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Phased Array

A Certification plan for a planar near-field range used for high-performance phased-array testing
M.H. Francis (National Institute of Standards and Technology),A. Repjar (National Institute of Standards and Technology), D. Kremer (National Institute of Standards and Technology), November 1992

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.

Design considerations for a planar near-field scanner
J.H. Pape (Scientific-Atlanta, Inc.),A.L. Wilcox (Scientific-Atlanta, Inc.), J.D. Huff (Scientific-Atlanta, Inc.), November 1992

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.

Implementation of a small planar near-field system
C.B. Brechin (Scientific-Atlanta, Inc.),R. Kaffezakis (Scientific-Atlanta, Inc.), November 1992

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.

Speed and accuracy for near-field scanning measurements
D.W. Hess (Scientific-Atlanta, Inc.),D.R. Morehead (Scientific-Atlanta, Inc.), S.J. Manning (Scientific-Atlanta, Inc.), November 1992

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.

Measurement receiver error analysis for rapidly varying input signals
O.M. Caldwell (Scientific-Atlanta Inc.), November 1991

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.

Determining faults on a flat phased array antenna using planar near-field techniques
A. Repjar (National Institute of Standards and Technology),D. Kremer (National Institute of Standards and Technology), J. Guerrieri (National Institute of Standards and Technology), N. Canales (National Institute of Standards and Technology), November 1991

The Antenna Metrology Group of the National Institute of Standards and Technology (NIST) has recently developed and implemented measurement procedures to diagnose faults on a flat phased-array antenna. First, the antenna was measured on the NISTplanar near-field (PNF) range, taking measurements on a plane where the multiple reflections between the probe and the antenna under test are minimized. This is important since the PNF method does not directly allow for these reflections. Then, the NIST PNF software which incorporates the fast Fourier transform (FFT) was used to determine the antenna’s gain and pattern and to evaluate the antenna’s performance. Next, the inverse FFT was used to calculate the fields at the aperture lane. By using this technique, errors in the aperture fields due to multiple reflections can be avoided. By analyzing this aperture plane data through the use of detailed amplitude and phase contour plots, faults in the antenna were located and corrected. The PNF theory and utilization of the inverse FFT will briefly be discussed and results shown.

Arc range test facility
P.R. Franchi (Rome Laboratory),H. Tobin (Rome Laboratory), November 1991

Problems exist with the measurement of large aperture antennas due to the far field requirement. This paper discussed a new method to measure a phased array at about 1/10 the normal far field. The basic idea involves focusing the test array at probe antenna a distance R away from the aperture. In the described measurement technique the probe antenna is placed on an arm that rotates 100º on the focal arc given by Rcos(?). This arc minimizes defocusing due to phase aberrations. To minimize the amplitude errors, the pattern of the probe antenna is carefully matched in order to compensate for the 1/R variation induced amplitude error. The application of this technique will enable arrays to be measured in anechoic chambers, allowing convenient classified testing, while avoiding the effects of weather, and will reduce the risks inherent in the high power testing on transmit. The results of a computer simulation is presented that characterizes the validity and limitations of the technique.

Application of beam space techniques to phased array calibration and fault compensation
H.M. Aumann (Massachusetts Institute of Technology),F.G. Willwerth (Massachusetts Institute of Technology), November 1991

Beamspace techniques are usually employed to synthesize phased array antenna patterns of arbitrary shape. In this paper a beamspace method is used to calibrate the pattern of a 32-element linear array with a conventional array taper. By measuring the antenna pattern in specific directions the beamspace technique permits the actually applied excitation function to be determined with little mathematical effort. Iterative corrections can then be made to the excitation function to maintain low sidelobe performance, or to compensate for element failures. Since local corrections to the array pattern result in global changes to the excitation function, explicit knowledge of where an element failure has occurred is not required. The beamspace analysis was carried out using antenna patterns obtained by electronically scanning the array past a far-field source. Such pattern measurements offer the possibility of maintaining phased array performance in an operational environment.

Performance measurements of an active aperture phased array antenna
L.D. Poles (Rome Laboratory),E. Martin (Rome Laboratory), J. Kenney (Rome Laboratory), November 1991

Transmit – receive modules (T/R) utilizing GaAs monolithic microwave integrated circuit (MMIC) technology for amplifiers, attenuators, and phase shifters are becoming integral components for a new generation of radars. These components, when used in the aperture of a low sidelobe electronically steerable antennas, require careful alignment and calibration at multiple stages along the RF signal path. This paper describes the calibration technique used to measure the performance of an active aperture 64 element S-band phased array antenna that employs T/R modules at every element. RF component performance and phased array sidelobe characeristics are presented and discussed.

