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Far Field
A Spherical near field system with a scanning probe
S.S. Dhanjal (General Electric Company),M. Cuchanski (General Electric Company), November 1990
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 testing using the Hewlett Packard 8510 automated network analyzer
R.R. Kunath (NASA Lewis Research Center),M.J. Garrett (NASA Lewis Research Center), November 1990
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.
Planar near-field codes for personal computers
L.A. Muth (National Institute of Standards and Technology),R. Lewis (National Institute of Standards and Technology), November 1990
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.
Spherical probing of spherical ranges
D.N. Black (Georgia Institute of Technology),E.B. Joy (Georgia Institute of Technology), G. Edar (Georgia Institute of Technology), M.G. Guler (Georgia Institute of Technology), R.E. Wilson (Georgia Institute of Technology), November 1990
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.
Antenna range performance comparisons
E.H. England (Defense Research Agency),H. Hezewijk (TNO Labs) J. Bennett (University of Sheffield) N. Williams (ERA Technology Ltd.), November 1991
The radiation patterns of a low (40dB) sidelobe antenna have been measured on a variety of antenna test ranges including Near Field, Far Field and Compact versions. Originally intended to validate new Near Field Ranges, some of the early results will be presented and the variations examined. The need for some form of range validation is shown. There is also some explanation of the fundamental effects that various ranges have on results.
A Methodology for diagnostics and performance improvement for large reflector antennas using microwave holography
D.J. Rochblatt (California Institute of Technology), November 1991
Microwave holography has proven to be a powerful technique for various evaluations, diagnostics, and RF performance improvements for large reflector antennas. The technique utilizes the Fourier Transform relation between the complex far-field radiation pattern of an antenna and the complex aperture field distribution. Resulting aperture phase and amplitude distribution data can be used to precisely characterize various crucial performance parameters, including panel alignment, subreflector position, antenna aperture illumination, directivity at various frequencies, and gravity deformation effects. The methodology of the data processing presented in this paper was developed at JPL and has been successfully applied to the NASA/JPL Deep Space Network (DSN) 34m beam waveguide antennas. The performance improvement of the antenna was verified by efficiency measurements and additional holographic measurements. The antenna performance was improved at all operating frequencies of the antenna (wide bandwidth improvement) by reducing the main reflector “mechanical surface” rms error to 0.43 mm. At Ka-band (32-GHz) the estimated improvement is 4.1 dB, resulting in aperture efficiency of 52%.
Measurement techniques for cryogenic KA-band microstrip antennas
M.A. Richard (Case Western Reserve University),K.B. Bhasin (NASA Lewis Research Center) C. Gilbert (Ball Communications Systems Division) S. Metzler (Ball Communications Systems Division) P.C. Claspy (Case Western Reserve University), November 1991
The measurement of cryogenic antennas poses unique logistical problems since the antenna under test must be embedding in the cooling chamber. In this paper, a method of measuring the performance o cryogenic microstrip antennas using the closed cycle gas-cooled refrigerator in a far field range is described. Antenna patterns showing the performance of gold and superconducting Ka-band microstrip antennas at various temperatures are presented.
Ship mounted antenna measurements using GPS
Millington. T.A. (Southwest Research Institute),J.H. Nixon (Southwest Research Institute), R.W. Robinson (Southwest Research Institute), November 1991
Antenna amplitude and phase pattern measurements on combat ships and other large ships have typically relied on traditional methods which include circling a fixed buoy in the far field, tracking a shore-based transmitter with an optical device, or circling the subject ship with a smaller boat outfitted with a transmitter. These techniques required the measurement of many independent variables using less than precise methods to compute antenna patterns relative to the ship’s structure. Using the global positioning system to precisely locate the ship relative to the transmitter site location and combining this with the ship’s heading, antenna measurements can be accurately and quickly obtained. This paper will describe the traditional fixed buoy and optical follower techniques and contrast these against the more accurate and faster GPS antenna measurement technique.
Antenna measurements for advanced T/R module arrays
J.S. DeRosa (Rome Laboratory), November 1991
Advanced airborne radar antennas will consist of ultra low sidelobe arrays of thousands of T/R modules and radiating elements. The detrimental effects of the aircraft structure on the antenna performance becomes increasingly important for ultra low sidelobe antennas will require large aperture, high fidelity antenna test facilities. In this paper, the major errors associated with measurement of an ultra low sidelobe antenna on the far field range are isolated and demonstrated by computer simulation. Data from measurements of a T/R module array on a scale model aircraft is provided to demonstrate typical sircraft effects on antenna performance.
