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We investigated two methods of probe-position error correction to determine how well the corrected results compare to the uncorrupted far field: the k-correction method and the Taylor-series method. For this investigation, we measured a 1.2 m dish at 4 GHz and a 1.2m by 0.9m phased array at 2.2 GHz. Measurements were made first without position errors and then with deliberate z-position errors. We perfonned probe position error correction using both methods and compared the results to the error-free far field. For errors up to A/4, the fifth-order implementation of the Taylor series correction was slightly better than the k-correction. For errors of ')..J2, the k-correction was better than the Taylor-series correction.
The feasibility of realizing a 500 GHz hologram type of compact antenna test range (CATR) for testing the 1.1 m antenna of the Odin satellite is studied. The quiet-zone field is analyzed theoretically by us ing an exact near-field aperture integration method. Due to fabrication errors the slots of the hologram are wider or narrower than in the ideal case. How ever, with reasonable value of fabrication errors the quality of the quiet-zone field is not degraded deci sively. The effect of the displacement of the different parts joined together to form a large hologram is the inclination of the amplitude and phase in the quiet zone. The analysis of the CATR feed scanning in the focal plane to avoid the need to rotate the AUT showed that at least a ±1° change of the direction of the plane wave in the quiet-zone is feasible.
There is a need for finite ground planes to test an tennas which are normally mounted on large struc tures. These ground planes are used to simulate a large structure such as an aircraft fuselage but are limited in size based on the available target zone di mensions. For example, TCAS antennas are tested on a 4' circular ground plane based on FAA require ments. Since the conducting ground plane creates significant diffraction errors which are not present in the intended application, these ground plane tests become difficult to interpret because one can not easily separate ground plane diffraction errors from antenna characteristics. A solution to this dilemma is to attach an R-Card (resistive sheet) to a con ductor (PEC) and form an R-Card ground plane. With a properly designed resistive profile, an R-Card ground plane can greatly reduce the edge diffrac tion errors. As a result, the desired antenna charac teristics without significant ground plane corruption terms can be obtained. This paper demonstrates this new concept through calculated and measured results. Also, a Genetic Algorithm (GA) to optimize the resistive profile is presented.
High performance antennas require very accurate measurements which are difficult to meet in the conventional compact antenna test ranges. This measurement errors are produced by the non perfect plane wave synthesized by the compact range system. By the application of the reaction between the antenna under test true pattern and the compact range incident field, a closed form relation is found for the measured radiation pattern. Under certain conditions, this measured pattern can be approximated by the convolution of the two diagrams. In this paper it is presented the inverse procedure: the deconvolution to numerically calculate either the true radiation pattern of the antenna under test or the plane wave spectrum of the compact range incident field . The effectiveness and limitations of the method are discussed by numerical simulations and tested by measurements.
This paper presents the various aspects involved in the design, development and establishment of Cylindrical Near-Field Measurement(cnfm) facility. A brief description of the hardware and the method of data acquisition are outlined. The capabilities of the CNFM system are brought into focus. The effects of alignment errors are presented. The patterns of various test antennas are presented over different frequency bands.
Ultra low sidelobe antenna measurements are very difficult to perform even in the best of ranges. This problem results from the fact that small stray signal errors within the range can be amplified by the antenna main beam gain and result in a error term that is larger than the desired ultra low sidelobe level. With this in mind, one can attempt to reduce the range stray signals, but it is only practical to reduce them so far. However, one can always desire to measure a lower sidelobe level than is feasible for the range. To correct this problem, a new measurement method has been developed that can significantly reduce these. It involves taking two measurements and properly processing the results. It has been shown that one can reduce complex range errors by as much as 35 dB in a real range environment.
Absorber is mounted in an anechoic chamber to attenuate stray signals. In this application the stray signals impinge on a whole continuous absorber wall. Consequently, to evaluate chamber performance, one must determine the reflection properties associated with an absorber wall instead of a finite absorber panel. Unfortunately, absorber is normally evaluated experimentally using a finite absorber sample. As a result, absorber measurements are corrupted by edge (end) effect errors. These errors have been observed in measured data using ISAR image techniques, especially for high performance absorbers. One can isolate these error terms by using image filtering. The corrected image is then transformed back to the frequency and angle domains, such that the resulting data will much better represent the true absorber performance. Measured and calculated results will be shown to validate this new method for high performance absorbers.
The process of recovering signal amplitude and phase from the in-phase (I) and quadrature (Q) signal components requires the I and Q channels to be perfectly balanced in amplitude and shifted exactly 90 degrees in phase. Existing I/Q correction algorithms on wide-band data generally work well when the channel imbalance errors exhibit little or no variation with frequency. Their effectiveness tends to decrease as the I/Q errors become more frequency dependent. In this paper, a software gating method of mitigating data errors resulting from I/Q imbalance will be presented. This approach to I/Q imbalance correction provides a method of mitigating frequency dependent I/Q errors over wide-band data without independently determining the imbalance at each frequency. The method has been shown to produce high quality amplitude and phase data from measured input with frequency dependent imbalance.
Large scanners used for near-field antenna measurements require careful attention to the design and fabrication process to maintain probe position accuracy. This paper discusses the design, implementation, and results of a novel optical probe position tracking system used by NSI on a number of large near-field scanners. This system provides measurement of the probe X, Y and Z position errors, and real-time on-the-fly position correction. The use of this correction can significantly enhance measurement accuracy, and can reduce the cost of building large near-field scanners.
