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Progress in Georgia Tech's research in Near-Field Spherical Microwave Holography (NFSMH) is reported. Previously, the amplitude resolution of Spherical Microwave Holography (SMH) was defined and demonstrated. The definition of resolution has been altered to include phase resolution. The resolution of phase is shown to be equivalent to the resolution of amplitude, and both depend on the highest mode order used in the spherical wave expansion. Previous measurements showed that SMH can easily achieve x/2 phase resolution where X refers to free space wavelengths. Current measurements show that the X/2 resolution limit of planar microwave holography can be surpassed by using evanescent energy in the NSMFH technique. Measurements of small, closely spaced, insertion phase defects placed on a hemispheric ally shaped radome are used to demonstrate the improved resolution. The measurement of evanescent energy is achieved by using a specially designed small aperture probe and a small separation distance between small aperture probe and a small separation distance between the radome surface and the measurement surface. The relationship between measured and theoretical insertion phase of a known radome defect is shown. Given the defect size and the maximum mode order used in the spherical wave expansion, measured insertion phase can be used to predict the actual defects electrical thickness.
The U.S.Army Redstone Technical Test Center (RTTC), Test and Evaluation Command, has developed a comprehensive antenna metrology and Radar Cross Section (RCS) evaluation facility. This facility features the compact antenna test range technique for millimeter wave measurements and the near-field scanning technique for microwave measurements. This paper described RTTC's use of these measurement techniques, instrumentation with PC Windows based automation software, anechoic chambers, and types of tests performed. Planned future thrust areas are also discussed.
A new multi-purpose antenna measurement facility was put into operation at the National Institute of Standards and Technology (NIST) in 1993. This facility is currently used to perform gain, pattern, and polarization measurements on probes and standard gain horns. The facility can also provide spherical and cylindrical near-field measurements. The frequency range is typically from 1 to 75 GHz. The paper discusses the capabilities of this new facility in detail. The facility has 10 m long horizontal rails for gain measurements using the NIST developed extrapolation technique. This length was chosen so that gain calibrations at 1 GHz could be performed on antennas with apertures as large as 1 meter. This facility also has a precision phi-over-theta rotator setup used to perform spherical near-field, probe pattern and polarization measurements. This setup uses a pair of 4 m long horizontal rails for positioning antennas over the center of rotation of the theta rotator. This allows antennas up to 2 m in length to be accommodated for probe pattern measurements. A set of 6 meter long vertical rails that are part of the source tower gives the facility that added capability of performing cylindrical near-field measurements. Spherical and cylindrical near-field measurements can be performed on antennas up to 3.5 m in diameter.
DASA's high precision Compact Range Program, which already was a breakthrough in new dimensions of RF measurements standards, will not be completed by a revolutionary new and one of the world's most unique types of Cylindrical Outdoor Near-Field Test Range. The most striking component of this new type facility will be its dominating fully air-conditioned, up to 50 m high diamond shaped concrete tower which is the integral part of the vertical probe scanner subsystem. Although this test range is located outdoor, it allows extremely precise characterization of all typical parameters for state of the art antenna systems.
DASA's high precision Compact Range Program, which already was a breakthrough in new dimensions of RF measurements standards, will not be completed by a revolutionary new and one of the world's most unique types of Cylindrical Outdoor Near-Field Test Range. The most striking component of this new type facility will be its dominating fully air-conditioned, up to 50 m high diamond shaped concrete tower which is the integral part of the vertical probe scanner subsystem. Although this test range is located outdoor, it allows extremely precise characterization of all typical parameters for state of the art antenna systems.
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.
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.
Antenna pattern measurements are dominantly influenced by the presence of extraneous fields in the test zone. A fast and simple way to recognize problems in pattern measurements provides the Antenna Pattern Comparison-technique (APC). This method usually consists of recording azimuthal patterns on different positions across the test zone. Differences in the amplitude data give a rough indication for the magnitude of the interfering signal. The "Novel APC-method" (NAPC) employs both amplitude- and phase-data so that it becomes possible to separate the direct and the extraneous signals from each other. It will be shown that this method is eminently suited to correct radiation patterns of high-gain and low-sidelobe antennas. For verification purposes corrected patterns are compared with time-dated ones and the resemblance is excellent. It is concluded that the NAPC-method is promising and powerful technique for accurate antenna pattern determination, mainly because it can be easily implemented for most applications.
The Hughes Aircraft Company Compact Range facility for antenna and RCS measurements, scheduled for completion in 1993, is described. The facility features two compact ranges. Chamber 1 was designed for a 4 to 6 foot quiet zone, and Chamber 2 was designed for a 10 to 14 foot quiet zone. Each chamber is TEMPEST shielded with 1/4 inch welded steel panels to meet NSA standard 65-6 for RF isolation greater than 100 dB up to 100 GHz, with personnel access through double inter locked Huntley RFI/EMI sliding pneumatic doors certified to maintain 100 dB isolation. While Chamber 1 is designed to operate in the frequency range from 2 to 100 GHz, Chamber 2 is designed for the 1 to 100 GHz region. Both RCS measurements and antenna field patterns/gain measurements can be made in each chamber. The reflectors used are the March Microwave Dual Parabolic Cylindrical Reflector System with the sub-reflector mounted on the ceiling to permit horizontal target cuts to be measured in the symmetrical plane of the reflector system.
