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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.
Phaseless measurements are going to represent a viable and less expensive alternative to standard near field techniques since they allow to reduce to a very large extent the complexity of an indoor set-up. In fact, they require "scalar" receivers, probe positioning systems with less strict mechanical requirements, and present no cabling problem. Furthermore the anechoic environment extension can be reduced and low dynamic range receivers used as "truncated" data can be managed. In this paper we outline the main advantages of an approach to the solution of the problem of the far field reconstruction from phaseless near field measurements. Conditions to reliably process the collected data can be put forward so circumventing the main difficulties of most solution algorithms for non linear inverse problems. Experimental results are also included for the planar geometry.
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.
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.
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.
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 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.
ORBIT's String Reel Target Manipulation System is used to support and rotate a target during RCS measurements. One of the challenges in this kind of RCS measurement is to accurately determine the position of the target in space, since the weight and moment of inertia of the target and the string flexibility do not allow measuring its position with conventional methods (linear encoder, etc.). In order to overcome this problem, the Non-Contact Optical Measurement System (NCOMS) has been developed and tested at ORBIT. The system provides the capability for precision tracking of the target position (X, Y, Z) and orientation (ROLL, PITCH, YAW). NCOMS is a computer-controlled system and operates by using two standard CCD cameras (stereo technique), as well as by use of a single camera with insignificant accuracy degradation. Another advantage of NCOMS is that the system operation does not require accurate camera positioning. The only requirements for CCD camera installation are target visibility and use convenience.
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.
New results from numerical simulations and preliminary experiments pertaining to the dynamics of conical rotation of a string-suspended target are reported. For the simplest case of a model target suspended by three strings at three points, we find that its equilibrium position and the pitch, roll and yaw are uniquely dependent on its center of gravity (CG). By assuming that the yaw angle is mainly provided by the rotation of the upper turntable (UTT), we study the change in pitch and roll by controlling the strings while using a precision optical measurement system (POMS) to monitor the actual position and angular orientation of the target. In addition, two natural modes of small oscillations for the transverse and the longitudinal motions of the model target are observed to be of periods 1.45 and 2.46 seconds, respectively. The electrical-mechanical response time of the string-reel systems is found to be about 1.8 + 0.2 seconds. The results will be useful for the conics algorithm development.
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.
The LX/LH organization of McClellan AFB has been using near-field (NF) technology for the past two years to test an Air Force phased array receiving antenna. McClellan uses both a close range surface RF scanner and a larger offset, fain and back-transform near-field scanner. NF testing is done for both trouble-shooting purposes to support repair efforts, and for acceptance-testing to certify the antenna. The purpose of this paper is to show how McClellan interprets its planar near-field data for diagnosing antenna faults. First the various near-field techniques used by the LX/LH organization will be discussed. Following, will be an examination of the antenna defects pointed out by the NF test date. Failures will be traced to the component level where possible. Techniques other than near-field, such as electronic test, will be used to verify these problems. Additionally, the repair methods will be discussed.
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.
With the recent use of dual-polarized transmission and reception on communications links, the capability to perform accurate polarization measurements is an important requirement of test-range systems. Satellite antennas are commonly measured in the clean, protected environment of compact and near-field ranges, and a circularly polarized feed/field probe is a primary factor in establishing their polarization properties. The feeds also provide excellent source-horn systems for tapered anechoic chambers, where their circular symmetry and decoupling of the fields from the absorber walls improve the often troublesome polarization characteristics of tapered chambers. Circularly polarized feeds are generally composed of four primary waveguide components: the orthomode transducer, quarter-wave polarizer, scalar ring horn, and circular waveguide step transformer. Linearly polarized feeds omit the quarter-wave polarizer. This paper discusses the design and performance of high-polarization-purity source feeds for evaluating the polarization properties of antennas under test. Circularly polarized feeds have been constructed which operate over 10- to 20-percent bandwidths from 1.5 to 70 GHz. Gain values are generally in the area of 12 to 18 dBi, with cross-polarization isolation in excess of 40 dB. Representative measured data are presented.
