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This paper describes the results of electrical boresight measurement comparisons between one far-field and two near-field ranges. Details are given about the near-field alignment procedures and the near-field error analysis. Details of the far-field measurements and its associated errors are not described here, since the near-field technique is of primary interest. The coordinate systems of the antenna under test and the measurement ranges were carefully defined, and extreme care was taken in the angular alignment of each. The electrical boresight direction of the main beam was determined at a number of frequencies for two antenna ports with orthogonal polarizations. Results demonstrated a maximum uncertainty between the different ranges of 0.018 deg. An analytical error analysis that predicted a similar level of uncertainty was also performed. This error analysis can serve as the basis for estimating uncertainty in other near-field measurements of antenna boresight.
A new measurement technique that is used to measure the polarization properties of dual port, circularly polarized antennas is described. A three antenna technique is used, and high accuracy results are obtained for all three antennas without assuming ideal or identical properties. This technique eliminates the need for a rotating linear antenna, reduces the setup time when gain measurements are also performed, and reduces errors for antennas with low axial ratios.
The weakest link in antenna metrology is the antenna range itself. Unknown reflections can cause large errors in antenna measurements and can change unpredictably. The planewave spectral (PWS) probe technique is one proposed method for identifying the location and magnitude of range scattering. This paper presents the results of a PWS probe of a compact range. The interpretation of the PWS plots is discussed in comparison with the range geometry. Nine separate scattering centers are identified. The meaningfulness of the PWS picture was tested by introducing a known dipole source.
The purpose of this paper is to discuss the accuracy requirement of a generic measurement system for in-flight antenna pattern evaluations. Elements of the measurement technique will be described. An attempt is made to distinguish the measurement requirement for a narrow beam radar antenna in contrast to that for broad beam communication antennas. Major elements of the measurement technique discussed include the flight path geometry, the multipath propagation problem, and the measurement errors. Instrumentation requirements consist of the ground segment, the receive and the tracking subsystems, and the airborne equipment, the radar components and the navigation and attitude sensors. Considering the in-flight antenna pattern testing as a generalized antenna range measurement problem, various sources of measurement errors are identified. An error budget assumption is made on each error component to estimate the overall expected accuracy of the in-flight antenna pattern measurement.
This paper evaluates the RCS errors associated with measuring a large flat plate which is illuminated by a compact range reflector with significant edge diffraction stray signals. This is done by evaluating the true fields incident on the plate and then using a physical optics technique to predict the backscattered fields. Results are compared with and without the edge diffracted fields present. A simple analytic expression is developed which can approximate the size of this potential error.
In the theory of inverse synthetic aperture imaging of radar targets the measuring distance is ordinarily supposed to be very much larger than the dimensions of the target. If this is not the case errors are introduced. We study these errors and means to decrease their influence by computation. The result is that the maximum tolerable target dimension can be substantially increased in a plane perpendicular to the axis of rotation if error correction is used.
A 400 element phased array antenna has been constructed at the GEC-Marconi Research Centre. Each radiating element is fed from its own phase shifter. The radiation patterns of this array have been measured using a recently constructed Cylindrical Near Field Test Facility. The radiation pattern is obtained on a two dimensional grid and contains both amplitude and phase information. It is therefore possible to transform these data back to the array aperture to obtain the array excitation amplitudes and phases. The spatial resolution obtained in the aperture is a function of the angular coverage of the radiation pattern used. The effect of deliberately introduced phase errors on the calculated aperture data is shown.
This paper describes techniques for coherently suppressing multipath and other error sources in planar near-field measurements. Of special interest is a simple, yet effective technique of suppressing axial multipath and mutual coupling between the nearfield probe and an antenna. This is of particular value in the testing of low sidelobe antennas. Traditionally, self comparison tests with different separations between the probe and the antenna under test are used to identify the magnitude of multipath errors. What is not generally realized is that these tests can be used to produce a coherent estimate of the induced error, which can often be suppressed. A series of tests was performed with a small X-band phased array antenna, resulting in a reduction of the sidelobe noise background from a 25 dB level to better than 50 dB.
A study of RF testing methods was conducted for the Radarsat SAR antenna. The implementation tolerances of a planar and a cylindrical near-field facility were computed, by simulation of the effects of different types of measurement errors on the reconstructed far field. The results are presented and the two types of near-field facility are compared.