The proposed synthesis method allows the calculation of the diffraction figure in the focal plane of the compact range, starting from a field distribution in linear polarization over a plane in the Fresnel zone. Applying this method (in only one dimension) to the ideal near field of a FFOC compact range, a linear array is synthesized which can be extrapolated to a planar array feeder design; providing excellent features in the quite zone.
Data collection is increasingly becoming the limiting factor in overall antenna and RCS measurement time. An equation for data collection time for multiple parameter measurements is presented along with and ordering function for determining the optimum nesting order for parameters. An example is used to demonstrate measurement speed enhancement techniques, reducing data collection time by 65 percent. Changing from stepped to linear near-field scanning reduced collection time by 75 percent.
The three cable method for removing the amplitude and phase variations of microwave cables due to temperature change and movement can offer a substantial improvement in antenna measurement accuracy. Implementation details of the method are provided for a planar near-field range. Items specifically addressed are range configuration, hardware requirements, data collection methodology, identification and assessment of error sources, and data reduction requirements.
The new antenna measurement facility in C.A.S.A. Space Division is described. The system, designed and installed by Grupo de Radiación of the Polytechnic University of Madrid , provides antenna measurement set-up for Far Field and both Planar and Spherical Near Field.
The effects of measurement distance on the sidelobe sum and difference patterns are examined. Highly efficient and robust aperture distributions, the Taylor ñ and the Bayliss ñ, are used to generate date representative of all such distributions. Patterns are obtained through numerical integration of the near-field inegral with exact phase term. Taylor ñ patterns are computed for sidelobe levels to -60 db (published in 1984), and Bayliss patterns for sidelobe levels down to -50 db (new results). For both sum and difference patterns, the change in first sidelobe height, in db, is linear with the log of the measurement distance normalized by 2D(squared)/(lambda). In both cases the lines for different sidelobe levels have the same slope. These results, and typical patterns showing sidelobe changes, will be presented.
A method has been developed by which the fair-field RCS of a target can be evaluated from its RCS measured in the near field. The method can compensate for the nonuniformity of the antenna pattern which can be a function of the angle, the frequency, and the target distance. A correction transform is evaluated which depends on the antenna pattern, the frequency, the target distance and the target size. The correction transform is independent of the target geometry. The RCS of a target is measured in the near field, in a band of frequencies around the frequency at which the far field RCS of the target is desired. The method can practically handle directional scattering elements, shading of the scattering elements by each other, and interactions among the scattering elements. The reconstructed RCS evaluated by this method shows excellent agreement with the actual far-field RCS.
Increasing use is being made of millimeter-wave systems and there is a need for improved antenna measurement facilities operating at these higher frequencies. Although the practical implementation of compact range and near-field/far-field techniques becomes increasingly difficult, by using a hybrid approach, the attributes of these existing schemes can be exploited and their limitations overcome. The technique uses a linear near-field probe to carry out an instantaneous integration of the field in the date acquisition requirement, together with a quasi-real-time prediction capability. This contribution reviews a number of implementation schemes for the semi-compact antenna test range (SCATR) approach which have been investigated over the past decade and presents the latest results. An implementation of the SCATR with amplitude-only data is presented as an economical and viable method for millimeter-wave frequencies.
A 4 foot by 4 foot near field planar scanner is used to evaluate the performance of a SA5751 compact range in CSIST. Using the far field patterns integrated from the scanned aperture fields, the coming directions of the clutters in the chamber can be determined. Often the clutter level is less than the side lobe level of the far field pattern, the scanned field is multiplied by a certain weighting function before integration to pop out the clutter signal. However the weighting method would broaden the main beam and hence clutters coming close along the reflected wave of the reflector are still can not be seen (sic). In this article, a method called main beam suppression, subtracting a constant filed (sic) on the scanned aperture, is introduced to solve this kind of problem and the result shows it serves well for finding those clutters hidden by the main beam and the side lobes nearer to it.
Considerable attention has been paid in recent years to the interpretation of measured field probe data in order to locate and quantify error sources present in the quiet zone of a compact range. This paper describes a new general purpose software package for that analysis. This software has been written to analyze data acquired in a plane-polar configuration. Analysis options include raw data analysis, near-field focusing of single or multiple line cuts, and plane wave spectrum propagation. A graphical user interface gives the operator extensive control over analysis and display parameters. The analysis algorithms used for multiple-cut processing can function with as few as two radial line cuts.
Traditional techniques for evaluating the performance of anechoic chambers, compact ranges, and far-field ranges involve scanning a field probe through the quiet zone area. Plotting the amplitude and phase ripple yields a measure of the range performance which can be used in uncertainty estimates for future antenna tests. This technique, however, provides very little insight into the causes of the quiet-zone ripple. NSI's portable near-field scanners and diagnostic software can perform quiet-zone measurements which will provide angular image maps of the chamber reflections. This data can be used by engineers to actually improve the chamber performance by identifying and suppressing the sources of high reflections which cause quiet-zone ripple. This paper will describe the technique and show typical results which can be expected.
Near field range design includes the placement of RF absorber in the test area. Absorber placement depends highly on the antennas being tested. A common approach is to design an expensive low-reflection chamber around the near-field scanner. The chamber and the additional floor space can sometimes cost more than the near-field scanning system itself. Another approach seeks to identify multi-path reflection to minimize cost by optimally placing absorberto meet specific antenna test requirements. The results is a lower cost range using less floor space. This paper describes a technique of evaluating near-field range multi-path.