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Much research has been done recently on 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 examines an alternative method of analyzing those data by applying spherical phase offsets to focus the field probe data to near-field distances. This method is applied to simulated field probe data for a large compact range. The technique yield the correct [x,y,z] coordinates of multiple scattering sources deliberately introduced into the simulated data.
The scanning-probe spherical near-field system at GE Aerospace uses a roll over azimuth positioner with probe horn on a cantilevered arm to scan spherical sector centered over a stationary antenna. The main sources of measurement errors in this system are: 1. Signal drift, 2. Deviations of recorded angles from commanded values, 3. Differences of actual sample positions from ideal ones. Unless corrected, these arrors may alter the transformed field.
The scanning-probe spherical near-field system at GE Aerospace uses a roll over azimuth positioner with probe horn on a cantilevered arm to scan spherical sector centered over a stationary antenna. The main sources of measurement errors in this system are: 1. Signal drift, 2. Deviations of recorded angles from commanded values, 3. Differences of actual sample positions from ideal ones. Unless corrected, these arrors may alter the transformed field.
The need to calibrate large antennas and radio stars is driven by needs in satellite communications systems, deep space communications systems and navigation systems. NIST presently is able to calibrate standard gain antennas up to 10 feet in diameter using their planar near-field facility and has sought means to extend their calibration services to larger antennas. During the last ten years, NIST developed an ETMS (Earth Terminal Measurement System) to measure the gain of large antennas using both radio sources and noise sources calibrated by NIST. This ETMS, however, requires that the flux density of the radio sources be accurately known. This often is not the case. NIST is currently involved in two measurement efforts using calibrated standard gain antennas, calibrated noise sources and the gain comparison method to accurately determine the absolute gains of large antennas and accurately determine flux densities of radio stars and planets. Recent progress on these efforts will be discussed.
A new implementation of the planar near-field back projection technique for phased array testing and aperture imaging is described. In the alignment of phased arrays, the aperture field is treated as a continuous distribution rather than using idealized array concepts. The continuous field is then sampled to obtain element excitations. In this way, nonrectangular arrays can easily be accommodated. The method also produces highly interpolated images of apertures that can offer much insight into their nature. Also, any polarization of the aperture field may be obtained if the probe pattern has been characterized. The technique uses large FFTs which are computed very quickly by a workstation located in the facility. Results from an iterative phase alignment of a 12x18 phased array are presented, as well as highly interpolated images of apertures and results which demonstrate the polarization selection.
To enhance the performance of existing near field techniques the new idea of far field pattern determination from only amplitude distributions of the near field is proposed. In this way the difficulties related to phase measurements are overcome. Some different algorithms are introduced and discussed. In particular, after recalling results for the planar geometry, cylindrical scanning surfaces are considered. The feasibility and the performances of the introduced algorithms are shown through numerical examples.
A method for obtaining the individual element excitations of an array antenna from measured radiation patterns is presented. Applications include element failure diagnosis, phased array antenna calibration, and pattern extrapolation. The measured far-field information is restricted to visible space which does not always contain the entire Fourier domain. A typical example is phased array antennas designed for large scan angles. A similar problem arises during near-field testing of planar antennas in which case the significant far-field domain is restricted by the scanning limitations of the near-field test facility. An iterative procedure is then used which is found to converge to the required solution. The validity of the approach has been checked both using the theoretical radiation patterns and real test cases. Good results have been obtained.
The Dassault Electronique flexible near-field antenna test facility, ARAMIS, has been used for test and calibration of state-of-the-art active phased-array antennas which were designed for military SATCOM operation. The 14-month successful program dramatically emphasized the benefits of a flexible antenna test facility such as ARAMIS. These benefits are the following: • Flexibility o Far-field mode (test of radiating elements and modules) o Planar near-field mode (test of sub-arrays and complete antenna) o High-resolution field mapping mode o Array Element testing • Speed: quick mode switching, “on the fly” multiplexed acquisition • Versatility: calibration of a module, a sub-array and the antenna; radiation patterns; gain; faulty element detection • Productivity: a single indoor facility performing different types of measurements, integrated software Test results gathered during this program and showing the ARAMIS contribution are presented.
