AMTA Paper Archive
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Communication satellite antenna measurement
RCA-Astro Electronics in Princeton, N.J. designs, develops and tests multiple-beam offset reflector antenna systems in the C and Ku frequency bands for satellite communications. Antenna measurements are performed at the antenna subsystem and the system level and on the complete spacecraft to demonstrate that alignment and performance meet their specification. This paper discussed the antenna range designs and test techniques involved in data acquisitions for contour patterns, cross-polarization isolation and antenna gain characterization. A description of the software required to obtain, analyze and present the data will be included in addition to typical test results.
Software and hardware for spherical near-field measurement systems
The following features have been added to the spherical near-field software set which is available for the Scientific-Atlanta 2022A Antenna Analyzer. Gain Comparison Measurement Probe Pattern Measurement and Correction Thermal Drift Correction Spherical Modal Coefficient Analysis Far-Field, Radiation Intensity, and Polarization Display The addition of the probe pattern correction permits antenna measurements to be made at range lengths down to within several wavelengths of touching. The addition of probe polarization measurement permits three antenna polarization measurements to be made and analyzed as well as two antenna polarization transfer measurements. Correction for phase and amplitude errors attributable to thermal drift is accomplished by the return-to-peak method. Reduction of antenna patterns to spherical modal coefficients is an essential feature of spherical near-field to far-field transforms and is offered as an augmentation to antenna design. Far field display features permit the far fields of antennas to be presented in both component and radiation intensity formats, in circular, linear and canted linear polarization components.
The Determination of near-field correction parameters for circularly polarized probes
In order to accurately determine the far-field of an antenna from near-field measurements the receiving pattern of the probe must be known so that the probe correction can be performed. When the antenna to be tested is circularly polarized, the measurements are more accurate and efficient if circularly polarized probes are used. Further efficiency is obtained if one probe is dual polarized to allow for simultaneous measurements of both components. A procedure used by the National Bureau of Standards for determining the plane-wave receiving parameters of a dual-mode, circularly polarized probe is described herein. First, the on-axis gain of the probe is determined using the three antenna extrapolation technique. Second, the on-axis axial ratios and port-to-port comparison ratios are determined for both the probe and source antenna using a rotating linear horn. Far-field pattern measurements of both amplitude and phase are then made for both the main and cross components. In the computer processing of the data, the on-axis results are used to correct for the non-ideal source antenna polarization, scale the receiving coefficients, and correct for some measurement errors. The plane wave receiving parameters are determined at equally spaced intervals in k-space by interpolation of the corrected pattern data.
Broad band feeds for new RCS ranges
Recent construction of RCS ranges has involved paraboloidal reflectors ranging from a few feet to sixty feet in diameter. These reflectors have required broad band feeds because the typical radar illuminator-receiver is capable of operating over an octave in frequency. This paper will describe a series of feeds which cover any octave in frequency from 100 mHz to 8 gHz, with coaxial line inputs. In addition waveguide-port feeds will be described which cover all of the standard waveguide bands up to 18 gHz. The four basic requirements for all of these feeds are: a) capable of handling the radar power, b) VSWR less than 2 to 1, c) orthomode operation with a 30 db isolation between the two linear polarizations and d) a radiation pattern which is constant with frequency. A fifth problem, for the reflectors which are truncated, is that of providing an elliptical cross section beam over the frequency band.
Effects of the alignment errors on ahorn's crosspolar pattern measurements. Application to L-SAT propagation package antennas.
When low crosspolar pattern measurements are required, as in the case of the L-SAT Propagation Package Antennas (PPA) with less than -36 dB linear crosspolarization inside the coverage zone, the use of good polarization standards is mandatory (1). Those are usually electroformed pyramidal horns that produce crosspolar levels over the test zone well below the -60 dB level typically produced by the reflectivity of anechoic chambers. In this case the alignment errors (elevation, azimuth and roll as shown in fig. 1) can become important and its efects on measured patterns need to be well understood.
Automated three-antenna polarization measurements using digital signal processing
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
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
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
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.
AUTOMATING THE 3 ANTENNA GAIN-POLARIZATION MEASUREMENT TO FIND SWEPT RESPONSES
The calibration of gain standards for antenna measurements requires path loss measurements between three antennas if the assumption of identical antennas is not made. The equipment finds the insertion loss for pairs of antennas as if the combination of the antennas and the free space between them were a two port network. The usual setup uses a network analyzer to measure the insertion loss. The Scientific Atlanta 2020 system can be operated as a network analyzer and used for these measurements. Part of the system is a synthesized signal source which allows frequency stepping, and along with leveling, enables the repetition of both amplitude and phase of the signals. The computer control of the equipment provides for rapid stepping through the frequencies, control of the receiver, ability to read amplitude and phase, and means of data storage for off-line analysis.
A Method of making fast high accuracy polarization measurements
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.  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
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
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
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
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
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
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.
Rotated feed horns in a compact range for RCS measurements
A way has been found to utilize the reflector return in a compact range as a source of continuous drift compensation. This is performed by translating receive polarizations 45 degrees with respect to the transmit polarizations to ensure returns in co- and cross-polarizations. An added benefit is the simplicity of alignment for the polarization calibration standard.
A Dual chamber Gregorian subreflector for compact range applications
A new dual chamber concept using a Gregorian subreflector system is being proposed for compact range applications. This concept places the feed and subreflector in a small chamber adjacent to the measurement range which contains the main reflector and target. These two chambers are connected together by a small aperture opening which is located at the focus of the main reflector. This system can potentially provide improved taper, ripple, and polarization performance. Because it uses a subreflector, the main reflector focal length can be decreased without a loss in performance. This in turn reduces the minimum length requirement for the main chamber. The design of this type of system plus the test results that have been performed will be presented at the conference.
Model 1603 compact range: a room sized measurement instrument
Harris Corporation has developed and introduced a miniature version of its shaped compact range called the Model 1603. This model is actually a scaled version of its very large compact ranges. The range features a three foot quiet zone in a very compact configuration, allowing the range to be set up in an anechoic chamber as small as a normal conference room. Performance features are equivalent to those achieved in large compact ranges by Harris, such as the Model 1640 with a forty foot quiet zone. Key features are very low quiet zone ripple, extremely low noise floor, and low cross polarization. This range can be used for the full gamut of precision RCS testing of small models or precision testing of antennas. It should also find wide application in production testing of these items. Harris can also provide turnkey compact range test systems based on the Model 1603 that use available radar instrumentation. Several of these miniature compact ranges have been delivered and are in use.
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