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Polarization

Communication satellite antenna measurement
C. Renton (RCA), November 1984

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
D. W. Hess (Scientific-Atlanta, Inc.),C. Green (Scientific-Atlanta, Inc.), B. Melson (Scientific-Atlanta, Inc.), J. Proctor (Scientific-Atlanta, Inc.), J. Jones (Scientific-Atlanta, Inc.), November 1984

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
A. C. Newell (Electromagnetic Fields Division),D. P. Kremer (Electromagnetic Fields Division), M.H. Francis (Electromagnetic Fields Division), November 1984

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
K.S. Kelleher, November 1984

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.
M. Calvo (Universidad Potitecnica de Madrid),J.L. Besada (Universidad Potitecnica de Madrid), November 1984

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.

Polarization measurements using the septum polarizer
H. E. Schrank (Westinghouse Electric Corporation), November 1983

The septum polarizer is a four-port waveguide device illustrated in its basic form in Figure 1. The square waveguide at one end constitutes two ports because it can support two orthogonal modes. A sloping (or stepped equivalent) septum divides the square waveguide into two standard rectangular wavelengths sharing a common broadwall. With a properly designed septum, this device has interesting and useful properties.

Automated, broadband antenna measurements
R.E. Hartman, November 1983

Today’s broadband electronic warfare systems are more sophisticated and complex than ever before. Many systems require that component and subsystems be characterized more extensively than in the past. This leads to the need for high-speed automated antenna measurement over a broad frequency band. For example, a program currently in progress requires that phase and amplitude measurements be made on the antenna system for four different polarizations at approximately 400 frequencies over a 9:1 bandwidth. This is achieved with an automated test system using broadband instruments which are capable of rapidly stepping through frequencies while maintaining measurement accuracy. This paper will review some of the current trends in test requirements, the problems associated with this increased demand for data and alternative solutions. Data will be presented to illustrate achievable performance.

The Ohio State University compact radar cross-section measurement range
E. Walton,J.D. Young, November 1983

This paper discusses the development and performance of a compact radar cross-section measurement range for obtaining backscattered signatures and patterns on targets up to 1.3 meters in extent, and at frequencies of 1 to eventually 100 GHz. The goal for the development was a general purpose but state of the art range which could obtain the complex radar signature vs. polarization, frequency, and target look angle for both Non-Cooperative Target Rcognition studies and Radar Cross-Section Control Studies. Since the facility was at a University, there were also concerns of cost, versatility, and ease of use in research programs by graduate students. The architecture and some design data on the system are discussed in section 2.

A Microwave interferometer technique for RCS and phase measurements
C. Coy,E. Lette, November 1982

The radar scatter matrix can be accurately characterized in magnitude, relative phase, and polarization for both far-field monostatic and bistatic conditions by means of a microwave interferometer. A separate transmitting antenna illuminates the target of interest while two adjacent receiving antennas measure magnitude and the combine in a phase comparator whose output is a phase differential caused by a changing target aspect angle. Using correct constants and scale factors this differential is integrated to provide target phase information. Different polarizations are obtained by switchable feeds. The technique can be used on an RCS range under static conditions or under dynamic conditions with a ground based radar and an airborne target. The advantage gained is that errors due to radar path length changes are eliminated.

The Orbiting Standards Package: A Recalibratable Satellite Instrument Assembly for Measuring Large Earth Station Antennas
A.J. Estin,R. C. Baird, November 1982

The concept of an Orbiting Standards Package (OSP) has been discussed as a means of making direct measurements of fields, patterns, and polarization states of signals radiated from large earth station antennas. It would also have the capability of producing test field of known intensities and arbitrary but well-defined polarization states, thereby enabling the determination of such parameters as G/T and Effective Receiving Area of earth stations. Recent developments in microwave six-port networks and in standard antennas would permit the all-electronic generation and detection of these signals. Moreover, it appears possible to recalibrate the satellite standards package to laboratory state-of-the-art accuracy following launch.