Accurate near-field cross-polarization measurements on circularly polarized (CP) antennas at millimeter-wave frequencies require well-characterized probes with low axial ratios. We have recently obtained and calibrated dual-port CP horns for use as near-field probes at frequencies of 40-50 GHz. These horns have axial ratios which are 0.3 dB or less over a 10% frequency bandwidth. With these good axial ratios the difference between vector and scalar probe correction is usually small. Additional advantages of the dual-port probes are the need for only a single alignment, more accurate knowledge of the relative phase between two ports of the same probe, and the ability to obtain both main and cross polarized data during one scan. The axial ratios of the dual port CP probes are also better than those of single-port CP Probes. In this paper we present some gain, axial ratio, and pattern measurements for these probes and show that they give accurate cross-polarization measurements.
Airborne or spaceborne radar systems often require adaptive suppression of interference and clutter. Before the deployment of this adaptive radar, tests must verify how well the system detects targets and suppresses clutter and jammer signals. This paper discusses a recently developed focused near-field testing technique that is suitable for implementation in an anechoic chamber. With this technique, phased-array near-field focusing provides far-field equivalent performance at a range distance of one aperture diameter from the adaptive antenna under test. The performance of a sidelobe-canceller adaptive phased array antenna operating in the presence of near-field clutter and jamming is theoretically investigated. Numerical simulations indicate that near-field and far-field testing can be equivalent.
The near field technique has grown from experimental systems of the early 1960s to sophisticated accepted means of testing antennas. Several schemes have been employed, namely planar, cylindrical and spherical scanning. The spherical scanning system chosen for one of the near field ranges at GE Aerospace is different from most near field systems in that the test antenna remains stationary while the probe is made to scan over a surface of an imaginary sphere surrounding it. The sampled field is corrected for positional, phase and amplitude errors and transformed to the far field. Radiation patterns, gain, EIRP, group delay and amplitude response were measured for a shaped beam communications antenna.
Near-Field antenna measurements were made using a Hewlett Packard 8510 automated network analyzer. This system features measurement sensitivity better than -90 dBm at measurement speeds of one data point per millisecond in the fast data acquisition mode. The system was configured using external, even harmonic mixers and a fiber optic distributed local oscillator signal. Additionally, the time domain capability of the HP 8510, made it possible to generate far-field diagnostic results immediately after data acquisition without the use of an external computer.
Effects of probe position errors in planar near-field measurements have been significantly reduced at NIST by accurate alignment of the scanner and an analytic error correction. Currently, the near-field range has probe position errors greater than 0.01cm only at the edges of the 4 x 4 m2 area, and less than that everywhere else. The position errors can be further removed by a theoretical procedure, which requires only the error-contaminated near-field and the probe position errors at the points of measurements. All necessary computations can be efficiently performed using FFTs. An explicit nth-order approximation to the ideal near field of the antenna can be shown to converge to the error-free near fied. Computer simulations with eriodic error functions show that this error-correction technique is highly successful even if the errors are as large as 0.2wavelength, thereby making near-field measurements at frequencies will abobe 60 GHz more practicable.
Effects of probe position errors in planar near-field measurements have been significantly reduced at NIST by accurate alignment of the scanner and an analytic error correction. Currently, the near-field range has probe position errors greater than 0.01cm only at the edges of the 4 x 4 m2 area, and less than that everywhere else. The position errors can be further removed by a theoretical procedure, which requires only the error-contaminated near-field and the probe position errors at the points of measurements. All necessary computations can be efficiently performed using FFTs. An explicit nth-order approximation to the ideal near field of the antenna can be shown to converge to the error-free near fied. Computer simulations with eriodic error functions show that this error-correction technique is highly successful even if the errors are as large as 0.2wavelength, thereby making near-field measurements at frequencies will abobe 60 GHz more practicable.