Compact range bistatic scattering measurements
E. Walton (The Ohio State University ElectroScience Laboratory),S. Tuhela-Reuning (The Ohio State University ElectroScience Laboratory), November 1991
This paper will show that it is possible to make bistatic measurements in a compact range environments using near field scanning. A test scanner is designed and operated. Criteria for the accuracy of positioning and repositioning are presented. Algorithms for the transformation of the raw data into bistatic far field calibrated RCS are presented. Examples will be presented where comparisons with theoretical bistatic sphere data are shown. Bistatc pedestal interaction terms will be demonstrated.
On the errors involved in a free space RAM reflectivity measurement
F.C. Smith (University of Sheffield),B. Chambers (University of Sheffield), J.C. Bennett (University of Sheffield), November 1991
Edge and corner diffraction and non-planewave illumination both cause measured free space relativity data to deviate from the infinite sample/planewave result which is predicted when using the Transmission Line Methos (TLM) for planar surfaces. The amount by which each of the two factors perturbs the measured data depends on the measurement system used; compact ranges, near field focused antennas and far field antennas on an NRL arch are all susceptible to the effects of non-planewave illumination and perimeter diffraction. Perimeter diffraction is virtually eliminated in the case of a near field focused system or where the sample is semi-infinite; however, the truncated illumination inevitable yields additional angular planewave components. In a far field system, the quadratic phase variation at the sample surface is shown to cause significant errors in the depth of resonant nulls. A uniform illumination is required to accurately map the depth of resonant nulls, but the consequent perimeter diffraction causes errors in null position. Perimeter diffraction does not cause errors in the null depth providing the illumination in uniform.
Error budget performance analysis for compact radar range
M. Arm (Riverside Research Institute),L. Wolk (Riverside Research Institute), R. Reichmeider (Riverside Research Institute), November 1991
The target designer using a compact range to verify the predicted RCS of his target needs to know what measurement errors are introduced by the range. The underlying definition of RCS assumes that the target is in the far-field, in free-space, and illuminated by a plane wave. This condition is approximated in a compact range. However, to the extent that these conditions are not met, the RCS measurement is in error. This paper, using the results of the preceding companion paper1, formulates an error budget which shows the typical sources that contribute to the RCS measurement error in a compact range. The error sources are separated into two categories, according to whether they depend on the target or not. Receiver noise is an example of a target independent error source, as are calibration errors, feed reverberation (“ringdown”), target support scattering and chamber clutter which arrives within the target range gate. The target dependent error sources include quiet zone ripple, cross polarization components, and multipath which correspond to reflections of stray non-collimated energy from the target which arrives at the receiver at the same time as the desired target return. These error contributors depend on the manner in which the target interacts with the total quiet zone-field, and the bistatic RCS which the target may present to any off-axis illumination. Results presented in this paper are based on the design of a small compact range which is under construction at RRI. The results include a comprehensive error budget and an assessment of the range performance.
Range field compensation
D.N. Black (Georgia Institute of Technology),E.B. Joy (Georgia Institute of Technology), M.G. Guler (Georgia Institute of Technology), R.E. Wilson (Georgia Institute of Technology), November 1991
The accuracy of antenna measurements can be improved by compensating for the effects of extraneous fields present in an antenna range using analytical compensation techniques. Range field compensation is a new technique to provide increased measurement accuracy by compensating for extraneous fields created by refection and scattering of the range antenna field from fixed objects in the range and by leakage of the range antenna RF system from a fixed location in the range. The range antenna field must be the dominant field in the range, and the range field cannot change for different AUTs. Existing compensation techniques are limited in the amount of compensation they can provide. The range field is measured over a spherical surface encompassing the test zone using a low gain probe. The measured range field is used in subsequent antenna measurements to compensate for the effects of extraneous fields. This technique is demonstrated using measurements simulated for an anechoic chamber far-field range.
Measurement receiver error analysis for rapidly varying input signals
O.M. Caldwell (Scientific-Atlanta Inc.), November 1991
An assessment of instrumentation error sources and their respective contributions to overall accuracy is essential for optimizing an electromagnetic field measurement system. This study quantifies the effects of measurement receiver signal processing and the relationship to its transient response when performing measurements on rapidly varying input signals. These signals can be encountered from electronically steered phased arrays, from switched front end receive RF multiplexers, from rapid mechanical scanning, or from dual polarization switched source antennas. Numerical error models are presented with examples of accuracy degradation versus input signal dynamics and the type of receiver IF processing system that is used. Simulations of far field data show the effects on amplitude patterns for differing rate of change input conditions. Criteria are suggested which can establish a figure of merit for receivers measuring input signals with large time rates of change.
