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This paper will discuss the need for performance verification, or calibration, of the transmitter and receiver systems used in an antenna or RCS range. Errors introduced by the range and positioning system means the instrumentation’s performance must be measured independently of the range and positioner. The performance verification should insure that the measurement system exceeds the manufactures’ specifications by a reasonable margin. The verification must be performed with the equipment installed on the range to insure adequate performance on the range. The system must als be verified as a system, rather than individual instruments. This guarantees that measurement errors in each instrument will not add together to exceed the system’s specifications. Testing of the system should be easy and repeatable to insure accuracy of the verification by the test technician. The tests should also be documented for later reference. The measurements should be traceable to a local standard such as NIST to certify the accuracy and stability of the measurement. The verification should be repeated on a regular basis to insure continued accuracy of the measurement system.
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.
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.
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.
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.
A procedure for the calculation of the effect of backwall reflections in a compact antenna test range (CATR) is shown. In the calculation, the magnitude of the backwall reflections known from previous work and the angular spectrum of the reflections measured in this work are used. The angular spectrum of the scattered reflections from the back part of an anechoic chamber was found to be very wide (>±45º), and the effect of backwall reflections was found insignificant, especially in a CATR with a long effective focal length.
It has been recently shown that an optimized blended rolled-edge compact range reflector can be successfully used to measure two or three foot targets at microwave and millimeter frequencies. In addition to the reflector design, one is faced with many other practical range design issues, such as absorber treatment, target mount and access, feed mount and access, etc. Each of these design aspects has been evaluated and an actual range has been constructed to illustrate the capability of such a system. The feed is mounted on a rotating side door for easy access. The target zone is approached from the rear of the chamber by rotating the backwall. These design concepts allow the range operator to quickly modify the measurement setup, yet still maintain extremely stable results. The simplicity of this design as well as its excellent measurement capability are presented.
This paper describes a radome measurement system based on a novel measuring technique, which quickly and accurately measures the electrical boresight shift, the boresight shift slope and the radome loss caused by the presence of a radome in front of a monopulse antenna. The system was required to measure the radome parameters at a spatial sector of 120º in front of the antenna. The conventional methods of radome characterization were useless for this range of relative angles (mechanically impossible). The unique radome measuring technique that was implemented is described, and the actual performance of the system as evaluated in the acceptance test procedures is presented, together with sample results.
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.
A performance simulation for analyzing the measurements of target RCS in a compact radar range has been applied to a small indoor range which will be installed at RRI. A dual reflector collimator has been examined with respect to both quiet-zone quality and the amount of stray energy in the chamber which eventually end up as clutter or multipath interference. The complicated ray geometries, beyond the reach of hand calculation, are discovered by complete tracing of all the rays from the feed source. The ray pats which interfere with target measurements are shown convincingly by graphical display. Vector clutter subtraction is widely used in compact ranges in order to reduce the background clutter to an acceptable level. Some of the effects which limit the effectiveness of clutter subtraction are also addressed in the paper. The sources of measurement errors which are obtained by this simulation are used in the measurement-error budget analysis, which is the subject of the follow-on paper.
A new radar cross-section (RCS) measurement facility has been designed and built at the Houston Advanced Research Center in Houston, Texas. This facility is capable of performing fully polarimetric RCS measurements over a frequency bandwidth of 2-40 GHz ad nearly an entire hemisphere of bistatic angles. What makes this facility unique is the fact that both the transmit and receive antennas are mounted on moveable platforms. The transmit antenna is fixed at 0º azimuth, but can be positioned anywhere from 10º to approximately 165º in elevation. The receive antenna can be positioned anywhere from 0º to 180º in azimuth and the same range in elevation as the transmit antenna. Monostatic measurements can be approximated by moving the transmit and receive antennas close together. The radar equipment is built around the HP 8510 vector network analyzer, and the measurement process is controlled and automated by an HP UNIX workstation running HP’s Visual Engineering Environment software.
