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The work described in this paper is devoted to measurement and analysis techniques for performing electromagnetic (EM) test environment assessments and error compensations for antenna performance testing and RCS testing at indoor and outdoor test sites. This paper is focused primarily on test articles and test facilities that are physically and/or electrically large and difficult to handle by conventional measurement and analysis techniques. The approaches discussed herein are based on the combined use of 1) arrays of EM field probes to rapidly measure the test zone fields, and 2) specialized EM spectral analysis techniques including the MUSIC high resolution imaging technique and the Spherical Angular Function (SAF) integral formulation of EM coupling and scattering.
We introduce a very efficient method for extracting from RCS measurements the cutoff frequency of modes propagating in a waveguide of arbitrary cross section. Based on a model of propagating mode, it offers the capability of identification of mode and it gives also an information about the frequency evolution of the mode excitation amplitude. The effectiveness of this method is illustrated by the analysis of measurements of different shapes of waveguide. The results obtained show that this representation widely improved the performance of time-frequency distributions usually used to analyze this kind of dispersive structure.
Analysis of in-orbit phased array antenna patterns measured from earth station requires a considerable examination of the in-orbit antenna operation. The antenna analysis should take into account the constant change of both observation angles and scan angles. The in-orbit phased array antenna pattern characteristics are mathematically analyzed. The coordinate transformation technique to calculate the time-varying trajectory of the observation angle in the antenna coordinate system is presented. The technique also encompasses the satellite track angle calculation as seen from the ground antenna. Data processing procedure of the dynamic antenna patterns and several test issues are discussed.
We have developed a methodology to determine the quality of a EMC test facility using equipment that may be generally available to RF testing services. By utilizing both the timeand frequency-domains, and accurate picture of the scattering and modal properties of the facility can be determined. This gives a much more information of the facility performance than a traditional scalar, frequency sweep of the facility. While the same frequency information is available with this dual-domain method, the causes of the irregularities can now be determined without guesswork and remediation to the facility can be preformed with more confidence.
The need to measure the boresight pointing direction of radar antennas to a high degree of accuracy yields a requirement for excellent positioning accuracy on near-field antenna ranges. Evaluation of this requirement can be accomplished by a full and complete sensitivity analysis. Alternatively, to gain an understanding of the effects of errors more simply, one can approach the question of accuracy required in the setup, by use of a physical model and straightforward physical reasoning. The approach starts with the assumptions of a collimated wave with planar phase fronts and the premise that the boresight direction of such a sum beam is along the normal to the phase fronts. A sensitivity analysis of the simple trigonometric boresight relationship between mechanical boresight and phase front normal, shows how accurate the receiver and the positioner must be to achieve a given boresight determination. Such an approach has been known for many years as it regards planar scanning; and, the results are known to be applicable. In this paper this consideration is extended to spherical scanners to arrive at estimates of the mechanical positioner accuracies and electrical receiver accuracies needed to make boresight measurements of radar antennas with spherical near-field ranges.
Compact RCS measurement ranges all suffer from some level of non-ideal field illumination. Stray fields from interactions with the chamber wall and diffraction effects are major contributors to the non-uniformity of the incident field at the target. This non-uniformity gives rise to unavoidable errors in RCS measurements. We present a detailed analysis of how non-uniform illumination manifests itself into RCS measurement errors. The analysis approach is based on the plane wave spectral decomposition of the illumination. We compute the energy scattered by the planar components of the illumination and determine how much of this energy is coupled backi nto the radar antenna. We model the target as a diffuse scatterer by using a collection of point scatterers distributed within a specified volume. We present uncertainty results based on a simulation as well as field probe data collected from AFRL’s Advanced Compact Range (ACR).
Two ways of modelling a compact range design are presented, and the coupling to a given antenna under test (AUT) is determined and compared to the AUT far field. The compact range models are both based on physical optics (PO). The first model applies a simple presentation of the serrations of the range reflector while the second model is based on a new feature of GRASP8, which allows a detailed description of the triangles of the range serrations. The AUT measurement is modelled by an accurate coupling analysis between the current elements on the compact range reflector and the antenna under test. This coupling pattern is compared to the real far-field pattern and the differences are discussed. By including known range imperfections in the AUT-torange coupling a better agreement to the measured patterns may be obtained. All computations are carried out by GRASP8.