Antenna engineers recognize that the planar near-field method for calibrating antennas provide accurate pattern and gain measurements. Bothe the pattern and gain measurements require some degree of probe position accuracy in order to achieve accurate results. This degree of accuracy increases for antennas that have structured near-field patterns. These are antennas in which the amplitude and phase change rapidly over a very small position change in the near-field scan plane. The National Institute of Standards and Technology (NIST) has recently measured an antenna with a very structured near-field pattern. This measurement was performed using a new probe positioning system developed at NIST. This measurement will be discussed and results will be presented showing how slight probe position errors alter the antenna pattern and gain.
We have developed planar near-field codes, written in Fortran 77, to serve as a research tool in antenna metrology. This new package has a highly modular structure and can be used to address a wide variety of problems in antenna metrology. We describe some of the inner workings of the codes, the data management schemes, and the structure of the input/output sections to enable scientists and programmers to use these codes effectively. The structure of the code is open, so that a new application can be incorporated into the package for future use with relative ease. A new module can rely on the large number of reusable subroutines currently in existence, and new routines are easily integrated into the existing library. Examples of applications of the codes to basic research problems, such as transformation of a near field to the far field and probe position error correction, are used to illustrate the effectiveness of these codes. Sample outputs are shown. The advantage of a high degree of modularization is demonstrated by the use of DOS batch files to execute Fortran modules in a desired sequence.
We have developed planar near-field codes, written in Fortran 77, to serve as a research tool in antenna metrology. This new package has a highly modular structure and can be used to address a wide variety of problems in antenna metrology. We describe some of the inner workings of the codes, the data management schemes, and the structure of the input/output sections to enable scientists and programmers to use these codes effectively. The structure of the code is open, so that a new application can be incorporated into the package for future use with relative ease. A new module can rely on the large number of reusable subroutines currently in existence, and new routines are easily integrated into the existing library. Examples of applications of the codes to basic research problems, such as transformation of a near field to the far field and probe position error correction, are used to illustrate the effectiveness of these codes. Sample outputs are shown. The advantage of a high degree of modularization is demonstrated by the use of DOS batch files to execute Fortran modules in a desired sequence.
A new spherical near field facility has been recently implemented at the Electromagnetic Propagation Area of INTA. The facility makes use of an existing big anechoic chamber (12 x 12 x 12 m.) and the near field/fair field transformation software developed by TICRA. This range has been calibrated by measuring an offset reflector antenna and comparing the results with those obtained in previous measurements of this antenna in other European testing facilities of different types. An experimental study has been carried out to check the dependence of the transformation software on the scanning parameters and different misalignments have been produced in order to determine the impact of the mechanical deformations on the accuracy of the system.
A near field synthetic aperture imaging technique using three main beam suppression methods is used to locate and quantify the sources of stray signals in a compact range. First, main beam cancellation by subtracting the complex average of the measured field for the overall probe aperture is used. Second, a software on-axis null is generated by preprocessing the data. Third, an antenna with a broadside null is used as the prober. It is shown that the software on-axis null enhances the resolution of the spurious scatterer images and is able to detect small spurious scattering centers, such as the surface discontinuity at the top of the reflector, which are otherwise undetectable. Probe data with two metallic tapes placed on the compact range reflector is used as another example to show the performance of the nulling technique.
A spherical range probing technique for the location of secondary sources in far-field compact and spherical near-field antenna measurement ranges are presented. Techniques currently used for source location use measurements of the range field on a line or plane to locate sources. A linear motion unit and possibly a polarization rotator are necessary to measure the range field in this manner. The spherical range probing technique uses measurements of the range field made on a spherical surface allowing the range positioners to be used for the range field measurement. The plane wave spectrum of the measured range field is used for source location in the spherical probing technique. Source locations in the range correspond to the locations of amplitude peaks in this spectrum. Source resolution limits of this technique is illustrated using simulated range measurements. Obtaining a plane wave spectrum from measured data is discussed.