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An accurate determination of the radiation pattern coverage is necessary for Flight Termination and Safety (FTS) systems. Vehicles such as the X-34 are physically large and can be difficult to handle and mount for full spherical characterization of the patterns. The question addressed here is "can a partial, full-scale mockup be used for FTS measurement purposes?" Simulations were performed to determine the percent spherical coverage for three candidate X-34 full-scale configurations: 1) Complete mockup, 2) full-length mockup without wings or tail and 3) a partial-length model without wings or tail. The radiation patterns were computed using NEC-BSC and post processing was done to determine the coverage. The FTS UHF percent spherical coverage calculations varied by less than 0.5 dB. For the three configurations, the level at 95% spherical coverage varied from -20.55 to -21.0 dBi for LHCP. Subsequent measurements of case 3) were within 0.5 dB of the values predicted.
This paper will explore the effects of the source antenna's axial ration on the apparent gain of an Antenna Under Test (AUT). A technique will be given to correct these errors. Finally, experimental test results will be given.
Radio source measurements of antenna G/T are well established, and can be anticipated to have more applications in the future. These techniques, however, sometimes have limitations that are overlooked and are not universally understood. Other valuable information about the antenna's performance can be determined. The measurement procedure is reviewed and common limitations are described.
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.
A diagnostic/prognostic method for phased-array antennas has been developed which uses a single, fixed position RF probe to detect and identify faulty array elements as the array operates normally. After system calibration, zero array down time is required. A fiberoptic RF probe which allows implementation of the technique while negligibly affecting array operation and performance, has also been developed. The system has been demonstrated both in various computer simulations of arrays to over 1000 elements and in recent experimental tests at NSWC, Crane, IN. Identified faults include array elements which were off, stuck at constant phase, low in power, including both single faults and large numbers of simultaneous faults. An RF radiating probe (a fiber-optic version of which has also been demonstrated) can be used to diagnose array receive mode operation. Results of both the simulations and the tests are reported along with the design of the fiber-optic probe.
A prototype broadband ground reflection range, to be used for measuring antenna patterns on full sized aircraft, was built and evaluated. The range was designed to evaluate an antenna at several arbitrary VHF/UHF frequencies simultaneously. This is a follow on to two previous papers that explored the design of such a range using numerical modeling and optimization by the use of genetic algorithms.
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.
One problem in a RCS ground bounce range is that the direct signal can be interfered with by the ground reflected signal. The undesired ground bounce signal will cause errors in the RCS measurement. This paper presents a study of ground bounce reduction using a tapered and stepped resistive sheet fence. In order to show that the proposed R-card fence technique can reduce the ground reflected signal significantly, both experimental and theoretical studies are performed. The resistance of the R-card varies based on a Kaiser-Bessel taper function. The experimentall results with and without the R-card fence show that the ground reflected signal can be attenuated by about 20dB. Both vertical and horizontal polarization cases are considered. This paper also the results of a simulation using NEC-BSC (Numerical Electromagnetic Code - Basic Scattering Code, developed at The Ohio State University ElectroScience Laboratory). Comparison of the results between measurements and simulations will be shown in this paper.
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.
We present a preliminary evaluation of a microwave measurement system that has been designed to determine electromagnetic fields in the quiet-zone of an antenna measurement range and to produce an image of the sources, intended and unintended, of the incident radiation. This information is of potential value in the processes of improving range perfor mance, correcting pattern results for non-ideal illumination , and evaluating measurement uncertainty.
Composite Optics Inc (COI) has designed and constructed a Portable Far-Field Antenna Test Chamber to complement their Large Compact Range. The need for this chamber arose after COI won a contract to design, build, and test hundreds of small broadband antenna elements. Because of the portability requirement, COI chose to procure and modify an industrial container, suitable for transportation on a standard flatbed trailer. This paper discusses the design, fabrication, and installation of a chamber, suitable for pattern measurements of small (<2 feet) antennas in the 6-18 GHz frequency range.
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.
