer for data acquisition and data reduction. The data reduction method is based on conventional digital Fourier transform techniques and has the advantage of inherent noise rejection. It utilizes a large number of sample points which greatly overdetermine the parameters to be measured. The method has been verified experimentally with measurements made on multiple overlapping sets of three antennas, as is conventional for this kind of procedure. The data are presented for broad-beam antennas of the type used as near field probe horns.

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.

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.

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.

A Dual shaped compact range for EHF antenna measurements
J.K. Conn (Harris Corporation),C. L. Armstrong (Harris Corporation), L. S. Gans (Harris Corporation), November 1984

A dual offset shaped reflector compact range is described. Improvements over the traditional single reflector, apex-fed compact range are outlined and discussed. A design plan for a dual offset shaped reflector compact range for EHF antenna measurement is presented.

Millimeter wave antenna measurements
M. S. Morse (Boeing Aerospace Company), November 1984

Millimeter wave antenna measurements are hampered by a lack of cost effective automated test equipment and the necessity of using unwieldy waveguide set-ups. This paper describes some practical considerations in using readily available test equipment to perform accurate, repeatable antenna measurements. Experimental results of gain, polarization and sidelobe level measurements will be discussed and compared with calculated results.

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.

Polarization correction of spherical near-field data
J.R. Jones (Scientific-Atlanta, Inc.),D.W. Hess (Scientific-Atlanta, Inc.), November 1984

This paper describes the relationship of probe polarization correction to probe-pattern corrected and non-probe-pattern-corrected spherical near-field measurements. A method for reducing three-antenna polarization data to a form useful for polarization correction is presented. The results of three-antenna measurements and the effects of polarization correction on spherical near-field measurements are presented.

Performance criteria for RCS measurement systems
J. Tavormina (Scientific Atlanta), November 1984

The purpose of an instrumentation radar is to characterize the Radar Cross Section (RCS) of a target as a function of target aspect and radar frequency. In addition, an instrumentation radar may be used to produce a high resolution radar image of a target which is useful in target identification work and as a diagnostic tool in radar cross section reduction. These purposes differ from those of a conventional radar, in which the objective is to detect the presence of a target and to measure the range to the target. Several different radars are currently used to perform radar cross section measurements. Common instrumentation radars may be classified as CW, Pulsed CW (Low-Bandwidth IF), Linear FM (FM-CW), Pulsed (High-Bandwidth IF) and Short Pulse (Very High-Bandwidth IF). These radars accomplish the measurement task in distinct manners, and it is sometimes difficult to determine where the strength or weakness of each radar lies. In this paper, a set of performance criteria is proposed for RCS measurements. The proposed criteria can be applied uniformly to any instrumentation radar independent of the type of radar design employed. The criteria are chosen to emphasize those performance characteristics that relate directly to RCS measurements and thus are most important to the user. Two instrumentation radars which have been designed at Scientific Atlanta, namely the Series 2084 (Linear FM) and the Series 1790 (Pulse), are used to illustrate the application of the performance criteria.

Preliminary development of a phased array near field antenna coupler
D. D. Button (Sanders Associates, Inc.), November 1984

End-to-end testing of electronic warfare (EW) equipment at the organizational or flight lines level is accomplished by use of an antenna coupler which is placed over the EW system antenna. The coupler is used to inject a stimulus signal simulating a signal emanating from a distant radar, and to receive and detect the EW system response (EW transmit) signal. The coupler is used to determine the EW receiver sensitivity over a swept frequency coverage and the EW transmit gain and effective radiated power (ERP) versus frequency characteristics, as well as to determine the operating integrity of the EW antenna and transmission lines.

Ultra low sidelobe testing by planar near field scanning
K. R. Grimm (Technology Service Corporation), November 1984

An innovative technique has been developed for accurately measuring very low Sidelobe Antenna patterns by the method of planar near field probing. The technique relies on a new probe design which has a pattern null in the direction of the test antenna’s steered bean direction. Simulations of the near field measurement process using such a probe show that -60dB peak side-lobes will be accurately measured (within established bounds) when the calibrated near field dynamic range does not exceed 40 dB. The desireable property of the new probe is its ability to “spatially filter” the test antenna’s spectrum by reduced sensitivity to main beam ray paths. In this way, measurement errors which usually increase with decreasing near field signal level are minimized. The new probe is also theorized to have improved immunity to probe/array multipath and to probe-positioning errors. Plans to use the new probe on a modified planar scanner during tests with the AWACS array at the National Bureau of Standards will be outlined.

Rolled edge modification of compact range reflector
W.D. Burnside (Ohio State University),B. M. Kent (Air Force) M. C. Gilreath (NASA), November 1984

The compact range is an electromagnetic measurement system used to simulate a plane wave illuminating an antenna or scattering body. The plane wave is necessary to represent the actual use of the antenna or scattering from a target in a real world situation. Traditionally, a compact range has been designed as an off-set fed parabolic reflector with a knife edge or serrated edge termination. It has been known for many years that the termination of the parabolic surface has limited the extent of the plane wave region or, more significantly, the antenna or scattering body size that can be measured in the compact range. For example, the Scientific Atlanta (SA) Compact Range is specified to be limited to four foot long antennas or scattering bodies as shown in their specifications. Note that the SA compact range uses a serrated edge treatment as shown in Figure 1. This system uses a parabolic reflector surface which is approximately 12 square feet so that most of the reflector surface is not usable based on the 4 foot square plane wave sector. As a result, the compact range has had limited use as well as accuracy which will be shown later. In fact, the compact range concept has not been applied to larger systems because of the large discrepancy between target and reflector size. In summary, the target or antenna sizes that can be measured in the presently available compact range systems are directly related to the edge treatment used to terminate the reflector surface.

Extension of the extrapolation method for accurate swept frequency antenna gain calibrations
A. Newell (National Bureau of Standards),A. Repjar (National Bureau of Standards), S.B. Kilgore (National Bureau of Standards), November 1984

For approximately 10 years the National Bureau of Standards has used the Extrapolation Technique (A. C. Newell, et al., IEEE Trans. Ant. & Prop., AP-21, 418-431, 1973) for accurately calibrating transfer standard antennas (on-axis gain and polarization). The method utilizes a generalized three-antenns approach which does not require quantitative a priori knowledge of the antennas. Its main advantages are its accuracy and generality. This is essentially no upper frequency limit and it can be applied, in principle, to any type of antenna, although some directivity is desirable to reduce multipath interence.







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