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Despite their high cost, phased array antennas are becoming popular for radar applications because of their ability to provide reliable information even in a hostile environment. Evaluation of these antennas requires parameters like gain, radiation pattern, beam width, sidelobe (both near and far off) azimuth and elevation null depth, etc. to be tested over the entire range of frequency spots and scan angles. Typically, if the number of frequency spots are 24 and the number of beam positions for which the measurement has to be done are about 100, then the total number of measurements needed to generate the required data are 7200. In addition, phased arrays with a space feed have to be initially collimated at all the spot frequencies. The outdoor testing of these many parameters may not be convenient, and at times it may even be impossible. The planar near field measurement technique provides a systematic and accurate method of measuring large array antennas for all the required parameters.
The National Institute of Standards and Technology (NIST) has written a certification plan to ensure that a proposed planar near-field facility is capable of measuring high-performance phased arrays. Generally for a complete plan, one must evaluate many aspects including scanner alignment, near-field probe alignment, alignment of the antenna under test, RF crosstalk, probe position errors RF path variations, the receiver's dynamic range and linearity, leakage, probe-antenna multiple reflections, truncation effects, aliasing, system drift, room multipath, insertion loss measurements, noise, and software verification. In this paper, we discuss some of the important aspects of the certification plan specifically written for the measurement of high-performance phased-array antennas. Further, we show how the requirements of each aspect depend on the measurement accuracies needed to verify the performance array under test.
An alternate method is presented for computing far-field antenna patterns from planar near-field measurements. The method utilizes near-field data to determine equivalent magnetic current sources over a fictitious planar surface which encompasses the antenna, and these currents are used to ascertain the far-fields. An electric field integral equation (EFIE) is developed to relate the near-fields to the equivalent magnetic currents. Method of moments (MOM) procedure is used to transform the integral equation into a matrix one. The matrix equation is solved with the conjugate gradient method (CGM), and in the case of a rectangular matrix, a least squares solution for the currents is found without explicitly computing the normal form of the operation. Near-field to far-field transformation for planar scanning may be efficiently performed under certain conditions by exploiting the block Toeplitz structure of the matrix and using CGM and Fast Fourier Transform (CGFFT) thereby drastically reducing comparison and storage requirements. Numerical results are presented by extrapolating the far-fields using experimental near-field data.
A theoretical comparison for the application and derivation of modal expansion and integral equation methods is presented. It is shown that one formulation can be transformed into the other one using Fourier transform. From this point of view it can be stated that both method solves the same integral equation but for the modal expansion approach the integral equation is solved in the spectral domain while for the integral equation method the same equation is solved in the space domain. It is shown that for most of the practical antenna types the integral equation method gives more accurate far-field estimation than the modal expansion method, particularly in the planar scanning case.
In recent years different techniques have been developed for measuring large aperture antennas on smaller ranges. Problems still exist with these techniques, though, such as impracticality and size restrictions. This paper presents a new method for measuring a phased-array antenna at approximately one tenth the far-field distance. This method involves focusing the test array to a probe a certain distance away, then moving the probe along an elliptical path. Since different elliptical paths can be easily generated with the same test hardware, this new method promises to yield a measurement technique that can be readily adapted to different sized antennas. This paper also presents the results of computer simulations showing the validity and limitations of this technique.
When the planar near-field method is used for antenna characterization, two probes are required to measure an antenna under test (AUT). The receiving patterns (both amplitude and phase) of these probes, obtained from planar near-field measurements, must be utilized to accurately determine the far field of the AUT. This process is commonly called planar near field probe correction. When the AUT is nominally circularly polarized (CP), the measurements are more accurate and efficient if nominally circularly-polarized probes are used. Further efficiency is obtained when only one probe which is dual-polarized is used to allow for simultaneous measurements of both components. However, when using dual-port CP probes to measure the antenna, we must apply the probe correction even for on-axis measurements.
When the planar near-field method is used for antenna characterization, two probes are required to measure an antenna under test (AUT). The receiving patterns (both amplitude and phase) of these probes, obtained from planar near-field measurements, must be utilized to accurately determine the far field of the AUT. This process is commonly called planar near field probe correction. When the AUT is nominally circularly polarized (CP), the measurements are more accurate and efficient if nominally circularly-polarized probes are used. Further efficiency is obtained when only one probe which is dual-polarized is used to allow for simultaneous measurements of both components. However, when using dual-port CP probes to measure the antenna, we must apply the probe correction even for on-axis measurements.
Near field measurement techniques have become popular now a days (sic) and they are preferred for many applications to the conventional testing. In particular, for testing large planar arrays with a variety of parameters to measure, planar near field technique proves to be useful. The design of the planar near field measurement system varies with the needs of the measurements. This flexibility makes it suitable for somespecial (sic) tests on array antennas. The important design parametersinclude (sic) the type of scan, positional error limit, receiver and source instabilities, cabling, etc. It discuss (sic) in detail the type of scanning when large number of parameters like sum, azimuth and elevation difference patterns of an array antenna for various scan positions are to be measured. This paper describes a planar field system of size 2.5X2.5 metres (sic) and discusses its design details and in particular the type of scanning and the methods of correcting instability of the entire system.
