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Traditionally the Compact Range is not considered a viable method for conducting low frequency (VHF/UHF) antenna or RCS measurements because of the limited electrical size of the collimating reflector system. Normally, compact range measurements are conducted in the extreme near-field or the collimating system where to reflector size is on the order of 25 to 30 wavelengths minimum with at least four wavelength edge treatments. This mode of operation limits measurements to the high UHF band (800 MHz) and above for typical sized reflector systems in use today. Recent research with compact range3s indicates that acceptable VHF.UHF measurements can be conducted in the quasi far-field region of the collimating system with reflectors as small as five wavelengths and with electrically short edge treatments. A good user knowledge of this mode of operation is required to maximize its utility. A qualitative measure of acceptable quiet zone performance must also be established. This paper addresses the theory of operation, practical implementation and inherent limitations of the non-conventional use of the indoor compact range for conducting low frequency measurements.
The Compensated Compact Range (CCR) has been proven to be a high performance test facility for payload and large satellite antenna measurements. To efficiently measure complete antenna farms with various types of antennas on the spacecraft in the same test campaign led to the growing demand for testing e.g. Global Horn antennas on the spacecraft in the CCR. As a matter of fact, medium gain antennas feature a small aperture simultaneously requiring larger test angles. Therefor, main interferer like "direct leakage" between the CCR feed and the antenna under test have to be quantified and their impact on the measurement accuracy have to be reconsidered. The presented paper will investigate theoretically the feasibility of testing high performance widebeam antennas in the Top-Fed-Cassegrain double reflector system. Qualified measurement results of INTELSAT Global Horn Antennas featuring medium gain and extreme crosspolarization purity will be presented.
A PTD (Physical Theory of Diffraction) code has been developed to analyze the quiet zone performance of serrated edge compact range reflectors. This PTD code utilizes a modified Ludwig's method to achieve fast convergence for the PO (Physical Optics) integration and the ILDC (Incremental Length Diffraction Coefficients) method to calculate the diffraction correction due to the edge current. In this paper, key formulations for this approach will be derived. Calculated results for an example range will be shown and compared with other published data.
The design, alignment and characterization of a compact range facility at the University of California at Los Angeles is summarized. The range is intended to operate from X-Band to 60 GHz, and to accommodate test items up to 3 feet in diameter. The compact range reflector is a circular aperture offset paraboloid which is devoid of edge treatment. Reduction of reflector edge diffraction effects is achieved using an array feed for narrowband test applications, or a horn feed for broadband test requirements. The array feed requires only one relative amplitude excitation coefficient and no phase shifters, so that a simple and cost-effective implementation using a radial transmission line beam-forming network is possible The array pattern displays a deep null at the reflector rim for diffraction reduction, and a flat-topped beam which results in minimum quiet zone field amplitude taper and high reflector aperture efficiently. Structures for support and alignment of the range reflector and feed are discussed, and alignment procedures are summarized. Range performance at X-Band using an array feed is determined using transverse and longitudinal pattern comparison methods.
Due to the new EMC regulations of the European Union (EU) the number of anechoic chambers recommended as EMC test facilities is rapidly growing. Very often, the size and shape of the chamber are dictated by the surrounding building, which may cause asymmetries in the chamber geometry. Sometimes, however the walls and/or the ceiling of the chamber are intentionally sloped to prevent modes from becoming dominant in the cross section. In this contribution, an accurate numerical analysis method, presented and verified at the AMTA '92 and '93 meetings, us applied to evaluate systematically the influence of these geometrical asymmetries on a specific example. It is shown, that the influences of the asymmetries are strongly dependent on the frequency and the actual shape of the chamber. Both parameters wok together in such a complex manner, that ordinary trial and error methods, often used for this purpose are unsuitable for an accurate analysis.
Test zone field (TZF) compensation increases antenna pattern measurement accuracy by compensating for non-plane wave TZFs. The TZF is measured over a spherical surface encompassing the test zone using a low gain probe. The measured TZF is used in the compensation of subsequent pattern measurements. TZF compensation is demonstrated using measurements taken in an anechoic chamber, far-field range. Extraneous fields produced by reflection and scattering of the range antenna field in the chamber causes the TZF to be non-planar. The effect of these extraneous fields on pattern measurements is shown. Measured TZFs are also shown. TZF compensation results for pattern measurements using a high-gain, X-band slotted waveguide array are presented.
