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With the recent use of dual-polarized transmission and reception on communications links, the capability to perform accurate polarization measurements is an important requirement of test-range systems. Satellite antennas are commonly measured in the clean, protected environment of compact and near-field ranges, and a circularly polarized feed/field probe is a primary factor in establishing their polarization properties. The feeds also provide excellent source-horn systems for tapered anechoic chambers, where their circular symmetry and decoupling of the fields from the absorber walls improve the often troublesome polarization characteristics of tapered chambers. Circularly polarized feeds are generally composed of four primary waveguide components: the orthomode transducer, quarter-wave polarizer, scalar ring horn, and circular waveguide step transformer. Linearly polarized feeds omit the quarter-wave polarizer. This paper discusses the design and performance of high-polarization-purity source feeds for evaluating the polarization properties of antennas under test. Circularly polarized feeds have been constructed which operate over 10- to 20-percent bandwidths from 1.5 to 70 GHz. Gain values are generally in the area of 12 to 18 dBi, with cross-polarization isolation in excess of 40 dB. Representative measured data are presented.
A new multi-purpose antenna measurement facility was put into operation at the National Institute of Standards and Technology (NIST) in 1993. This facility is currently used to perform gain, pattern, and polarization measurements on probes and standard gain horns. The facility can also provide spherical and cylindrical near-field measurements. The frequency range is typically from 1 to 75 GHz. The paper discusses the capabilities of this new facility in detail. The facility has 10 m long horizontal rails for gain measurements using the NIST developed extrapolation technique. This length was chosen so that gain calibrations at 1 GHz could be performed on antennas with apertures as large as 1 meter. This facility also has a precision phi-over-theta rotator setup used to perform spherical near-field, probe pattern and polarization measurements. This setup uses a pair of 4 m long horizontal rails for positioning antennas over the center of rotation of the theta rotator. This allows antennas up to 2 m in length to be accommodated for probe pattern measurements. A set of 6 meter long vertical rails that are part of the source tower gives the facility that added capability of performing cylindrical near-field measurements. Spherical and cylindrical near-field measurements can be performed on antennas up to 3.5 m in diameter.
In polarimetric RCS measurements, the cross-polarization levels which are required in the test zone, correspond closely to those which are realizable with most Compact Antenna Test Ranges (CATR). On the other hand, such a performance may not satisfy the accuracy requirements in cross-polarization measurements of high performance microwave antennas. These applications include spacecraft antennas, ground stations for satellite communications or microwave antennas for terrestrial applications, where two polarizations are used simultaneously.
Composites of the electrically conducting polymer polypyrrole with paper, cotton cloth and polyester fabrics have been evaluated for use in radar absorbing structures. Reflectively measurements on the composites in the range 8-18 GHz and transmission line modelling have revealed impedance characteristics with a common transition region. Relationships between substrate material, polymer loading and electrical performance have been explored. Polarization characteristics have also been measured. The electrical model has been successful in predicting the performance of Salisbury screen and Jaumann multi-layer designs of RAM.
To gain greater insight into the design of surface ships with reduced radar cross-section characteristics, a structure resembling a ship deckhouse was physically modeled and measured. The structure was represented as a truncated pyramid. Four scaled pyramids were fabricated, all identical except for the radii of the four vertical (slanted) edges. The pyramids were measured at the University of Massachusetts, Lowell Research Foundation, submillimeter laser compact range. Measurements were made a scaled X-band using a laser-based system that operates at 585 GHz with the pyramids scaled at a ratio of 1:58.5. These shaper were measured at 0.75 degrees depression angles on a smooth metal ground plane at both HH and VV polarizations. The goal of this study was to determine if small changes in the radius of the curvature of the slanted edges could significantly affect the radar cross-section of the pyramid. In this paper the results of measurements of the pyramids will be presented. The data are compared with computer code predictions and the differences are discussed.
The proposed synthesis method allows the calculation of the diffraction figure in the focal plane of the compact range, starting from a field distribution in linear polarization over a plane in the Fresnel zone. Applying this method (in only one dimension) to the ideal near field of a FFOC compact range, a linear array is synthesized which can be extrapolated to a planar array feeder design; providing excellent features in the quite zone.
