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An efficient and accurate spherical near field to far field transformation without probe correction is presented. The indices m of the Legendre polynomials is summed up analytically, thereby reducing the computation time. Computations with both synthetic and experimental data illustrate the accuracy of this technique.
The interdependence of accuracy and speed should be considered when analyzing measurement requirements. Tradeoffs can be made to optimize the measurement when accuracy is of primary importance, or where speed is critical. Several different measurement modes of the HP 8530A Microwave Receiver are presented, each with different measurement speed and accuracy tradeoffs. Examples are given that illustrate which acquisition modes would be appropriate to optimize the acquisition speed and accuracy in a variety of applications
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.
The feed support struts often cause noticeable strut lobes in the patterns of reflector antennas. For example, strut lobes are apparent in the measured and calculated patterns presented in Ref. [1] for the 8-foot diameter reflector with a prime focus feed. As pointed out in [1], the calculated strut lobes are higher than the measured ones. The reason for the difference is secondary scattering by the oppositely located strut, which was not modeled in the calculated pattern in [1]. Detailed examination showed a difference of about 2 1/2 dB caused by the secondary scattering for this reflector antenna design. The purpose of this paper is to present measured and calculated patterns which explicitly demonstrate the quantitative effect of the secondary strut scattering. This effort is shown by comparing the measured strut lobe levels with the oppositely located strut removed, i.e., by using 3 struts instead of 4 struts. Calculated patterns are also given in which the secondary scattering is modeled.
A 585 GHz compact range has been developed for obtaining full scale RCS measurements on scale model targets. The transceiver consists of two CW submillimeter-wave gas lasers along with two colled-InSb heterodyne mixers. Coherent detection has been implemented to maximize sensitivity and allow for a vector measurement capability. In addition, the target can be rapidly translated in range to generate a doppler modulation which is used to reject background signals during low-RCS measurements. Although most scaling has evolved to develop non-metallic materials with scaled dielectric properties as well as validation and test program, RCS measurements are made on scaled simple and complex shapes and compared with full-scale measurements and computer predictions. A description of the 585 GHz compact range along with measurement examples are presented in this paper.
The antenna analyzer is specifically designed to make use of measurement techniques that have been difficult to use until now The analyzer is an original vectorial receiver design, based upon the analysis of one of the sidebands of the marked RF measurement signal. Thanks to the RF marking process, the antenna analyzer is not the only equipment that allows characterization (in amplitude, phase or return loss) of all devices in a transmitting chain, including the high power elements, without cutting off the transmission. Originally introduced for the analysis of wired antennas in UHF-VHF bands, its use is now extended to microwave antenna measurements, especially printed circuit antennas. A special characteristic of the new analyzer, ESTAR 2110 is its capacity to measure the phase of RF signal with power levels as low as -120dBm. The analyzer is ideal for elaborate analysis of fundamental antenna parameters such as RF current distribution, close field, antenna pattern, impedance and phase balance of antenna network. The paper describes the marking principle and its use in making antenna parameter measurements.
The field present in the test zone of an antenna measurement range can be calculated from the range field measured on a spherical surface containing the test zone. Calculated test zone fields are accurate only within a spherical volume concentric to the measurement surface. This paper presents a technique for determining the probing radius necessary to create a volume of accuracy containing the test zone of the range. The volume of accuracy radium limit is caused by the spherical mode filtering property of the displaced probe. This property is demonstrated in the paper using measured field data for probes of differing displacement radii. This property is used to determine the volume of accuracy radium from the probing radius. This is demonstrated using measured far-field range data.