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.
Absorber material us used in antenna measurements to reduce multiple reflections and multipath effects. However, in some cases the effect of the absorber can still have an uncertainty larger than the desired uncertainty of the measurement. For accurate antenna gain measurements, using the planar, cylindrical and spherical near-field methods and the extrapolation technique, insertion-loss measurements should be accurate to within + 0.03dB. To satisfy this requirement it is important to minimize the multiple reflections between the probe and antenna under test. If the multiple reflections are too large, the insertion loss becomes very position sensitive and uncertainties on the order of 1 dB can occur. It is imperative that absorber be used to cover all metal surfaces of the antenna mounts. Uncertainties can also occur if the absorber is not used carefully. The effects on antenna fain data measured with and without absorber will be shown. Measurement results showing the effect of the placement of the absorber on the antenna under test will also be presented. This will include absorber distance from the antenna's aperture, the rotation of absorber about the antenna's coordinate system, and the use of different types of absorber.
The Three-Antenna gain method is commonly used on far-field ranges to determine an antenna's absolute gain. This is especially true when no other calibrated antenna is available. This method has been used for years by calibration laboratories such as NIST to calibrate probes and gain standards for far and near-field ranges. In some cases, the calibration is too costly or does not meet the schedule requirements of the near-field test range. An alternative is to calibrate the probe or gain standards directly on the near-field range. In this paper we present the results of a study done to show the accuracy of the Three-Antenna gain method when used on a near-field range. An extensive error analysis is presented validating the utility of this method.
It is shown that a phase-tapered aperture may be used to produce a uniform plane wave at Fresnel zone distances. This allows one to perform antenna/RCS measurements at reduced distances relative to a far-field range, but without the illuminator complexity and cost associated with a compact range. An asymptotic expression is obtained for the Fresnel field of a circular aperture field source distribution characterized by a large quadratic phase taper. The field is shown to be equivalent to that produced by a uniform ring source and central radiator, so that the design equations for the ring source and central radiator can be applied to plane wave synthesis using a circular phase-tapered aperture. The asymptotic expression for the field as compared with a numerical evaluation obtained using aperture integration. A simple implementation of a phase-tapered aperture using a radiating source which illuminates an aperture at a distance is presented. A quiet zone field extent which is approximately 70-80% of the source aperture extent is demonstrated.
Two techniques of antenna diagnostics based on the knowledge of partial information about the near field are discussed. In the first approach, the problem of the characterization of the source from the knowledge of the near field over a limited scanning domain is formulated as a linear inverse one. The second problem concerns the antenna diagnostics from the knowledge of only amplitude distributions over limited regions of two planar surfaces. In both cases effective and reliable solution procedures are discussed.
The advantages of mesh antennas include good storability and low mass for large on-board antennas over 10M in diameter. Their weak point is that surface adjustment is necessary to attain high accurate surface. Surface adjustment traditionally involves the repeated measurement of surface node position with a theodolite system and subsequent cable adjustment. These steps take much time. This paper describes a surface adjustment scheme that uses near field measurement for a modular mesh antenna composed of mesh, cable network and supporting structure. The node positions of the antenna are obtained by back projection of the far field pattern generated from the near field pattern. The cable network has low sensitivity to changes in local node position. The results of tests show that the surface accuracy needed to achieve the required RF performance can be obtained quickly without theodolite systems.
Phaseless near-field antenna measurements, and the ensuing required processing for retrieving the phase are becoming an increasingly important part of modern antenna measurements. This paper discusses recent investigations of phaseless near-field measurements using the UCLA bi-polar planar near-field antenna measurement system. A "table-top" implementation of this technique is proposed as a resizable and cost-effective means for performing phaseless near-field measurements at millimeter wave frequencies. This paper will present a brief overview of the phase retrieval problem and the algorithms which have been utilized in the microwave and millimeter wave regimes. A more detailed examination of an iterative Fourier transform algorithm for solution of the phase retrieval problem with application to the bi-polar near-field measurement technique will be given. Measurement results will be presented and compared to results obtained using both the measured amplitude and phase data.