This paper first presents an overview of the types of errors present in phase length measurements. Next, a minimum-error test method is formulated. The first test results using the test method are included, demonstrating accuracies on the order of plus-or-minus 3 electrical degrees at 18 GHz. Measurement time using currently available ANAs is accomplished in less than 1/2 minute when run in the reflective, time-domain mode.
A unique RCS field probe system is described which determines: 1) the two way phase and amplitude field taper, and 2) the RCS measurement error within the quiet zone. The RCS of a suspended target is measured by the radar at selected locations or while moving in the quiet zone. The field taper is obtained from a time gated target return. The quiet zone RCS error for a target is obtained by comparing RCS measurements from anywhere in the quiet zone with the target RCS measured at the center of the quiet zone. A quiet zone containing a high quality illumination field was measured and found to have more than a 5 dB quiet zone RCS error. The RCS error magnitude is dependent upon the radar variables which are determined by the target size. There is a significant difference between the implied RCS error based on the illumination field quality and RCS measurement error caused by the additional contributions of multipath and target dependent clutter that are peculiar to each facility. Accurate RCS measurements require detailed knowledge of the test facility's multipath, target dependent clutter characteristics, and the target's bistatic signature.
In the last few years, the interest in Radar Cross Section (RCS) measurements has increased rapidly. The development of high-performance Compact Ranges (CR) has made possible measurements on large targets down to very low RCS levels (below -70 dBsm). RCS imaging is a powerful tool to determine the location of scattering sources on a target. The response of the target is measured as a function of the frequency and aspect angle. A two-dimensional Fourier transform then gives the reflection density as a function of down-range and cross-range. If the response is measured vs. azimuth and elevation, even a complete 3-D image is possible. For high-resolution imaging (large bandwidth, wide aspect-angle span) a direct 2-dimensional Fourier transform gives rise to errors caused by the movement of the scatterers during the measurement. These errors can be corrected by applying a coordinate transformation to the measured data, prior to the Fourier transforms. This so called focused imaging allows further manipulation of measured data. However, the measurement accuracy can be a limiting factor in application of these techniques. It will be shown that the Compact Range performance as well as positioning accuracy can cause serious errors in high-resolution imaging and thus in interpretation of processed data.
Three test methods have been developed and validated for characterizing materials at VHF and UHF in an indoor environment. The first method employs a resonant strip-line cavity for the independent determination of permittivity and permeability from .15-2 GHz. The planar field geometry and sample configuration permit evaluation of material anistropy. Measurements are taken on an Automatic Network Analyzer (HP 8510 ANA). The second method measures the reflection/transmission (R/T) of planar material samples at UHF. This is a free space measurement performed in an anechoic chamber. Data is taken from .2-2 GHz using two dual ridged horn antennas and the ANA. A calibration method has been developed for the ANA to correct for measurement errors. Off-set shorts and thru delays are used in this technique. The third technique evaluates reflection performance of materials from 150-250 MHz. This technique employs a custom designed corner reflector antenna. Only one such antenna is needed due to the calibration technique. These methods allow a synergistic approach to material development. Candidate material can be evaluated using the cavity or R/T systems. Material designs can then be tested on either the UHF and/or VHF systems.
The deleterious effect of tilting the pylon on the measured RCS of a low level target is shown. A two scatterer computer model is developed to demonstrate the harmful effect of the pylon on the target signature. Predicted RCS plots are provided for the pylon to target ratios of -20, -10, 0, and +10 dB. The familiar error curve for two interfering signals is shown as applicable to bound the RCS errors of two scatterers. A method for computing the pylon RCS from linear motion RCS measurements is described with sample data plots. A knowledge of the pylon RCS allows the inclusion of measurement confidence levels on all RCS plots which is very valuable to the analyst. All radar data that is below the known RCS of the target support structure can be blanked from the plotted data to prevent confusion since these RCS values are an artifact of the measurement system and are not a true representation of the target RCS.
The weakest link in antenna metrology is the antenna range itself. Unknown reflections can cause large errors in antenna measurements and can change unpredictably. Conventional range probing methods typically provide a go/no go test with very little information about the location of the range scatterers. A number of techniques show promise for locating antenna range scattering centers. This paper describes the theory of a probe analysis method being implemented at Lockheed Missiles and Space Company. The method is based on planewave spectral analysis. A specialized probe system to test the planewave spectral theory will be described.