G/T measurement technique for low directivity wide beam antennas
R.P. Heon (Texas Instruments),S. Sanzgiri (Texas Instruments), November 1990

The end-to-end G/T performance of an antenna system has typically been measured using celestial bodies. The technique requires high G/T performance ( 10 dB/K) to obtain accuracies of 0.5 dB. In addition, the measurement is dependent on several atmospheric and environmental conditions. This paper describes a technique for measuring the G/T performance of a low directivity, wide beamwidth antenna. The discussion details the measurements, extrapolation technique to demonstrate performance at specified atmospheric conditions, measurement uncertainties, and test results. This new G/T measurement technique offers advantages over the existing technologies. Measurements are limited to the output port of the antenna so as to include all interactions between components within the system. This allows for accurate characterization of phased array performances. In addition, testing can be performed on indoor antenna ranges under environmentally controlled conditions.

A New implementation of the planar near-field back projection technique for phased array testing and aperture imaging
D. Garneski (Hughes Aircraft Company, Radar Systems Group), November 1990

A new implementation of the planar near-field back projection technique for phased array testing and aperture imaging is described. In the alignment of phased arrays, the aperture field is treated as a continuous distribution rather than using idealized array concepts. The continuous field is then sampled to obtain element excitations. In this way, nonrectangular arrays can easily be accommodated. The method also produces highly interpolated images of apertures that can offer much insight into their nature. Also, any polarization of the aperture field may be obtained if the probe pattern has been characterized. The technique uses large FFTs which are computed very quickly by a workstation located in the facility. Results from an iterative phase alignment of a 12x18 phased array are presented, as well as highly interpolated images of apertures and results which demonstrate the polarization selection.

Adaptive alignment of a phased array antenna
H.M. Aumann (Massachusetts Institute of Technology),F.G. Willwerth (Massachusetts Institute of Technology), November 1990

A technique for aligning a phased array is described. Array element attenuation and phase commands are derived from far-field patterns measured without calibrations. The technique is based on iteratively forming mulls in the antenna pattern in the directions specified by a uniform array illumination. It may be applied in situations where array elements are not individually accessible, or where an array contains no build-in calibration capacity. The alignment technique was evaluated on a far-field range with a linear, 32-element array operating at L-band. The array containing transmit/receive modules with 12-bit amplitude and phase control. Insertion attenuation and phase measurements were comparable to those obtained by conventional techniques. However, the alignment procedure tends to compensate for the effects of nonuniform element patterns and range multipath. Thus, when used to implement other excitation functions, the array sidelobe performance with adaptive calibrations was substantially better.

On-line bite to accurately monitor beam position, beam shape, and system performance of electronically scanned phased array antennas
J.H. Acoraci (Allied-Signal Aerospace Company), November 1990

Electronically scanned phased array antennas typically have a large number of beam positions. Accurate on-line monitoring of phased array beam positions can be used to ensure proper antenna and total system performance. Bendix has developed and successfully implemented a beam-position monitoring technique designated the “RF Integral Monitor System”. Use of this on-line technique does not interfere with normal system operation and yields results that are comparable to results obtained on an actual far field antenna range. The RF Integral Monitor technique and specific hardware implementations, for both linear and circular electronically scanned phased arrays, will be described in this paper.

Array antenna diagnosis and calibration
M. Johansson (Ericsson Radar Electronics AB, Antenna Systems),B. Svensson (Ericsson Radar Electronics AB, Antenna Systems), November 1990

A method for obtaining the individual element excitations of an array antenna from measured radiation patterns is presented. Applications include element failure diagnosis, phased array antenna calibration, and pattern extrapolation. The measured far-field information is restricted to visible space which does not always contain the entire Fourier domain. A typical example is phased array antennas designed for large scan angles. A similar problem arises during near-field testing of planar antennas in which case the significant far-field domain is restricted by the scanning limitations of the near-field test facility. An iterative procedure is then used which is found to converge to the required solution. The validity of the approach has been checked both using the theoretical radiation patterns and real test cases. Good results have been obtained.

Near-field testing of adaptive radar systems
A.J. Fenn (Massachusetts Institute of Technology), November 1990

Airborne or spaceborne radar systems often require adaptive suppression of interference and clutter. Before the deployment of this adaptive radar, tests must verify how well the system detects targets and suppresses clutter and jammer signals. This paper discusses a recently developed focused near-field testing technique that is suitable for implementation in an anechoic chamber. With this technique, phased-array near-field focusing provides far-field equivalent performance at a range distance of one aperture diameter from the adaptive antenna under test. The performance of a sidelobe-canceller adaptive phased array antenna operating in the presence of near-field clutter and jamming is theoretically investigated. Numerical simulations indicate that near-field and far-field testing can be equivalent.