This paper considers an electromagnetic field simulation of an anechoic chamber with experimental verification. The simulation is a Geometric Optics (Ray Tracing) mathematical model of the direct path between two antennas and interfering scattering. There are two separate models due to the frequency dependent nature of the pyramidal radar absorbing material (RAM). The model for the frequency range of 30 to 500 MHz was used to characterize the specular scattering. The specular scattering was modeled as a lossy, tapered, TEM transmission line in an inhomogeneous anisotropic tensor material. The frequency range from 500 MHz to 18 GHz was characterized by dominant tip diffraction of RAM patches and the model made use of a Uniform Theory of Diffraction code for a dielectric corner. The measurements and simulations were based on an azimuthal cylindrical scan. Diagnostic measurements were also performed by a cylindrical scan of a directional horn antenna to locate scattering sources in the chamber. A cylindrical wave, modal expansion of the diagnostic data which included a one dimensional Fast Fourier Transform with Hankel function expansions.
A Ground to Air Imaging Radar system (GAIR) used to perform diagnostic imaging and total RCS measurements on low observable airborne targets has been developed by the Environmental Research Institute of Michigan (ERIM). In order to ensure accurate measurement of the scatterers contributing to a target's radar signature, proper calibration in imperative. The use of external calibrators to measure the end-to-end system transfer function is the ideal way to perform a system calibration. However, this is a more difficult and challenging task with a ground based radar viewing an airborne target, as opposed to a traditional airborne SAR which views an array of ground based trihedral corner reflectors. This paper will discuss the internal and external calibration methods used in performing an end-to-end system calibration of the GAIR. Primary emphasis is placed upon the external calibration of the GAIR and the three independent measurements utilized: a ground based corner reflector, a sphere drop, and an in-scene calibrator. The system calibration results demonstrate that the GAIR is an accurately calibrated radar system capable of providing calibrated images and total RCS data. Moreover, only the ground and internal measurements are required on a daily basis in order to maintain system calibration
A small compact range measurement facility has been installed at the Environmental Research Institute of Michigan (ERIM) for research aimed at improving RCS measurement and radar imaging techniques. This paper describes the facility, which is referred to as the Experimental Range Facility (ERF). The ERF has two instrumentation radars; a Flam & Russell FR959 gated CW radar and a Hughes MMS-300 pulsed radar. The radars are connected to a suite of workstations, which support a variety of internally and externally developed radar imaging and data exploitation software. The ERF is also equipped with sophisticated target positioning control and sensing equipment.
The presence of the sea surface has a powerful influence on the scattering characteristics of marine targets during radar cross section (RCS) measurements. To obtain accurate RCS measurements of a large, distributed marine target, the radar site must satisfy various requirements. The major requirement is to provide quality RCS data without strong multipath distortion of the target return signal. In this paper multipath effects on a large scatterer measured at both low-and high-elevation radar sites are summarized. It is observed that multipath effects contribute strongly to the RCS of the target measured at a low elevation radar site. The data show large RCS fluctuations of more than 15 dB when a scatterer is measured at difference altitudes or ranges. The quality of the data measured at a low-elevation radar site then becomes questionable, which creates difficulties in assessing the true RCS of the target. For diagnostic purposes, it may be necessary to change the target range or altitude several times to make a credible assessment of RCS. The same target measured at a high-elevation site has less multipath influence on the RCS data, making assessment of the true RCS feasible.
Aeronautical SATCOM systems for INMARSAT typically employ circular polarized electronically or mechanically steered multi beam antennas. Characterization of thee antennas requires extensive measurements that differ from conventional antenna pattern measurements. Some of these are: A. Multiple frequently CP gain, axial ratio, and discrimination measurements over a hemisphere for a large number of beams. B. Noise temperature and G/T measurements C. Carrier to multipath rejection D. Intermodulation characteristics E. Receiver and Transmitter system characteristics Details of instrumentation and procedure for these tests are presented with special emphasis on issues such as measurement speed, accuracy and processing of large amounts of data.
In this paper, a new error correction technique is introduced to improve the accuracy and efficiency of the traditional NRL Arch method. The use of this integrated technique allows one to correct the error terms in the traditional NRL arch setup so that a broadband evaluation of the performance of the absorber product can be performed with much better accuracy and efficiency. This technique also allows one to conduct large bistatic angle evaluation of absorbers without the cross talk and other error signal interferences. Design guidelines for a broadband NRL test arch are provided so as to successfully implement this improved NRL Arch method for a broadband evaluations of anechoic absorbers. Sample test results from Ray Proof's broadband test arch (0.5-6 GHz) are also presented.