TRW, working with Nearfield Systems Inc., is building a state-of-the-art near-field antenna measurement system to test the Astrolink payload antenna system. Astrolink is the next generation broadband satellite network that wm deliver high speed Internet connections to the business desktop. TRW is building the Astrolink on board communications payload which includes the antenna system. For this multi-reflector antenna payload, TRW is building a 40 ft. x 30 ft. horizontal near-field measurement system to operate from 1 to 50 GHz using NSl's high speed Panther receiver and Agilent Technologies high speed VXI microwave synthesizers. The system will be capable of performing conventional raster scans, as well as directed plane polar scans tilted to the plane of a specific reflector. Completion of the range is scheduled for the first quarter 2001. This paper will describe the near-field antenna measurement system that will test the Astrolink antenna payload and provide an overview of the specifications and test requirements for this test system. This paper will also describe the tilted plane-polar scanning capability, the 1 to 50 GHz RF subsystem, and the facility plans and progress.
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.
Future European Space Agency (ESA) earth observation and space science missions such as MASTER and PLANCK will have instruments and associated antennas working well up into the Terahertz frequencies. The large sizes of the antenna apertures and the need to accurately verify their performance, place high demands on test facilities and test techniques. In recent decades, different types of facilities have been developed. ESA has identified that for measurements up to at least 500 GHz, existing facilities and techniques could be applied with a relatively modest investment. A trade-off between the cylindrical near-field and compact antenna test ranges at Astrium has been carried out to identify which of the two existing ranges would provide better accuracy.
A radar transceiver operating at 1.56 THz has recently been developed to obtain coherent, fully polarimetric W-band (98 GHz) RCS images of 1:16 scale model targets. The associated optical system operates by a scanning a small focused beam of swept frequency radiation across a scale model to resolve individual scattering centers and obtain the scaled RCS values for the centers. Output from a tunable microwave source (10 - 17 GHz) is mixed with narrow band submillimeter-wave radiation in a Schottky diode mixer to produce the chirped transmit signal. Two high-frequency Schottky diode mixers are used for reception of the V-pol and H-pol receive states, with a fourth mixer providing a system phase reference. The full 2x2 polarization scattering matrix (PSM) for each resolved center is obtained following off-line data processing. Measurement examples of five simple calibration objects and a tank are presented.
The purpose of millimeter wave RCS measurements is often to evaluate the performance of scale model aircraft. To representative ISAR it is important that also the resolution cell size is scaled in proportion to the frequency. A typical bandwidth used for full scale aircraft measurements at 10 GHz is 2 GHz. This means that for at a 1:10 scale model measured at 100 a bandwidth of 20 GHz should be used. By modifications of a HP83558A W-Band antenna measu rement equipment, a powerful RCS measurement equipment covering 75 - 103 GHz with high receiver have been achieved. The hardware modifications and the radar and turntable performance are presented. This paper also shows the W-Band requirements for the SAAB indoor RCS measu rement facility in Linkoping, Sweden, and how these requirements are fulfilled. RCS measurements have been performed on 1:50 and 1:10 model aircraft. These measurements are discussed and ISAR images with resolution cell sizes down to 10 mm x 10 mm are presented.
We evaluate radiation characteristics of a millimeter wave reflector antenna by using two sets of phaseless data measured in its near field. This is called a near field phase retrieval method (NFPRM). To apply this method to millimeter or submillimeter antennas, we have to pay more attention to the relations how the estimation errors and convergence are affected by the interval of two planes and SIN ratios of the measurements. This is because the difference between the two measured amplitude distributions is usually very small. In this paper, this method is applied to a case for 90-GHz reflector antenna with an extremely small interval between the two planes. The results show some clear correlation between the estimation errors and measurement conditions.
Considerable progress has recently been made in the application of phase retrieval methods for phaseless near-field antenna measu rements. These techniques have sufficiently matured so that accurate antenna measurements can be performed when the phase information is either unavailable or inaccurate. A comparison of conventional (amplitude and phase) and phaseless (amplitude only) planar near-field measurements for non-ideal measuring probe locations is examined via simulated array antenna case studies involving both random and systematic errors. It will be demonstrated that the presented phase retrieval algorithm can more accurately reproduce the true pattern of the antenna under test because of the diminished sensitivity of the amplitude of the near field, as compared to the phase, with respect to the measuring probe locations. This phase retrieval approach requires no knowledge of the actual measurement locations, other than the nominal location of the two required measurement planes, and is suitable for relatively large probe position errors.