Accurate gain measurements using any measurement technique require a calibrated gain standard, and the uncertainty in the gain of the standard is usually the largest term in the error analysis. To reduce the uncertainty, gain standards are often calibrated using a three- antenna measurement technique and the resulting gain values are generally certified to have an uncertainty of approximately 0.10 dB1-11. For near-field measurements, the gain standard may be the probe that is used to obtain the near-field data or it may be a Standard Gain Horn (SGH). Since the calibration of the gain standard is time consuming and often costly, it is desirable to verify that the gain of the standard is stable over long periods of time. This paper will describe tests to verify the gain stability of the standard and will also illustrate the terms in the error analysis that have the major effect on the uncertainty of any near-field gain measurement. With proper attention to the major error terms, the stability of the gain standard can be verified to approximate the original calibration uncertainty.
The requirement to calibrate and test large active pulsed planar array RADAR antennas, such as the one developed for the advanced synthetic aperture radar (ASAR), places certain requirements on the measurement facility and analysis software that are perhaps not encountered in other areas of application. This paper gives a brief overview of ASAR and an introduction to some of the difficulties encountered during the test and measurement campaign. Results are presented that compare measurement with theoretical prediction. Good agreement has been obtained for both far and near field data.
This paper describes error analysis and measurement techniques that have been developed specifically for cylindrical near-field measurements. A combination of analysis and computer simulation is used to show the comparison between planar and cylindrical probe correction. Error estimates are derived for both the pattern and probe polarization terms. The analysis is also extended to estimate the effect of position errors. The cylindrical measurement geometry is very useful for evaluating the effect of room scattering from very wide angles since scans can cover 360 degrees in azimuth. Using a broad beam AUT and scanning over a large y-range provides almost full spherical coverage. Comparison with planar measurements with similar accuracy is presented.
Wireless communication is occupying new application areas all the time. Data intensive applications require high quality of services level. This emphasizes the need for better solution in every part of wireless communication. In spite of increasing requirements these components should be low priced and robust especially in industrial environments. All this set up an interesting and challenging framework for antenna design. The aim of this paper is to present parameters that have to be considered when new antennas are designed to harsh environments with strong multipath propagation. These parameters are demonstrated by measurements of phase error and attenuation in underground tunnel environment with a few different antenna designs including a new photonic bandgap design.
Analyzing very large ISAR RCS data files using traditional processing software is often a cumbersome experience. The user is often forced to print out hundreds of images manually to get an overview. We propose a solution to this problem. A generalized ISAR algorithm is utilized to automatically generate a series of complex images, creating a "movie" of images with all the information in every pixel. Regions of interest can be zoomed in or scaled to the desired range. Regions can be gated out and the corresponding RCS. presented. The time to perform analysis tasks can be reduced by factors of 10-100. The implementation, which also contains modules for filtering and statistics, has been named Columbus. The use of Matlab and C provides portable code and a flexible platform for further development.
The measurement of the frequency response of complex targets of interest for the purpose of radar cross section (RCS) analysis has become a common task for modern radar ranges. When carefully done to avoid transients, the stepped frequency continuous wave (CW) method directly measures the frequency response of the target. On the other hand, dechirp-on-receive processing utilized by linear frequency modulated (LFM) radars introduces certain distortions to the measurement that are rarely fully considered. In this paper, we derive the relationship between the true frequency response of a target and what is measured with an LFM radar utilizing dechirp-on-receive. One can use this relationship to analyze the effects of the LFM processing as a function of the target geometry or scattering mechanisms and radar parameters. Radar parameters may then be selected so as to minimize the differences between the LFM measured response and the true frequency response of the target.
The Radar Signature Management Group of Racal Defence Electronics Limited specializes in the measurement, prediction and analysis of radar signatures. Types of measurement ranges used by the Group fall into three categories: • Indoor instrumented ranges • Outdoor measurement ranges • Full-scale trials, in which dynamic measurements are made of the target in its normal operational environment This paper describes a methodology used for characterizing the uncertainties within data from one of the outdoor RCS measurement ranges, at frequencies from 8 to 12 GHz. The results are summarized and uncertainties arising from the following sources are quantified: • Linearity • Absolute Accuracy • Stability and Repeatability • Polar Diagram The effects of background and target-to-pylon support interface are also discussed. The individual uncertainties are combined in a simple manner in order to obtain an overall uncertainty bound for the range, and recom mendations are made for reducing uncertainties against the difficulty and cost of implementation.
