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The application of analytical photogrammetry to the measurement of microwave antenna reflectors is discussed. The basic concepts of photogrammetric triangulation are outlined and accuracy considerations are reviewed. Recent developments in close-range photogrammetric systems, which have greatly enhanced both the accuracy and economy of antenna mensuration, are briefly discussed and advantages of the photogrammetric approach are highlighted. Three recently conducted antenna measurement projects are reviewed.
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 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.
Enhanced accuracy in antenna pattern measurements using the HP-8510 is possible by using a novel calibration procedure. By circumventing antenna dispersion, this procedure leads to better resolution of multipath responses and thus increases the effectiveness of gated measurements. Measured patterns of a dipole antenna are presented to illustrate the effectiveness of this procedure.
This paper addresses two major issues that impact long-range outdoor antenna measurements with the HP 8510 network analyzer: using a radiated reference signal to provide phaselock reference, and using harmonic mixers with a phase locked local oscillator (LO). The measurements were made at microwave frequencies on a 700 ft outdoor antenna range using a reference antenna in antenna test configurations with the HP 8511A frequency converter and with a harmonic mixer configuration using the new HP 8510 "Remote Phaselock" option developed by Hewlett-Packard. In addition to CW antenna patterns, the use of time domain and gating to reduce the effects of ground reflections was investigated. Measurement considerations and results are discussed. The favorable outcome of this investigation is applicable to a broad variety of antenna measurements.
As with all areas of production testing, it is desirable to be able to perform anechoic chamber testing of transmit antennas with a fully automatic test system. This paper has been prepared to describe how automating anechoic chamber testing of transmit antennas will yield data that is accurate, repeatable and cost-effective. The heart of the automated system is a Hewlett-Packard Model 8510 Network Analyzer controlled by a desktop computer.
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.
A large, tapered anechoic chamber exists at the NASA Johnson Space Center (see Figure 1). This chamber has been used to test antennas mounted on full-size replicas of the Apollo moon lander. Also, antennas mounted on a scale model of the Space Shuttle have been tested in this facility. The chamber will have extensive utilization in the future for testing proposed Space Station antennas and other satellite antennas.
Conventional pattern measurements are difficult to apply when the aperture is very large (250 lambda or more), particularly in the case of a relatively fragile antenna structure intended for a space application. Near field techniques can offer a solution, but may need a relatively large R.F. enclosure and custom instrumentation. This paper examines various alternative approaches in the case of the 15 m planar array under development at CAL for Radarsat. Specifically, the techniques under consideration include planar probing, cylindrical probing, planar cylindrical probing, intermediate range spherical probing, and some special variants. It is shown that the fact that the Radarsat antenna generates shaped beams as opposed to pencil beams impacts the relative accuracies achieved by these techniques to a very significant extent. The data collection and processing time, the size of the anechoic chamber needed, and the instrumentation requirement are also important considerations.
The Radarsat synthetic aperture radar (SAR) antenna is baselined to be a large slotted waveguide planar array, with a rectangular aperture of dimensions 1.5m x 15m. At the nominal frequency of 5.3 GHz, the dimensions in terms of wavelengths are approximately 26.5 lambda x 265 lambda. The usual 2D(squared) divided by lambda formula yields a far-field range length of at least 7.96 Km, which is far beyond the range lengths currently available to the program. A more conservative design would at least double that number, rendering a far-field measurement concept all but impracticable. (*) This work was carried out for the Radarsat Project Technical Office of the Communications Research Centre, Canadian Department of Communications, under DSS contract OSR84-00175, funded by the Canadian Department of Energy, Mines, and Resources.
Spar Aerospace, along with other aerospace companies, have experienced an evolution in the development of spacecraft antennas over the past 20 years. Spacecraft antennas originated as either simple antennas providing figure of revolution patterns for spin stabilized communication satellites or simple monopoles for telemetry and command purposes. Communication satellite antennas later evolved to shape beam reflector type configurations. Spaceborne antennas are now moving to even larger reflector antennas and to planar arrays for radar applications. This evolution in spaceborne antennas has been followed by a parallel evolution in antenna test facilities and facilities requirements.
With the development of precise antenna measurement systems and the increasing demands on antenna performance, attention is more and more turning towards the requirements of antenna measurement systems and their verification. One way a verification can take place is by measuring on several systems one common antenna. There are a number of advantages and disadvantages associated with such an exercise, however several purposes are served, not least amongst which is the confidence gained on the part of a customer that the range in question has demonstrated certain capabilities.
A planar and cylindrical near field facility is described. The facility was designed, constructed and integrated at RAFAEL as an extension to its veteran S-A 2020 Antenna Analyzer. The system utilizes open loop stepper motors for linear motion. Operation modes include cartesian, plane-polar and cylindrical measurements. All measurement control and data acquisition functions are performed by the 2020 computer. Conversion routines are being run on a host CDC CYBER mainframe computer and include a new algorithm for polar probe correction.
Fundamental research in the behaviour of the electromagnetic field in the immediate vicinity of radiators, scatterers or diffracting objects, indicates the need for equipment capable of scanning the three dimensional volume surrounding such objects. Such equipment requires the ability to determine the amplitude and phase of the vectorial components of the fields in a variety of ways, such as on a three dimensional grid of uniformly or otherwise spaced points, on regular (such as spherical, paraboloidal) or on arbitrarily defined surfaces or along various loci.
Near-field measurement techniques are widely used today for antenna far-field pattern characterization. Since the 60's, much has been done concerning accuracy. The three main coordinate systems, planar, cylindrical, and spherical have been investigated. probe corrections have been introduced [1] - [6].
The planar scanning system is commonly used in the near field testing of high gain antennas, where the rectangular measurement grids are used. The polar grids are also used, which are more convenient when the antenna aperture is circular. In the planar scanners the measurements are carried out in the x-y plane in increments of both x and y. The result of the measurement is an mxn matrix of the near field data consisting of m cuts with n data points per each cut. The far field patterns may then be calculated, using the near field data, by the aperture field integration or the modal expansion methods [1]. In this paper the aperture field integration method is studied, where the far field components can be calculated from [1] - [2].
The NBS has developed a test for estimating the effects of multiple reflections between the probe and antenna on the far field using a near-field measurements. The essence of this test is to take near-field data at more than one separation distance. For each separation distance the far field is obtained using a Fourier transform. The different far fields are then averaged in a complex manner. The difference between the average far field and each of the other far fields is due to multiple reflection effects.
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.
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.
The National Bureau of Standards, in Boulder, has been doing various types of swept frequency antenna measurements for a number of years. Included in these are swept gain measurements on the Extrapolation range and fixed point Near-Field swept measurements.