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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.
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.
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.
Obtaining far-field radiation patterns of high frequency antennas (>80Ghz) from near-field measurements has been an important issue in the last twenty years. However with frequencies increasing into the millimetre and sub-millimetre bands, questions have been raised about possible limitations on the assessment of such antennas and in particular the measurement of phase. The PTP phase retrieval algorithm addresses the problem by extracting the phase from the knowledge of two amplitude data sets in the near-field. The accuracy of the algorithm is studied by simulation and measurement by means of a numerical/statistical approach. Pseudo-random phase apertures can be generated using Zernike polynomials, which in turn can be used as initial estimates for the algorithm. This paper shows some simulated and measured results for various separations. It can be seen that different pseudo-random phase functions can affect the accuracy of phase retrieved results in particular when the distance between planes is considerably small in relation to the AUT size.
In this paper, an iterative algorithm for the retrieval of the radial component of the electric field is described to be used in matrix source reconstruction methods that deal with spherical measurement. A source-field decoupled integral equations are presented, making it necessary the use of a radial field retrieval algorithm to calculate the equivalent magnetic currents (EMC) in the antenna plane from the angular components of the electric field. The technique is applied in near field to far field (NF-FF) transformations using spherical ranges. With the presented technique, some drawbacks, inherent to the intensive resolution of the integral equations that appears in the methods based on equivalent currents, are overcome. Verification with simulated results as well as measurement results are presented.
The complexity of modern antennas has resulted in the need to increase the productivity of ranges by orders of magnitude. This has been achieved by a combination of improved measurement techniques, faster instrumentation and by increased automation of the measurement process. This paper concentrates on automated measurement systems, and describes the requirements necessary to make such systems effective in production testing, and in research and development settings. The paper also describes one such implementation - the MI Technologies Model MI-3000 Acquisition and Analysis Workstation - that was designed specifically to cmnply with these requirements The paper discusses several important factors that must be considered in the design of automated measurement systems, including: (1) Enhancing range productivity; (2) Interfacing with instrumentation from a large number of suppliers; (3) Providing a uniform front-end for the measurement setup and operation that must be largely independent of the choice of the hardware configu rations or the type of range (near-field or far-field); (4) Making the test results available in a format that simplifies transition to external commercial and user program med applications; (5) Providing powerful scripting capability to facilitate end-user program ming and customization; (6) Using a development paradigm that allows incremental binary upgrades of new features and instruments. The paper also discusses computational hardware issues and software paradigms that help achieve the requirements.
The majority of precision spherical positioner alignment techniques used today are based on procedures that were developed in the 1970's around the use of precision levels and auto-collimation transits. Electrical alignment techniques based on the phase and amplitude of the antenna under test are also used, but place unwanted limitations on accurately characterizing an antenna's electrical/mechanical boresight relationship. Both of these techniques can be very time consuming. The electrical technique requires operator interpretations of data obtained from amplitude and phase measurements. The autocollimation technique requires operator interpretations of optically viewed measurement data. These results are therefore typically operator dependent and the resulting error quantification can be inaccurate. MI Technologies has recently developed a mechanical alignment technique for Spherical Near-Field antenna measurements using a tracking laser interferometer system. Once the laser system has been set-up and stabilized in the operational environment; the entire spherical near-field alignment may be completed in a few hours, as compared to the much more lengthy techniques used with level/transit or electrical techniques. This technique also simplifies the quantification of the errors due to the inaccuracy of the alignment. This paper will discuss the effect of the alignment error on results obtained from spherical near-field measurements, and the procedures MI Technologies developed using a tracking laser interferometer system to obtain the precision alignment needed for a spherical near-field measurement.
The majority of precision spherical positioner alignment techniques used today are based on procedures that were developed in the 1970's around the use of precision levels and auto-collimation transits. Electrical alignment techniques based on the phase and amplitude of the antenna under test are also used, but place unwanted limitations on accurately characterizing an antenna's electrical/mechanical boresight relationship. Both of these techniques can be very time consuming. The electrical technique requires operator interpretations of data obtained from amplitude and phase measurements. The autocollimation technique requires operator interpretations of optically viewed measurement data. These results are therefore typically operator dependent and the resulting error quantification can be inaccurate. MI Technologies has recently developed a mechanical alignment technique for Spherical Near-Field antenna measurements using a tracking laser interferometer system. Once the laser system has been set-up and stabilized in the operational environment; the entire spherical near-field alignment may be completed in a few hours, as compared to the much more lengthy techniques used with level/transit or electrical techniques. This technique also simplifies the quantification of the errors due to the inaccuracy of the alignment. This paper will discuss the effect of the alignment error on results obtained from spherical near-field measurements, and the procedures MI Technologies developed using a tracking laser interferometer system to obtain the precision alignment needed for a spherical near-field measurement.
Toshiba Corporation, working with Nearfield Systems Inc., has a fully digital antenna measurement system for digital beam-forming (DBF) antennas. The DBF test facility is integrated with the large 35m x 16m vertical near-field range installed at Toshiba in 1997 [3], and includes the NSI Panther 6500 DBF Receiver as the primary measurement receiver. The DBF system was installed in March 1999 and has been used extensively to test and characterize a number of complex, high performance DBF antennas. A DBF antenna typically incorporates an analog-to digital (AID) converter at the IF stage of the transmit/receive (T/R) module. The digital IF signals are transferred to a digital beam-forming computer, which digitally constructs, or forms, the actual antenna pattern, or beams. Since the interfaces to the DBF antenna are all digital, the usual microwave mixers and down-converters are incompatible. The NSI Panther 6500 is designed to interface directly with DBF antennas and allows up to 8 channels of I and Q digital input (16 bits each) with 90 dB dynamic range per channel. The NSI DBF receiver solves the DBF interface problem while providing enhanced performance over conventional microwave instrumentation. [2].