In the outer space environment, microstrip antennas and microstrip lines may exhibit changed electrical properties due to the severe solar and galactic cosmic ray fluence and the diurnal temperature change on the microstrip antennas. These radiation and thermal effects may alter the permittivity of the microstrip dielectric substrate, and in turn the electrical performance of the microstrip antennas. The radiation and thermal effects on the microstrip substrate are usually accumulative and irreversible. Thus a proper evaluation of the accumulated radiation effects on microstrip antennas is an important task in the spaceborne microstrip antenna performance analysis. To assess the radiation effects on NASA's Tracking and Data Relay Satellite (TDRS) Multiple Access (MA) microstrip antennas in the outer space environment over the lifetime of the spacecraft, a life cosmic ray radiation test and a life thermal test were devised and performed on the MA microstrip antenna elements. This paper presents the test description and the analysis of the test results. The success of these life tests provided the necessary step for the full flight qualification of the MA array elements.
High Power Superposition (HPS) is a method for measuring high power active array antennas in the transmit mode using the near-field technique. Due to the substantial field emitted by array antennas it is not practical, due to safety reasons, to power all of the elements at once. Therefore, the nearfield of smaller element groups are measured individually and the results are added together using complex mathematics. This forms the mathematical equivalent of the full power nearfiield, which is then processed using conventional near-field techniques. The results from the HPS testing method will be discussed with consideration of all the errors introduced. In addition requirements, issues, and solutions for accurate HPS testing will be discussed.
Nearfield Systems, Inc. in Carson, California delivered a vertical 23' by 22' (7.0m x 6.7m) near-field test range to Raytheon Electronic Systems in Sudbury, Massachusetts. This planar and cylindrical measurement system is capable of characterizing antennas of various physical sizes in continuous wave or in pulse mode from 800MHz to 50GHz. The near-field measurements are computer controlled and capable of multiple frequency, multiple beam and multiple polarization in AUT transmit or receive modes. The precision robotic system uses a Data Acquisition Controller running NSI software to provide four-axes for probe positioning and three-axes for antenna positioning. The RF subsystem is based on the HP 8530A microwave receiver, HP 83630B RF source, HP 83621A LO source and HP 85309A LO/IF Distribution Unit. The test range was evaluated using the NIST 18- term error analysis on a 45GHz 54" diameter left-hand circular polarized reflector antenna.
A model-based approach is presented to estimate the desired planar wavefront (DPW) component in range probe data. The estimated DPW component at the center of the quiet zone can be used effectively to calibrate frequency domain range probe data. The calibration is required when the range probe data is used for stray signal analysis. Using a simulated range probe data set and an experimental range probe data set, it is shown that the model-based DPW estimate is better than the DPW estimate obtained using simple smoothing. This is especially true at low frequencies where the quiet zone of a range is limited to 5-6 wavelengths.
For greatest efficiency and accuracy in measuring patterns of a circularly polarized antenna on a planar near field range (NFR), a recommended procedure is to use a fast switched, dual circularly polarized probe. With such equipment one obtains complete pattern and polarization data from a single scan of the antenna aperture. For our task of measuring high gain shaped beam apertures, measurement efficiency is further improved by using a moderately high gain (about 12 dBi) probe that has been accurately calibrated for patterns, polarization, and gain over the test frequency band. Such a probe allows scan data point spacing to be typically at least one wavelength, thus keeping scan time minimized with acceptably small aliasing (data spacing) error. The measured near field amplitude and phase data is transformed via computer to produce the angular spectrum that is further processed to remove the effect of the probe patterns, i.e. probe correction. The final output is a set of (principal and cross) circular polarized far field patterns. However on one occasion, due to fast breaking changes in requirements, we were unable to obtain a calibrated circular polarized probe in the available time. For this test we used an available calibrated 12 dBi fast-switched dual linear-polarized probe with software capable of processing principal and cross circular-polarized far field patterns. As anticipated, we found from preliminary tests that the predicted low cross-polarized shaped beam pattern was not achieved when using the calibrated fast Ku band probe switch. Further tests showed the problem to be due to small errors in calibration of the probe switch. This paper will discuss test and analysis details of this problem and methods of solution.
When mounted on spacecraft , pattern of some antennas are perturbed by the presence of satellite body. The prediction of antenna performances including satellite structure effect is generally done at early stage of antenna design but is limited in terms of model complexity. The test on full spacecraft & in far field condition is then necessary. This solution is very expensive as it means for test at satellite level to use Compact antenna Test Range in order to satisfy cleanliness aspects. For the Meteosat Second Generation (MSG) program test on the toroidal antennas need to be performed on different model including a flight model. A good compromise was to use the external omnidirectional antenna range and a part of satellite structure representing the major contributor for the antenna pattern as identified via numerical analysis. The external range offer possibilities that cannot be reached in Compact range, e.g. low cost, full sphere pattern, low frequency range.