Calibration of large antenna measured in small quiet zone area
D-C. Chang (Chung Shan Institute of Science and Technology),M.R. Ho (Chung Shan Institute of Science and Technology), November 1991
Compact range systems have been widely used for antenna measurements. However, the amplitude taper can lead to significant measurement errors especially as the dimension of antenna is larger than quiet zone area. An amplitude taper removing technique by software implement is presented for compact range system. A 12 feet by 1.0 feet S-band rectangular slot array antenna is measured in SA5751 compact range system, which provides a quiet zone area with a 4 feet diameter. Results of corrected far-field patterns from compact range are compared with that taken by planar near-field range.
The Application of a small compact range to the testing of millimeter antennas
J.D. Huff (Scientific-Atlanta, Inc.),D.W. Hess (Scientific-Atlanta, Inc.), November 1991
Since the first commercial compact range was introduced by Scientific-Atlanta in 1973, the compact range has become a very popular alternative to far-field ranges. In recent years larger and larger compact ranges have been built, increasing the size of antennas that may be tested and lowering the operating frequency. However little has been done in the other direction, to increase the operational frequency and to decrease the size of the compact range. This paper reports on the design and fabrication of a small compact range having a 1 foot test zone and operating at 95 GHz.
Quiet zone scan of the single-plane collimating range
C.R. Birtcher (Arizona State University),C.A. Balanis (Arizona State University), V.J. Vokurka (Eindhoven University), November 1991
The prototype of the March Microwave Single-Plane Collimating Range (SPCR) has been in operation at Arizona State University’s ElectroMagnetic Anechoic Chamber (EMAC) facility for approximately three years. The unique SPCR produces a cylindrical-wave test region by bouncing spherical wavefronts off a parabolic cylindrical reflector. Consequently, a simplified algorithm can be applied to determine antenna far-field patterns. Both computation and acquisition times can be reduced considerably when compared to classical NF/FF cylindrical scanning techniques. To date, this is the only SPCR in operation. Some of the fundamental quantities which characterize an antenna/RCS measurement range are the size and quality of the “quiet zone”, usually expressed in terms of ripple and taper of the illuminating fields relative to an ideal planar wavefront. Direct one-way probing of the quiet zone fields in the vertical and horizontal planes has been recently completed at ASU. An overview of the range geometry, the field probing methodology, and the data processing will be presented. The results of the quiet zone scan will be presented as amplitude ripple, amplitude taper, and phase ripple versus frequency from 4 GHz to 18 GHz in four bands. The vertical-scan phase deviations are relative to an ideal planar wavefront, while those of the horizontal scan are relative to an ideal cylindrical wavefront.
Near-field measurement experience at Scientific-Atlanta
D.W. Hess (Scientific-Atlanta, Inc.), November 1991
The experience with near-field scanning at Scientific-Atlanta began with a system based upon a analog computer for computing the two-dimensional Fourier transform of the main polarization component. When coupled with a phase/amplitude receiver and a modest planar near-field scanner this system could produce far-field patterns from near-field scanning measurements. In the 1970’s it came to be recognized that the same advances, which made the more sophisticated probe-corrected planar near field measurements possible, would enable conventional far-field range hardware to be used on near-field ranges employing spherical coordinates. In 1980 Scientific-Atlanta first introduced a spherical near-field scanning system based upon a minicomputer already used to automate data acquisition and display. In 1990, to meet the need of measuring complex multistate phased-array antennas, Scientific Atlanta began planning a system to support the high volume data requirement and high speed measurement need represented by this challenge. Today Scientific-Atlanta is again pursuing planar near-field scanning as the method of choice for this test problem.
Probe correction coefficients derived from near-field measurements
G. Masters (Nearfield Systems Incorporated), November 1991
Probe correction is necessary in near-field measurements to compensate for non-ideal probes. Probe compensation requires that the probe’s far-field pattern be known. In many cases direct far-field measurements are undesirable, wither because they require dismantling the probe from te near-field range set-up or because a far-field range is not available. This paper presents a unique methos of deriving probe-correction coefficients by measuring a probe on a near-field range with an “identical” probe and taking the square root of the transformed far field. This technique, known as the “robe-square-root” method can be thought of as self-compensation. Far-field comparisons are given to show that this technique is accurate.
Probe correction coefficients derived from near-field measurements
G. Masters (Nearfield Systems Incorporated), November 1991
Probe correction is necessary in near-field measurements to compensate for non-ideal probes. Probe compensation requires that the probe’s far-field pattern be known. In many cases direct far-field measurements are undesirable, wither because they require dismantling the probe from te near-field range set-up or because a far-field range is not available. This paper presents a unique methos of deriving probe-correction coefficients by measuring a probe on a near-field range with an “identical” probe and taking the square root of the transformed far field. This technique, known as the “robe-square-root” method can be thought of as self-compensation. Far-field comparisons are given to show that this technique is accurate.


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