A software's user-interface design determines how productive someone will be in a accomplishing a given task. This is particularly true in the area of antenna measurement and analysis. The MiDAS software package is used as an example of how software can be specifically designed to focus on enhancing efficiency by implementing an advanced human-machine interlace. Simple yet critical aspects such as minimized menu access, integrated, user friendly error checking and help, and clear, consistent, and integrated features help to improve productivity, reduce errors and save time. In addition, design principles such as having only one interface for all antenna measurement disciplines (e.g., near-field and far-field), reduces the time needed for training which, in turn, lowers costs. This paper explores how the implementation of such a user interface can be used as a paradigm for increasing efficiency in the field of antenna measurement and analysis.
An important source of error in a compact range antenna pattern measurement is the deviation of the quiet-zone field from the perfectly fiat amplitude and phase of a plane wave field. Although some guidelines and rules of thumb exist that relate the quiet-zone field to the error in the measured antenna patterns, the error or perturbation is dependent on the particular type of antenna that is being measured. For example, the non-ideal quiet zone field will produce very different errors for a small horn than for a large phased array. A realistic error budget or uncertainty analysis of the compact-range measurement requires knowledge of the antenna pattern uncertainty as a function of the quiet-zone field and the particular antenna of interest. A simulation method is derived using reciprocity that allows one to quantify the perturbations induced in a given antenna pattern when the quite-zone field distribution is known. This is particularly useful, since one typically has a fair estimate of the antenna pattern and has measured data of the quiet-zone field. The simulation is tested by modelling the antenna as a collection of elemental current sources and simulating the quiet-zone field as generated by elemental current sources. Using this simple simulation model, a closed-form near-field antenna pattern may be calculated for comparison with the more general computer simulation derived from reciprocity.
A reconstruction method that calculates bi-dimensional equivalent magnetic currents from the tangential electric field components over a hemispherical region is presented. The method is applied for diagnosis as well as for near field to far Field (NF-FF) transformation. The method is well suited for antenna radiation pattern measurement using a near-field spherical acquisition system in anechoic chamber.
A reconstruction method that calculates bi-dimensional equivalent magnetic currents from the tangential electric field components over a hemispherical region is presented. The method is applied for diagnosis as well as for near field to far Field (NF-FF) transformation. The method is well suited for antenna radiation pattern measurement using a near-field spherical acquisition system in anechoic chamber.
An efficient algorithm for the RCS evaluation of the Monostatic Radar Cross Section (RCS) from a reduced set of bistatic near-field data is proposed. The algorithm allows to evaluate the monostatic RCS from near field data collected in an angular region centered on the direction of interest, whose amplitude depends on the size of the scatter and the distance of the measurement zone. Numerical examples on two dimensional elliptical cylinders show the effectiveness of the proposed technique.
An efficient algorithm for the RCS evaluation of the Monostatic Radar Cross Section (RCS) from a reduced set of bistatic near-field data is proposed. The algorithm allows to evaluate the monostatic RCS from near field data collected in an angular region centered on the direction of interest, whose amplitude depends on the size of the scatter and the distance of the measurement zone. Numerical examples on two dimensional elliptical cylinders show the effectiveness of the proposed technique.
This paper describes the alignment procedure for using a spherical near-field measurement facility to determine incident fields throughout a spherical volume. This information can be used, for example, to characterize an anechoic chamber or the quiet zone of a compact range. A probe is mounted on a standard roll-over-azimuth positioner and aligned looking out of the sphere so its aperture maps out the surface of a sphere. The probe measures the amplitude and phase of the fields incident on the sphere. This method differs from the standard spherical near-field measurement where the source antenna serves as the probe and is looking into a sphere containing the test antenna.
In this paper we discuss the experimental issues encountered in the measurement of the local electromagnetic fields in the reactive region of a scattering or radiating body using the NRL Near-Field Facility. Our investigations require high-resolution measurements, reaching spatial resolutions of small fractions of wavelength, high polarization sensitivity, and broad bandwidth. We present techniques currently successfully being used in these investigations. The complexity of the reactive zone fields imposes difficult requirements on probe designs. Currently, the NRL probes include coaxial and coplanar configurations. We will discuss their properties and characterization. Design trade-offs to reach spatial resolutions of one-tenth of a wavelength, bandwidths of several giga-hertz and limitations on polarization sensitivity will be addressed. We will correlate these observations with the results of the back-propagation algorithms being developed at the Naval Postgraduate School.
The accuracy of near field measurements have in the past largely been judged by inspection however the authors have developed an objective measure of the accuracy and repeatability of such measurements. This paper illustrates the measurement process and the techniques associated with statistical image classification used to confirm its accuracy and repeatability. The technique will be illustrated via the correlation of data sets acquired over a variety of different frequencies and scan plane areas. The examination of these measurements will demonstrate the applicability and sensitivity of the technique when the accurate assessment of highly correlated patterns is required.