The Georgia Tech Research Institute has designed and built a field probe for the U.S. Army Electronic Proving Ground Compact Range. The field probe is an R-0 scanner covering a 59-foot diameter area. It includes a laser-referenced Z-axis correction servomechanism, a polarization positioner, and a cable handling system for one-way data acquisition.
The dynamic, polarization/frequency diverse, Instrumentation Radar System (IRS) described herein combines the features of an X-band radar tracker with a wideband, fully polarimetric coherent data collection system. Mounted in a transportable trailer, the system can be towed to virtually any site to acquire radar signature measurements on moving aircraft. Specifically, this system can collect the complete, polarimetric target scattering matrix as a function of frequency in real time from all three traditional monopulse channels, as well as from the usually terminated diagonal difference channel. The acquired data can be used for multidimensional images, or for studying the characteristics and performance of monopulse trackers following real targets.
Coherent subtraction algorithms, such as specular subtraction, require precision target alignment with the imaging radar. A few degrees of phase change could significantly degrade the performance of coherent subtraction algorithms. This paper provides an analysis of target position measurement errors have on ISAR data. The paper addresses how traditional position errors impact phase and image focusing. Target rotational positioning errors are also evaluated for their impact on magnitude errors from specular misalignment and polarization sensitive scattering and image phase errors from height-of-focus limitations. Several tables of data provide a useful reference to ISAR data experimenters and users.
The Antenna Metrology group of the National Institute of Standards and Technology (NIST), working in cooperation with McClellan Air Force Base (MAFB), Sacramento, CA, have examined-measurement techniques to test a large phased-array antenna using planar near-field (PNF) scanning. It was necessary to find methods that would be useful in both field and production testing and could provide gain and diagnostic information in a simple and timely manner. This paper will discuss several aspects of the PNF measurement cycle that impact effective testing of the antenna array. These aspects include the use of a polarization-matched probe, the effect of scan truncation both on the transform to the far field and the transform to the aperture plane, and use of gain prediction curves as a diagnostic tool.
A dual-linearly polarized probe developed for use in planar near-field antenna measurements is described. This probe is based upon Scientific-Atlanta's Series 31 Orthomode Feeds originally developed for spherical near-field testing. The unique features of this probe include dual orthogonal linear ports, high polarization purity, excellent port-to-port isolation, an integrated coordinate system reference, APC-7 connectors, and a thin-wall horn aperture to minimize probe AUT interactions. The probe was calibrated at the National Institute of Standards and Technology (NIST) and the calibration data consisting of the probe's complete plane-wave spectrum receiving characteristic s'02(K) were imported directly into the Scientific-Atlanta Model 2095/PNF Microwave Measurement System. This paper describes the dual-ported probe and its application in a planar near-field range.
A reflector antenna has been designed for the RCS measurements. The antenna is dual linearly polarized and exhibits constant beamwidth over an octave bandwidth. The design principle has been to keep the effective antenna aperture constant in terms of the wavelength over an octave bandwidth. The theoretical design lead into the choice of the antenna and the feed. The reflector was an offset parabolic reflector. The feed was a ring-loaded conical corrugated horn. The measurement results of the designed reflector antenna showed very good agreement with the computer results. The V- and H- polarization characteristics of the antenna are almost identical.
Performance trade-offs are investigated between the use of clustered waveguide bandwidth feeds and the use of one multi-octave bandwidth single aperture feed in a prime focus compact range for dual linear polarization. The results show that feed structure may be used for advantage for the particular test requirements of compact range systems for Radar Cross Section Measurement.
A comparative analysis of different geometries of dual compact antenna test ranges is done looking at the cross-polarization level and the scanning capability of the system. The analysis is based on a very simple and quick computation of the fields over the main refector [sic] projected aperture.
Radar Cross Section (RCS) measurements are typically made at linear polarizations (usually horizontal and vertical) and the transmit and receive polarizations are the same (co-polarized). In addition, however, it is sometimes desirable to measure the cross-polarized RCS of a target (i.e., transmit horizontal, receive vertical or vice-versa). A complete set of both co-and cross-polarized RCS of a target is called a scattering matrix. This paper describes the algorithm used for calibrating a scattering matrix measurement in the McDonnell Douglas Technologies Inc. (MDTI), Radar Measurement Center (RMC). Verification data collected at Ka band on various targets is included to validate the algorithm and implementing computer code.