Scale model aircraft/phased array measurements
M. O'Brien (Loran Randtron Systems),R. Magatagen (Loran Randtron Systems), November 1989

This paper describes the techniques applied to a fully automatic computer controlled, HP8510 based, range gated digital data acquisition system used to provide scale modeled large aperture synthesis, evaluation of aircraft blockage effects, array patterns, element cancellation ratios, as well as providing a large accurate data base for radar simulation exercises.

Measurement of phased array patterns by near-field focusing
H.M. Aumann (Massachusetts Institute of Technology),F.G. Willwerth (Massachusetts Institute of Technology), November 1989

Performance verification of an adaptive array requires direct, real-time sampling of the antenna pattern. For a space-qualified array, measurements on a far-field range are impractical. A compact range offers a protected environment, but lacks a sufficiently wide field of view. Conventional near-field measurements can provide antenna patterns only indirectly. This paper shows how far-field antenna patterns can be obtained in a relatively small anechoic chamber by focusing a phased array in the near-field. The focusing technique is based on matching the nulls of far-field and near-field antenna patterns, and is applicable to conformal or nonuniform phased arrays containing active radiating elements with independent amplitude and phase control. The focusing technique was experimentally verified using a 32-element, linear, L-band array. Conventionally measured far-field and near-field patterns were compared with focused near-field patterns. Very good agreement in sidelobe levels and beamwidths was achieved.

Alignment measurements using a special purpose phased array antenna
L.D. Poles (Rome Air Development Center), November 1989

A special purpose 80 element linear phased array antenna was aligned using an iterative phase cycling method. First, the array was aligned to yield maximum main-beam power in the reactive near-field zone and then in the far-field zone. A record of the phase-shifters settings achieved for each zone was kept for use as look-up table during operation. In situ electronic main-beam steering was performed to compare sidelobe performance for the two cases. This report describes the measured results obtained using the phased cycling alignment procedure and compares the measured one-way radiation pattern for the two distance conditions.

Measurement of phased array gain-to-temperature ratio using the solar method
J. Harris (Georgia Tech Research Institute), November 1989

The gain-to-temperature ratio (G/T) is a fundamental parameter used in describing the performance of a receiving antenna. This single parameter is sufficient to describe the antenna's contribution to the sensitivity of the receiving system because it includes the antenna gain as well as noise producing factors such as sidelobes and ohmic losses. The G/T of a phased array antenna is determined by the size of the aperture and the aperture distribution as well as the ohmic losses, electronic gains and noise figures of the components in the array assembly. Measurement of phased array G/T is an important means of verifying that the integrated assembly is performing to the level equivalent to the sum of the subassemblies. One method of measuring antenna G/T uses the sun as a source of radio frequency noise. This method is equivalent to the hot/cold source method used to measure the noise figure of a two port device. The sun is used as the hot source and the clear sky is used as the cold source. The difference in the noise output from the antenna with it pointed at the hot sun and the cold sky is used along with the known solar flux density and appropriate correction factors to calculate the G/T. The measured G/T is corrected for the antenna beamwidth and losses in the post antenna measurement system. This paper describes the solar method used to measure the G/T of a large S-Band airborne phased array designed and built by the Georgia Tech Research Institute (GTRI). The measurement results are compared to the G/T values calculated knowing the performance of the components that make up the array assembly.

Design of a short range for testing large phased arrays
L. Goldstone (Norden Systems), November 1989

Large arrays require large separations between the transmit antenna and the antenna under test (AUT) to measure pattern parameters in the far field. For the subject AUT, a range of 6 miles with a spurious signal level of -58 dB was necessary to obtain the required accuracy. Measurements have been performed on a significantly shorter range without serious degradation. The antenna was focused for the angle of electronic scan and the resulting pattern measured. The theoretical far field patterns were compared with the calculated focused patterns for the short range. The maximum sidelobe error of 1/2 dB occurred at 60 degrees scan. There was no noticeable degradation in beamwidth, gain, or foresight at any scan angle. A 6-mile range would have produced a 2-dB sidelobe error. The measured range reflection level was -50 dB. The transmit dish with sidelobes of 22 dB was replaced with an array that had 40 dB sidelobes. This change reduced the reflections to below the required -58 dB. The antenna was focused using a range calibration technique and the measurements substantiated the theory.







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