An Airborne S-band telemetry antenna system which uses a Luneberg lens aperture
W.O. Copeland (Kentron International, Inc.), November 1982

An S-band telemetry antenna system was designed and fabricated using a 30-inch diameter lightweight Luneberg lens as the aperture. It is equipped with four feeds in the azimuth plane to achieve single beam patterns or multiple beam patterns. Initial measurements with the lens without a radome were made with various feeds and feed combinations in the compact range of the Georgia Tech Engineering Experiment Station. The final design also done by Georgia Tech to Kentron Specifications, uses a custom designed quad ridged circular feed with orthogonal linear polarization outputs which are converted to left- and right-hand circular polarization using 90o hybrid couplers. A control panel permits the operator to manually select a single beam coverage of 11o x 11o, twobeams combined for 22o x 11o sector coverage, or four beams combined for 44o azimuth x 11o elevation sector coverage. A automatic mode permits the full gain of a single beam, about 22 dB, to be attained and switched automatically to the RF feed containing the greatest signal power as sensed by eight total power radiometer receivers; one for each orthogonal polarization for each of the four antenna feeds. Selectable integration time constants are 0.1, 1, 10, and 100 milliseconds. Dependable switching is obtained for signals of -99 dBm or greater. The RF switching is achieved by PIN-diode switches in 10 nanoseconds. The system employs eight state-of-the-art gain and phase matches GaAs FET low-noise preamps which have a noise figure of 1.1 dB and gain of 51 dB. External limiters at the input of each LNA protect the devices from accidental RF inputs up to six watts average power. The system was designed as a removable package to be flown aboard the U.S. Army’s C-7A Caribou aircraft with an opened rear cargo ramp to collect terminal TM data from missile reentry vehicles (RV’s) impacting near the Kwajalein Missile Range. Flight testing of the system against target of opportunity missions began the third week of June 1982. The system is expected to be declared an operational system in support of ballistic missile testing by December 1982.

A CW radar cross-section measurement facility in X-band
A.K. Bhattacharvya (Indian Institute of Technology),D.R. Sarcar (Indian Institute of Technology), S. Sanyal (Indian Institute of Technology), S.K. Tandon (Indian Institute of Technology), November 1982

A monostatic C.W. radar cross-section facility in the X-band at the Radar and Communication Centre, Indian Institute of Technology, Kharagpur, India is described. This set up is capable of automatically measuring the c.w. monostatic radar cross-section over the range of aspect angle 0 to ±180o for both TE and TM polarizations. The transmitting/receiving antenna and the rotating target is housed on the roof-top of the building and the microwave circuit with recording arrangement is in the air-conditioned laboratory. It is capable of handling a target of arbitrary shape of maximum size equal to 70 cm and uses a two-stage background (without target) cancellation technique employing Magic-T. A typical value of effective isolation between the transmitted and received signals is of the order of 70 dB and a dynamic range of 35 dB. Measurements made in this set up with different types of targets show a fair agreement with the results obtained by analytical investigations. The same set-up with necessary modifications for measuring the phase of the scattered field along with the amplitude data is expected to provide the amplitude and phase information for target identification and classification problems.

Design, calibration and performance of a full-sized aircraft antenna range from 30 MHz to 40GHz
J.F. Aubin (Flam & Russell, Inc.),R.E. Hartman (Flam & Russell, Inc.), November 1981