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 field present in the test zone of an antenna measurement range can be calculated from the range field measured on a spherical surface containing the test zone. Calculated test zone fields are accurate only within a spherical volume concentric to the measurement surface. This paper presents a technique for determining the probing radius necessary to create a volume of accuracy containing the test zone of the range. The volume of accuracy radium limit is caused by the spherical mode filtering property of the displaced probe. This property is demonstrated in the paper using measured field data for probes of differing displacement radii. This property is used to determine the volume of accuracy radium from the probing radius. This is demonstrated using measured far-field range data.
A complete description is given of the unique radar cross-section (RCS) measurement facility built at the Houston Advanced Research Center in The Woodlands, TX. The uniqueness of this chamber comes from its ability to independently move the transmit and receive antennas, which can each be moved to any position within their respective ranges of motion to a resolution of about 0.05 degrees. The transmit antenna is fixed in azimuth, but can be moved in elevation: the receive antenna is free to move in both azimuth and elevation. Additionally, the target can be rotated in azimuth by means of an azimuth positioner. Analysis has been performed to determine the impact of chamber effects on measurement accuracy. The most notable chamber effect comes from the two large aluminum truss structures, which are the mounting supports for the transmit and receive antennas. Fortunately, the scattering from these structures can be readily separated from the desired target return through the use of range (time) gating. Time domain results are presented showing the effects of these structures.
Numerous monostatic radar cross-section (RCS) calibration routines exist in the literature. Many of these routines have been implemented at the RCS measurement facility built at the Houston Advanced Research Center in The Woodlands, TX. Key monostatic results are presented to give an indication of the measurement accuracy achievable with this chamber. Unfortunately, bistatic calibration routines are not nearly as common in the literature. As with the monostatic routines, a number of bistatic routines have been implemented and typical results are presented. Additionally, descriptions are given for some of the reference targets along with their support structures that are used during calibration.
A large deployable multibeam antenna for communication satellites operating in the Ka band with 2.5 GHz transmit/receive bandwidth was developed and measured. The antenna is an offset Cassegain system with a 4.7 m diameter mail reflector divided into a central and 24 rigid deployable panels. One application studied in detail was the continuous illumination of the FRG with 16 beams. Spherical nearfield measurement techniques were used to validate the predicted performance. Because the gravity influence would cause inadmissible deformations, a compensation device must be used. To take into account the influence of the remaining deformations varying with the elevation position of the antenna, a special analysis software was developed which uses measured surface coordinates. Because measured and computed values agree well, it is possible to predict the performance in orbit precisely. A pointing accuracy of 0.01 degrees was achieved by adjustment of the sub reflector using a monopulse tracking system.
In ISAR applications data is acquired on a circular grid. In further processing, data on a rectangular grid is obtained by interpolation. This causes the loss of data outside the interpolated area. The latter can be corrected by extrapolation, but this can give incorrect information. A new technique s proposed which uses a larger rectangular area than in the above mentioned case. Some parts of this rectangle are calculated by extrapolation. Because most of the data in the larger rectangular area consists of original data, only minor parts are extrapolated. Consequently, this method is expected to be more reliable than traditional extrapolation techniques. Simulations have shown that the data obtained by the new interpolation - extrapolation scheme provide a considerable improvement to the amplitude - and phase accuracy across the enlarged rectangular grid.
In polarimetric RCS measurements, the cross-polarization levels which are required in the test zone, correspond closely to those which are realizable with most Compact Antenna Test Ranges (CATR). On the other hand, such a performance may not satisfy the accuracy requirements in cross-polarization measurements of high performance microwave antennas. These applications include spacecraft antennas, ground stations for satellite communications or microwave antennas for terrestrial applications, where two polarizations are used simultaneously.