The RAFAEL general purpose radome measurement range has been modernized and refurbished, maintaining its capability to accommodate all range of radome sizes up to 1.2 meters in diameter. It is based on a 3-axis positioner placed in an open anechoic chamber with a null seeker placed 20 meters away and about 10 meters above the ground. All the positioner’s axes are controlled by an automatic positioner controller. The receiver and source are based on a HP-8510B system. The X-Y null seeker serves for boresight error measurements. It has a 0.7m x 0.7m total motion span, which is about 2º. It is controlled by a dual-motor controller, so that the scanning antenna can be moved in any kind of motion. Instrumentation control and data acquisition and analysis is performed using a HP-330 UNIX controller. Present software handles monopulse antennas with or without a comparator, and can implement the comparator in software. There are two major measurement modes: One for BSE measurements and the other for radiation patterns.
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.
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.
This paper reports on a spherical near-field measurement technique currently being researched at Georgia Tech. Spherical Near-Field Microwave Holography (SNFMH) has been successfully used to locate defects in radome walls. Defects with size greater than or equal to .5 free space wavelengths (?0) in diameter were detected [1]. Present efforts have established theoretical resolution limits for the SNFMH process. The SNFMH technique is used to process spherical near-field measurements of a (.23 ?0 x .12 ?0) open-ended waveguide probe. The probe consists of a section of WR92 waveguide, tapered down to one fourth of its standard dimensions. The results are compared to the theoretical mode-limited impulse response. The SNFMH technique is also used to process spherical near-field measurements of a plastic, 8 ?0 radius, hemisphere radome with various plastic defects attached. Results of a case with two .5 ?0 diameter plastic defects are compared to the theoretical mode-limited pulse response. Contour plots of uniquely shaped defects are shown.
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.
Portable near-field measurement systems can provide significant flexibility to both large companies seeking to increase their antenna test capabilities, and small companies looking for their first investment in a test range. There are many unique applications for portable near-field antenna measurement systems in addition to their use for standard antenna performance measurements. Some additional applications include flight-line testing, anechoic chamber quiet zone imaging, and EMI testing. Many of NSI’s near-field systems have been portable designs, capable of being set up in a small lab or office and easily relocated. Key features required for use of a portable system are rapid setup, simplicity of use, low cost, and accuracy. This paper will be focused on practical experience with installing, calibrating, and operating portable near-field measurement systems. It will also cover tradeoffs in their design, and usage in a variety of applications.
A recently developed analytic technique that can correct for probe position errors in planar near-field measurements to arbitrary accuracy [1] is shown to be also applicable to spherical near-field data after appropriate modifications. The method has been used to successfully remove errors in the near-field, hence leading to more accurate far-field patterns, even if the maximum error in the probe’s position is as large as 0.2?. Only the error-contaminated near-field measurements and an accurate probe position error function are needed to be able to implement the correction technique. It is assumed that the probe position error function is a characteristic of the near-field range, and that it has been obtained using state-of-the-art laser positioning and precision optical systems. The method also requires the ability to obtain derivatives of the error contaminated near-field defined on an error-free regular grid with respect to the coordinates. In planar geometry the derivatives are obtained using FFTs [1], and, in spherical geometry, one needs to compute derivatives of Hankel functions for radical errors, and derivatives of the spherical electric and magnetic vector basis functions for errors in the ? and Ø coordinates. The error-correction technique has been shown to work well for errors in and of the spherical coordinates r, ? or Ø. Efficient computer codes have been developed to demonstrate the technique using computer simulations.
This paper describes measurement system performance parameters that were considered during the design phase of a planar near-field measurement range for Spar Aerospace Limited. All aspects of the planar near-field measurement system are addressed. These include; instrument selection, scanner interface hardware, system controller/computer hardware, software for data collection, near-field to far-field transformation, data analysis, networking and system configuration. The Scientific-Atlanta Model 2095 Microwave Measurement System with its near-field options is used as the basis for meeting the Spar requirement. The various data collection parameters of the Model 2095 are described with special emphasis on how the factors relate to near-field requirements such as fixed grid sampling. Examples of typical test scenarios are presented as an aid in exploring detailed data collection system timing.
A novel bi-polar planar near-field measurement range is described. This range is mechanically simple and has a reduced implementation cost compared to other planar techniques. The particular physical implementation and comparison with the plane-polar range is presented. Development aspects of the customized bi-polar range at UCLA are summarized. An optimal near-field interpolation is used to enable the near-field to far-field (NF-FF) processing via fast Fourier transform (FFT). Computer simulated near-field and far-field results are given.