Planar Near-Field scanning is becoming the method of choice for testing many types of antennas. These antennas include planar phased arrays, space deployable satellite antennas and other antennas either too large to move during the test or otherwise sensitive to the gravity vector. The planar scanner is a major component of the measurement system and must provide an accurate and stable platform for moving the RF probe across the test antenna's aperture. This paper describes basic design requirements for a planar near-field scanner. Based on recent development activity at Scientific-Atlanta several design considerations are presented. Scanner parameters discussed include basic scanner concepts and geometry, scanner accuracy and stability, RF system including cabling and accuracy, load carrying requirements of the RF probe carriage, position and readout systems and drive and control systems. A scanner will be presented which incorporates many of the design features discussed.
This paper describes a novel planar near-field measurement system designed to test a beam-steered flat face phased array antenna. This system is unique in its ability to measure multiple beams during a single scan of the aperture. The system utilizes a very fast planar scanner with six foot by six foot of travel combined with fast beam-steering commands to significantly reduce the test time of multiple-beam phased array antennas. These features combined with software based on algorithms developed by the National Institute of Standards and Technology provide state of the art measurements of planar phased array antennas.
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 series of measurements to validate the performance of a Planar Near-Field (PNF) Antenna Test Range located at the Satellite and Aerospace Systems Division at Spar Aerospace Limited were made by Scientific-Atlanta during the month of February 1992. These measurements were made as a part of a contract to provide Spar with a Model 2095 Microwave Measurement System with planar near-field software options and related instrumentation and hardware. The range validation consisted of a series of self-tests and far-field pattern comparison tests using a planar array antenna provided by Spar that had been independently calibrated at another range facility. This paper describes the range validation tests and presents some of the results. Comparisons of far-field patterns measured on the validation antenna at both the Spar PNF facility and another antenna range are presented.
A novel planar near-field antenna measurement and diagnostic system is described. This bi-polar near-field system offers a large scan plane size with reduced "real estate" requirements and a simple mechanical implementation resulting in a highly cost effective antenna measurement system. A brief description of the bi-polar near-field range and its associated data processing methods are given. Measured results are compared with those obtained on a far-field range and a plane rectangular planar near-field range. It is shown that the UCLA facility produces highly accurate results which rival those of modern production antenna measurement facilities. Holographic images produced from measured data are provided to demonstrate the diagnostic capabilities of the antenna range and to provide electromagnetic field visualization for educational purposes.
Rapid data acquisition is crucial in making comprehensive near-field scanning tests of electronically-steered phased array antennas. Multiplexed data sets can now be acquired very rapidly with high speed automatic data acquisition. To obtain high speed without giving up accuracy in probe position a feature termed subinterval triggering has been devised. To obtain simultaneously reliable thermal drift or tie scan data a feature termed block tie scans has been devised. This paper describes these two features that yield speed and accuracy in planar near-field scanning measurements.
Increasing demands on antenna design characteristics have led to corresponding increases in test requirements, particularly in the need for high speed multi-frequency or multi-beam measurements. Special steps are required in the data acquisition process to maintain synchronization of the data to insure accurate results are achieved. This paper will describe techniques used by NSI for a planar near-field measurement system using a Hewlett Packard 8530A with multiple frequencies and multiple beams acquired in the inner loop of the scan pattern.
A Planar Near Field to Far Field (PNF/FF) Transformation Program has been developed. This PNF/FF package includes probe correction, spectral filtering, position errors correction and sampled data expansion. In order to evaluate how measurement system errors affect PNF/FF transformation results, a whole set of simulation routines have also been implemented. In this paper, main modules of the PNF/FF package are discussed and error simulation models together with correction routines are described.
A new near-field far-field transformation procedure, based on only amplitude measurement, is tested from both simulated and measured data. The measurements have been collected at Marconi Research Center and refers to a parabolic reflector working at 9 Ghz. This first experimental validation of the procedure fully support (sic) the feasibility of phaseless near field measurement in the antenna testing.
A new near-field far-field transformation procedure, based on only amplitude measurement, is tested from both simulated and measured data. The measurements have been collected at Marconi Research Center and refers to a parabolic reflector working at 9 Ghz. This first experimental validation of the procedure fully support (sic) the feasibility of phaseless near field measurement in the antenna testing.
The anechoic chamber housing the TUD-ESA Spherical Near Field Far Field Antenna Test Facility at the Technical University of Denmark dates back to 1967 while the present RF and data collection and control systems were designed and installed in several stages between 1978 and 1985. This paper undertakes to describe the definition and realization of a refurbished and upgraded radio anechoic facility for antenna measurements given as a starting point the already existing facility. In a parallel effort, both the RF and data collection and control subsystems are being renewed and upgraded.
Some of the novel mechanical and electronic subsystems features on a recently installed high specification planar near-field scanner are described together with a discussion of the problems encountered during the commissioning period. The test facility incorporates a number of novel design concepts both in terms of its instrumentation, control and processing subsystems. Features of the facility are the speed of data acquisition and the accuracy of the acquired near-field data. Scan speeds of up to 0.8 m/s and positional accuracies of 30 microns in the Z-axis have been achieved, and the near-field data is acquired, displayed and measured on the fly, hence allowing a typical 3m x 3m scan to be executed and the measured near-field results to be displayed and processed within a period of thirty minutes.