With the recent use of dual-polarized transmission and reception on communications links, the capability to perform accurate polarization measurements is an important requirement of test-range systems. Satellite antennas are commonly measured in the clean, protected environment of compact and near-field ranges, and a circularly polarized feed/field probe is a primary factor in establishing their polarization properties. The feeds also provide excellent source-horn systems for tapered anechoic chambers, where their circular symmetry and decoupling of the fields from the absorber walls improve the often troublesome polarization characteristics of tapered chambers. Circularly polarized feeds are generally composed of four primary waveguide components: the orthomode transducer, quarter-wave polarizer, scalar ring horn, and circular waveguide step transformer. Linearly polarized feeds omit the quarter-wave polarizer. This paper discusses the design and performance of high-polarization-purity source feeds for evaluating the polarization properties of antennas under test. Circularly polarized feeds have been constructed which operate over 10- to 20-percent bandwidths from 1.5 to 70 GHz. Gain values are generally in the area of 12 to 18 dBi, with cross-polarization isolation in excess of 40 dB. Representative measured data are presented.
Northrop Corporation's Business and Advanced Systems Development Group has recently completed a very successful Radar Cross Section (RCS) measurements program on the USAF/Northrop B-2 bomber. One of the capabilities spawned from the program is a measurements radar system, comprised largely of off the shelf hardware, which provides high resolution whole body two-dimensional RCS images of large targets on the ground in the near field. Its high gain antennas allow operation in a space limited area and utilizes Synthetic Aperture Radar (SAR) data collection techniques. The system, though designed for use at VHF, has been expanded to operate from 100-2000 MHz in three bands. The hardware, associated signal processing, its applications and limitations are discussed.
Design and implementation of a large horizontal planar near-field system delivered to Space Systems/Loral for satellite antenna testing will be discussed. The 22' x 22' scan plane is 25' above the ground and employs real-time optical compensation for the X, Y, Z corrections to the probe position. The system provides high speed multiplexed near-field measurements using NSI's software and the HP-8530A microwave receiver. System throughput is enhanced through the use of a powerful and flexible test sequencer software module.
In the past, various companies have installed large permanent Near-field antenna measurements systems. In many instances, a test range has been constructed for a particular project or purpose. After the conclusion of the project, the range may become dormant or under-utilized. In addition, a dormant range quickly becomes a potential source for spare parts. These factors combine quickly to render the once functioning range useless. With the current industry emphasis on cost reduction, minimizing new capital purchases, and utilization of existing resources, an upgrade of a dormant test facility is a preferable path. NSI has recently upgraded an existing Near-field antenna measurement system at Hughes Space and Communications Co. hereinafter referred to as Hughes S&C. This paper focuses upon the design considerations undertaken during the upgrade process.
The planar near-field measurement technique is a proved technology for measuring ordinary antennas operating in the microwave region. The development of very low-sidelobe antennas raised the question whether this technique could be used to accurately measure these antennas. We show that data taken with an open-ended waveguide probe and processed with the planar near-field methodology including the probe correction, can be used to accurately measure the sidelobes of very low-sidelobe antennas to levels of -55 to -60 dB relative to the main-beam peak. We discuss the major sources of error and show that the probe antenna interaction is one of the limiting factors in making accurate measurements. The test antenna for this study was a slotted-waveguide array whose low sidelobes were known. The near-field measurements were conducted on the NIST planar near-field facility
This paper will discuss one method of characterizing the scan plane for planar near-field measurements. The method uses a theodolite auto-collimator, a laser interferometer, an electronic level and an optical square. The data obtained using these techniques are first used to make alignment corrections to the scan plane; then new data are used to determine the best fit for the realigned scan plane. The normal to this place is referenced using a permanently placed mirror. In addition, the final data obtained can be used in probe position-correction techniques, developed for planar near-field measurements.