One critical issue in designing absorber for an anechoic chamber is the bistatic scattering performance of the absorber and its effect to the quiet zone field quality. The bistatic scattered fields from the absorber side walls, floor and ceiling of the range result in undesired stray signals which can cause significant measurement errors. Consequently, it is very important to analyze the performance of the absorber from the overall system point of view; i.e., the performance of the absorber in the range environment. This paper will address this issue and present calculated results of absorber wall performance for a compact range with a blended rolled edge reflector.
A novel bi-polar near-field range has been constructed at UCLA recently. The purpose of this article is the evaluation of the bi-polar measurement of a large reflector antenna using simulation methodologies. Bi-polar measurement of such an antenna is simulated and a parametric error study is reported. The study shows that a bi-polar near-field range for measuring large reflector antennas can be designed to provide accurate measurements with reasonable hardware requirements. The measured on-axis gain is found to be highly tolerant to probe position errors which occur in the plane of the measurement. The z-positional error has a greater effect on the gain, however, this error can be minimized with careful alignment of the bi-polar axes.
This paper describes a 550 GHz planar near-field measurement system developed for flight qualification of the radio telescope carried onboard the NASA submillimeter wave astronomy satellite (SWAS). The very high operating frequency required a new look at many near-field measurement issues. For example, the short wavelength mandated a very high precision scanner mechanism with the accuracy of a few microns. A new thermal compensation technique was developed to minimize errors caused by thermally induced motion between the scanner and spacecraft antenna.
ISAR images are formed by Fourier processing coherent wideband responses collected with angle diversity. Unfortunately, physical and practical considerations limit the frequency and angle diversities achievable. The finite diversities induce sidelobes, which are usually mitigated by application of tapered windows in the spectral domain. This procedure reduces image sidelobes at the cost of increased mainlobe width, thus degrading resolution. Spatially-Variant Apodiz.ation (SVA), a new non linear method developed at ERIM to improve the quality of SAR imagery, reduces sidelobe levels while preserving the mainlobe width corresponding to unwindowed data. In contrast to conventional window techniques which simply apply the same window function to every image element, SVA operates on the image by adaptively applying a window optimized for each spatial element. The algorithm uses phase information available from the coherent RCS data to distinguish processing sidelobes from correct responses. Mainlobes are passed using rectangular weighting, while sidelobes are reduced or eliminated entirely. This paper discusses the concept, theory, and implementation of SVA for ISAR imaging, and summarizes capabilities and limitations of the method. Results using SVA are presented and compared to conventionally windowed one- and two-dimensional images. The sensitivity of the procedure to additive noise and phase errors is investigated
Martin Marietta has developed a new, automated facility for high-volume production testing of the Longbow millimeter wave missile. Two dedicated far field anechoic chambers were designed, both automated to support component test and analysis in the production environment. One standard far field chamber is used to perform the complete characterization of the antenna and rac1orne; it allows very accurate measurements of power sidelobes, monopulse errors, and cross polarization isolation. The completed radar missile sensor group is evaluated in the second far field chamber, which can reach higher-level parameters of the antenna, transceiver, and gimbal. This paper describes chamber and test station capabilities; time reduction benefits; and the novel, new assembly technique which allows for future portability of these chambers with limited downtime.
Techniques for editing RFI from antenna measurements are developed for vector network analyzer instrumentation, and include the processing within the analyzer. An algorithm was devised for identifying data that may contain RFI; this algorithm is based on the electrical size of the antenna. Once data containing RFI are identified, extrapolation techniques based on the electrical size of the antenna are used to produce continuous data.
A method is presented to qualify a compact range measurement system for low sidelobe antenna measurements. The method uses the target zone fields (probe data) of the compact range. Using the method, one can identify the angular regions around which the measurement errors can be significant. The sidelobe levels which can be measured around these angular regions with less than a 3 dB error are also defined.
This paper contrasts indoor and outdoor implementation of efforts during upgrades of VHR RCS measurement capabilities. Sites studied are two McDonnell Douglas Technologies Incorporated, Range Measurements Services facilities. Indoor. Radar Measurement Center (San Diego, CA) is a large compact range. Equipment-Harris Corporation Model 1630 Collimator System, Scientific Atlanta Model 2090 radar. Outdoor. Microwave test facility (Victorville, CA), large ground plane facility. Equipment-Steerable dipole feed dish, System Planning Corp, Mark III radar.
Phased array antenna sidelobe levels are evaluated based on the statistics of the differences in element patterns. It is shown that the differences can be treated as random errors. The standard formula for predicting the average sidelobe level of an array due to random errors is valid if the interaction between the element patterns and the excitation function is taken into account. Sidelobes of a linear array with a variety of near-field perturbations are considered. The statistics indicate that for an N-element array, adaptive calibrations may lower the average sidelobe level by a factor of N.
The Precision Airborne Measurement System (PAMS) is a flight test facility at Rome Laboratory which is designed to measure in-flight aircraft antenna patterns. A capability which provides antenna pattern measurements for multiple VHF and UHF antennas, at multiple frequencies, in a single flight, has recently been demonstrated. A unique half space VHF/UHF long periodic antenna is used as a ground receive antenna. Computerized airborne and ground instrumentation are used to provide the multiplexing capability. The new capability greatly reduces time and cost of flight testing. The design, construction, and calibration of the half-space log-periodic ground receiving antenna is discussed and the ground and airborne segments of the instrumentation are described.