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.
A system performance upgrade program of Taiwan CSIST 26' by 20' horizontal planar near-field measurement system was completed recently. The program includes replacement of the original RF subsystem with new HP8530A based microwave receiver and HP83630A synthesized sweepers in external mixers configuration. Other parts of the program introduce new data acquisition computer, HP9000/382 work station, running X-windows under U-NIX environment providing better networking capability and friendly user graphic interface. Efforts has been made to handle the operation between host computer and RF subsystem and to incorporate with the system hardware, in particular on timing requirement and synchronization among processes. This paper briefly introduces the upgraded system hardware configuration and software architecture and describes the results of system overall RF performance along with the upgrade program.
The need for accurate measurements of antenna parameters particularly for satellite antennas considering their relationship to satellite system budgets has become increasingly important. As a multinational agency, the Space Agency has, over the years, introduced the concept of inter-range comparisons of measurements made on "standard" antennas in order to ensure a of measurement quality from ranges throughout This process has evolved into the concept and application of test range validation and accreditation where the antenna being measured by the range is used not to calibrate the range, but rather as a means of assessing the ability of the range to perform accurate measurements. To this end, ESA has developed a Validation Standard (VAST) antenna which will shortly form the basis of accreditation for european test ranges used for ESA satellite antennas. This paper wilt describe the evolution, objectives, and the proposed application of the ESA antenna range accredition programme and its relevance to the activities of the European Union.
This paper describes the of the National Association of Authorities (NATA) which is for the accreditation of calibration and aboratories in Australia. The development of accreditation criteria in the area of antenna measurements is with the roles of NATA's Electrical Registration Advisory Committee and the National Measurement Laboratory being defined. The evolving EMC regulatory environment which has driven the demand for calibration and testing facilities is outlined. Specific issues addressed include the acceptability of calibrations in terms of traceability from either within Australia or overseas, the validation of test sites, the determination of uncertainties of measurement and the relationship of uncertainties to test limits and specifications. Some of the specific problems encountered at laboratory assessments are highlighted. Finally, NATA's international linkages, which have been established via mutual recognition agreements, are discussed together with their significance for accreditation and the acceptability of measurement results.
This paper describes the implementation of the National Voluntary Laboratory Accreditation Program (NVLAP) for calibration laboratories at the National Institute of Standards and Technology (NIST) NVLAP Office in Gaithersburg Maryland. It chronicles the efforts of the National Conference of Standards Laboratories (NCSL) Total Quality Management (TQM) Committee for Calibration Systems Requirements in demonstrating the need for the program, and NIST's efforts in reaching the decision to implement the program in response to a petition from the NCSL. The development of Calibration Laboratories Program Handbook (NIST Handbook 150), and a companion Calibration Laboratories Technical Guide (NIST Handbook 150-2) for the eight fields of calibration covered by the program are discussed. The recruitment and training of Technical Experts (TEs) who are used in the assessment of laboratory competence are outlined. The issue of proficiency testing as it relates to determination of laboratory competence, and the importance of the program as it relates to international markets via recognition by international accreditation community, are discussed.
Radar Cross Section (RCS) measurements are often performed in discrete frequency bands for a variety of reasons. Although some indoor ranges are capable of performing very wide-band measurements (with bandwidths up to or exceeding 9: 1), some are designed with very rigid illumination requirements on the coIIimating reflector(s) that can only be met over a narrow band. In addition, the bandwidth available on most outdoor ranges is limited by "ground plane" effects which make it impossible to maintain an adequate broadband field over the target. Often, RCS measurements are limited to half an octave at most. Since resolution in RCS imaging is directly proportional to bandwidth, there exists a need for concate nati ng several discrete bands of measurements into a single continuous band. This resulting band must be free of both amplitude and phase discontinuities that would affect the quality of the resultant image. This paper discusses the sources of discontinuities between measured bands on both indoor and outdoor ranges, and provides algorithms for removing them using linear filtering methods. Data is presented from an outdoor range illustrating the results on targets up to 70-feet in length.