While near field antenna test techniques are well understood, published methods for high volume testing are rare. This paper addresses special requirements for production testing of satellites at the Hughes Aircraft Company Space and Communications Group facility in El Segundo, California. The El Segundo facility has the capability of testing antennas which employ multiple beams and polarization isolation for frequency spectrum reuse. It is required that the measurement techniques and equipment be able to test this type of antenna during a single traverse of the planar near field scanner. Serious demands are placed on the system to meet these requirements: * Maximum dynamic range and linearity must be maintained in an environment of rapidly shifting signal levels. * Isolation of signals must be maintained while allowing rapid switching for beam and polarization sampling. * Equipment settling time must be minimized to maintain scan rate at the highest possible speed. * RF interfaces must be repeatable, and capable of rapid reconfiguration. * Calibration and system checkout techniques must be accurate, quick, and capable of detecting malfunctions and costly setup errors. * Data transfer and processing must not be a limitation to the availability of the system for measurement. * System growth capability must be maintained, but not allowed to interfere with 'old and valued' customers. Some of the trades and pitfalls in meeting these requirements will also be presented.
A method for the calculation of the errors induced through target-wall-target interactions is presented. Both near-field and far-field situations are considered. Far-field calculations are performed both with Fraunhoffer diffraction theory and target antenna analogies. Absorber is considered as both a specular and a diffuse scatterer. The equations developed permit trade studies of chamber size versus performance to be made.
Reducing ripple in the aperture field of the parabolic reflector is one of the main considerations in the design of a compact range, since it determines the "usable" target zone for RCS and antenna measurements. The usable target zone is typically defined as the aperture region where the ripple is less than 0.1 dB [1]. Studies [2,3] have shown that edge diffractions and therefore ripple can be significantly reduced by using blended rolled edges such as in Figure 1. For low aperture field ripple, it is assumed that the junction between the parabolic surface and the blended rolled edge is smooth. In practice, however, the rolled edges may be machined separately and then fitted to the main reflector. If this is done, small wedge angle errors (Figure 2) or step discontinuities (Figure 3) may be mechanically introduced at the junctions. Typically, angle deviations of plus-or-minus 0.5 degrees and steps of plus-or-minus 0.005 inches may be expected. If the parabola and part of the rolled edge is machined as a unit, diffraction due to discontinuities in the mechanical junction between this surface and the rest of the rolled edge can have less effect on ripple in the aperture field. Now, the questions to be answered are: * How much of the target zone is lost due to discontinuities at the edge of the parabola? * How much of the rolled edge need to be machined with the parabola to prevent mechanical discontinuities from decreasing the usable target zone? * What range of discontinuities can be tolerated? In this paper, these questions are answered for a 12 foot radius semi-circular compact range reflector with cosine-blended rolled edges.
Microwave holographic metrology is considered to be a key technique for achieving improved performance from large reflector antennas, especially at the shorter wavelengths. An important benefit of microwave holography is that the mathematically transformed data yields precise information on panel alignments on a local scale [1-5]. Since the usage of the holographic technique requires both the amplitude and phase data of the measured far-field patterns, one must carefully assess the impact of systematic and random errors that could corrupt the data due to a variety of measurement error sources.
The holographic antenna measurement system developed for the COMSAT Labs far-field range was tested with various antennas including axis-symmetric reflector antennas, offset single and dual reflector antennas, and phased-array antennas. Numerous examples which demonstrate the value of holographic measurement as an antenna diagnostic tool are presented. Microwave holography utilizes the Fourier transform relation between the antenna radiation pattern and the antenna aperture electromagnetic field distribution. Complex far-field date are collected at sample points and a Fourier transform is performed to give amplitude and phase contours in the antenna aperture plane. These contours facilitate reflector antenna diagnosis. The feed illumination and blockage pattern are provided by the amplitude distribution. The aperture phase distribution allows simple determination of deviations in the reflector surface and feed focusing. For phased-array antennas, the contours provide a measure of the complex element excitation. Measurement system parameters including pointing accuracy, phase stability, and measurement dynamic range were studied and refinements implemented to increase speed, accuracy, and resolution of the contour plots. To prevent aliasing errors, sampling criteria were explored to determine the optimum parameter ranges. For most antenna positioners, the antenna center is displaced from the rotation center. The importance of properly accounting for this displacement is discussed in the final section.