A bistatic calibration technique for wide-band, full-polarimetric instrumentation radars is presented in this paper. First general bistatic measurement problems are discussed, as there are the coordinate systems, the definition of polarization and the bistatic scattering behavior of convenient calibration targets. In chapter two the new calibration approach is presented. The general mathematical and physical description of errors introduced in the bistatic system is based on the radiation transfer matrix. The calibration procedure is discussed for the application with a vector network analyzer based instrumentation radar. For verification purposes measurements were performed on several targets.
Recently, the theory and computer programs on the Periodic Moment Method (PMM) for scattering from both singly and doubly periodic arrays of lossy dielectric bodies have been developed. The purpose is to design microwave wedge and pyramid absorber for low reflectivity so that one can improve measurements and/or reduce the size of the anechoic chamber. With PMM, the reflection and transmission coefficients of periodically distributed bodies illuminated by a plane wave have been accurately calculated on the Cray Y-MP supercomputer at the Ohio Supercomputer Center. Through these studies, some wedge and pyramid absorber configurations have been designed, fabricated and tested in the OSU/ESL Anechoic Chamber. Very good agreement between calculations and measurements has been obtained. In the 1990 AMTA meeting, several wedge absorber designs and results for the TM case and normal incidence were presented. In this paper, the measured and calculated frequency responses of some experimental wedge designs, as well as an 8” and 18” commercial wedge and pyramid absorber panels will be reported for both TM and TE polarizations. Time domain responses will also be shown for both measurements and calculations.
The target designer using a compact range to verify the predicted RCS of his target needs to know what measurement errors are introduced by the range. The underlying definition of RCS assumes that the target is in the far-field, in free-space, and illuminated by a plane wave. This condition is approximated in a compact range. However, to the extent that these conditions are not met, the RCS measurement is in error. This paper, using the results of the preceding companion paper1, formulates an error budget which shows the typical sources that contribute to the RCS measurement error in a compact range. The error sources are separated into two categories, according to whether they depend on the target or not. Receiver noise is an example of a target independent error source, as are calibration errors, feed reverberation (“ringdown”), target support scattering and chamber clutter which arrives within the target range gate. The target dependent error sources include quiet zone ripple, cross polarization components, and multipath which correspond to reflections of stray non-collimated energy from the target which arrives at the receiver at the same time as the desired target return. These error contributors depend on the manner in which the target interacts with the total quiet zone-field, and the bistatic RCS which the target may present to any off-axis illumination. Results presented in this paper are based on the design of a small compact range which is under construction at RRI. The results include a comprehensive error budget and an assessment of the range performance.
An assessment of instrumentation error sources and their respective contributions to overall accuracy is essential for optimizing an electromagnetic field measurement system. This study quantifies the effects of measurement receiver signal processing and the relationship to its transient response when performing measurements on rapidly varying input signals. These signals can be encountered from electronically steered phased arrays, from switched front end receive RF multiplexers, from rapid mechanical scanning, or from dual polarization switched source antennas. Numerical error models are presented with examples of accuracy degradation versus input signal dynamics and the type of receiver IF processing system that is used. Simulations of far field data show the effects on amplitude patterns for differing rate of change input conditions. Criteria are suggested which can establish a figure of merit for receivers measuring input signals with large time rates of change.
The experience with near-field scanning at Scientific-Atlanta began with a system based upon a analog computer for computing the two-dimensional Fourier transform of the main polarization component. When coupled with a phase/amplitude receiver and a modest planar near-field scanner this system could produce far-field patterns from near-field scanning measurements. In the 1970’s it came to be recognized that the same advances, which made the more sophisticated probe-corrected planar near field measurements possible, would enable conventional far-field range hardware to be used on near-field ranges employing spherical coordinates. In 1980 Scientific-Atlanta first introduced a spherical near-field scanning system based upon a minicomputer already used to automate data acquisition and display. In 1990, to meet the need of measuring complex multistate phased-array antennas, Scientific Atlanta began planning a system to support the high volume data requirement and high speed measurement need represented by this challenge. Today Scientific-Atlanta is again pursuing planar near-field scanning as the method of choice for this test problem.