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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.
A technique for multipaction analysis based on finite element modeling of the electromagnetic fields within a device is demonstrated. A multipaction device is modeled with HFSS to determine the field solution for use in multipaction analysis. The resultant field magnitudes within the critical gap region were compared with the measured breakdown events for 4 different gap sizes of the device. The relationship between the scattering coefficient convergence and field solution convergence is examined, and some indicators of the latter are established. The correlation between the data and the predictions indicates that the technique represents s reasonable analytical tool for such analysis.
Progress in Georgia Tech's research in Near-Field Spherical Microwave Holography (NFSMH) is reported. Previously, the amplitude resolution of Spherical Microwave Holography (SMH) was defined and demonstrated. The definition of resolution has been altered to include phase resolution. The resolution of phase is shown to be equivalent to the resolution of amplitude, and both depend on the highest mode order used in the spherical wave expansion. Previous measurements showed that SMH can easily achieve x/2 phase resolution where X refers to free space wavelengths. Current measurements show that the X/2 resolution limit of planar microwave holography can be surpassed by using evanescent energy in the NSMFH technique. Measurements of small, closely spaced, insertion phase defects placed on a hemispheric ally shaped radome are used to demonstrate the improved resolution. The measurement of evanescent energy is achieved by using a specially designed small aperture probe and a small separation distance between small aperture probe and a small separation distance between the radome surface and the measurement surface. The relationship between measured and theoretical insertion phase of a known radome defect is shown. Given the defect size and the maximum mode order used in the spherical wave expansion, measured insertion phase can be used to predict the actual defects electrical thickness.
Composites of the electrically conducting polymer polypyrrole with paper, cotton cloth and polyester fabrics have been evaluated for use in radar absorbing structures. Reflectively measurements on the composites in the range 8-18 GHz and transmission line modelling have revealed impedance characteristics with a common transition region. Relationships between substrate material, polymer loading and electrical performance have been explored. Polarization characteristics have also been measured. The electrical model has been successful in predicting the performance of Salisbury screen and Jaumann multi-layer designs of RAM.
The use of electronically scanned phased array antennas in demanding rolls such as satellite communications and radar systems has led to an increasing desire to analyze the sources of error present in the boresight alignment of such systems. Not the least among these errors are those introduced by thermal effects on the various components which comprise the array structure. In an effort to understand this mechanism, this paper will discuss a technique which uses an infrared camera system to analyze the beam deflection errors caused by the effects of temperature gradients present in the antenna system.
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.
This paper deals with the development of an approach to the design of triad steering antenna arrays which are used in anechoic chambers for hardware-in-the-loop testing of monopulse antenna seeker systems. In the design of a large array, such as those used for hardware-in-the-loop of guided weapons, it is important to optimize the array element spacing. An excessively narrow spacing results in an unreasonable number of required antennas and increased cost, while an excessively wide spacing will induce angle measurement errors in the seeker under test which can be significant. The specific objective of this effort is to quantitatively describe the monopulse discriminant efforts which result when a non-planar field, radiated by an antenna triad, illuminates a monopulse seeker under test. The approach to this problem is to calculate the triad field at the aperture of the monopulse seeker assuming various levels of triad element phase and amplitude error. Using this illumination field and the illumination function of the monopulse antenna, the resulting sum and difference patterns are calculated along with the monopulse discriminant. Software has been developed to perform these calculations. The resulting patterns are compared with the ideal far field pattern and the discriminant bias, or angle measurement error, is quantified.
This paper briefly reviews existing compact range performance characterization methods showing the limitations of each technique and the need for an accepted and well understood technique which provides efficient and accurate characterization of compact range measurement accuracy. A technique known as the transverse pattern comparison method is then described which has been practiced by the author and some range users for the past several years. The method is related to the well known longitudinal pattern comparison method, however, comparisons are conducted in the transverse planes where the required span of aperture displacement is much smaller and does not exceed the dimensions of the quiet zone. This method provides several advantages for characterizing compact range performance as well as enables range users to improve achievable measurement accuracies by eliminating the impact of extraneous signal errors in the quiet zone.
The plane wave quality of a compact range (CR) is usually specified in terms of the crosspolar level and the magnitude and phase ripple in the test zone. The way these deviations from the ideal plane wave affect the measurement of different antenna types can be treated by the application of the reciprocity principle between the transmitting and receiving antenna in a measurement set-up. By the application of the sampling theorem, it is found that the measured antenna pattern can be expressed as a summation of the plane wave spectrum components of the field at the test zone weighted by the true radiation pattern of the antenna under test (AUT) evaluated at the CR plane wave directions in the rotated coordinate system of the AUT. The inverse procedure can be used to extract the CR plane wave information (and therefore the CR field at the test zone by means of the Fourier series) from the measurement of a standard antenna with a known radiation pattern.