This paper summarizes the results of work performed for the Naval Air Development Center (NADC) on a new full-sized aircraft antenna range located in Warminster, PA. Because of the ever-increasing sophistication of aircraft systems, a facility capable of testing full scale mock-ups has become necessary to fully characterize the system in its operating environment. There are, however, several unique problems associated with such a range. Many systems of interest have a wing-tip to wing-tip baseline, which requires that the incident illumination be “uniform” over a significant aperture (approximately 40x15 feet for tactical aircraft). Differential path loss between wing-tip ends, as the aircraft is rotated, can be a source of large error, as can the parallax created by off-center rotation. Also, since today’s military aircraft carry a wide variety of systems, the range is required to be a “general use” range, operational over a wide frequency spectrum from 30 MHz to 40 GHz. A thorough examination of design trade-offs was performed relating the critical parameters of source beamwidth, specular reflection, path loss, phase error, and receive aperture size in order to choose the proper source antenna type, source height, and separation distance between source and test antennas for each frequency band of interest. Other factors in the range design were a maximum possible source height of 40 feet (approximately the height of the pedestal), and a desire to keep the separation distance fixed over the entire frequency range. Results are presented with indicate excellent performance over an 18 x 18 foot aperture for various polarizations. It was found that the range operates effectively as a ground reflection range from 30 MHz to 3 GHz, and as an elevated range at higher frequencies. Peak-to-peak amplitude ripples over the test aperture of 1.0 dB (corresponding to a reflection level of –25 dB) were acheived over a significant portion of the frequency spectrum.

Antenna test facility at ISAC-Bangalore
S. Pal (ISRO Satellite Centre),V.K. Lakeshmeesha (ISRO Satellite Centre) V. Mahedevan (ISRO Satellite Centre) L. Nicholas (ISRO Satellite Centre) R. Ashiya (ISRO Satellite Centre), November 1980

The paper describes a simple but unique antenna test facility suitable for aerospace antenna developments. The total idea can be easily adopted by organizations who wish to carry out antenna measurements with minimum required instrumentation. The facility majorly caters for omni and wide beam antenna measurements, has been set up at ISRO Satellite Centre, Bangalore, India. It has been extensively used for omnidirectional antenna developments in VHF, UHF, L, S, and X-bands for India’s various space programs. Radiation pattern, gain, polarization and impedance measurements can be carried out both in near free space conditions as well as the ground reflection modes. The main feature of the facility is the use of large fiber-glass mounting structures for avoiding reflections and perturbations in radiation patterns due to impressed surface currents, specially in VHF ranges. Field probing is done by the use of a fiber-glass X-Y probe positioner. The facility used Scientific Atlanta 1752 Receiver and 1540 Recorder. Suitable software has been added to the facility for contour plotting of radiation levels, calculation of efficiency isotropy, and polarization properties.

An Automated Precision Microwave Vector Ratio Measurement Receiver Offers Solutions for Sophisticated Antenna Measurement Problems
F.K. Weinert, November 1980

This paper describes a new, automated, microprocessor controlled, dual-channel microwave vector ratio measurement receiver for the frequency range 10 MHz to 18 GHz. It provides a greater than 120 dB dynamic range and resolutions of 0.001 dB and 0.1 degree. Primarily designed as an attenuator and Signal Generator Calibrator, it offers solutions to antenna measurement problems where high accuracies and/or wide dynamic measurement ranges are required such as for broadband cross-polarization measurements on radar tracking antennas, highly accurate gain measurements on low-loss reflector antennas, frequency domain characteristics measurements on wide-band antennas with resulting data suitable for on-line computer conversion to time domain transient response and dispersion characteristics data and wideband near field scanning measurements for computing far field performances. The measurement data in the instrument is obtained in digital form and available over an IEEE-488 bus interface to an outside computer. Measurement times are automatically optimized by the built-in microprocessor with respect to signal/noise ratio errors in response to the measurement signal level and the chosen resolution. Complete digital measurement data amplitude of both channels and phase, is updated every 5 milliseconds.

Antenna Polarization measurements
R. Heaton, November 1979

In recent years there has been an increasing requirement for more extensive and precise measurements of the polarization properties of antennas. Some of the more conventional polarization measurement techniques are no longer applicable because of the required measurement time or the achievable accuracy. This presentation is an overview of polarization measurement methods which may be employed on far-field antenna ranges. Instrumentation requirements and sources of error are also included.







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