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This paper examines the possibility of increasing the speed of Near-Field measurement of an Antenna, by reducing the number of measurement points and by determining the degree of truncation permissible while maintaining a prescribed degree of precision of the reconstructed far-field. The Near-Field of a planar radiating array is analysed in depth. A formulation and a procedure to correct the spectral domain of the field are established. It is shown that correction in the spectral domain can improve the accuracy of the Far-Field while using the same amount of Near-Field data. The technique has a good potential to be applied to Near Field data of large radiating Antennas leading to new information about the accuracy and speed of measurement achievable.
Portable scanners used for near-field antenna measurements are usually incapable of providing a large scan area with a high degree of probe position accuracy. This paper discusses a 4.5m x 2.0m portable scanner developed by NSI with a probe position accuracy on the order of 2 mils (0.050 mm) rms. An NSI patented optical measurement system measures the X, Y, and Z position, and provides real time position correction capability. This lightweight, portable scanner combined with optical correction provides enhanced accuracy while reducing overall antenna measurement system costs and improving test chamber flexibility.
An analytic technique recently developed at NIST [1] [2] to correct for probe position errors in planar near-field measurements has been implemented to arbitrary accuracy. The nth-order correction scheme is composed of an mth-order ordered expansion and an n - m higher-order approximation, where both n and m are arbitrary. The technique successfully removes very large probe position errors in the near-field, so the residual near-field probe position errors are substantially below levels that can be measured on a near-field range. Only the error-contaminated near-field measurements and an accurate probe position error function are needed for implementation of the correction technique. 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 coordi nates. In planar geometry the derivatives are obtained using FFTs [1], giving an approximate operation count of (3 • 2=- 1 - 1 + (n - m)) N log N, where N is the number of data points. Efficient computer codes have been developed to demonstrate the technique. The results of simulations are more accurate than those obtained us ing the well-known k correction [3), which can correct for position errors in some direction in k space, but further contaminates the sidelobe levels.
Hologram measurements are becoming more and more popular as a reliable method for identifying bad elements and the tuning of active phased array antennas. Relying on holographic data to adjust phase shifters and attenuators in these antennas can give undesired results if the accuracy of the data is poor. Often measurements can be improved if the error sources can be isolated and quantified. This paper presents an approach to producing a hologram accuracy budget based on the NIST 18-term error budget created for near-field measurements. A set of hologram accuracy terms is identified and data is presented showing the typical hologram accuracy that can be expected from a near-field scanner.
Modern low profile and conformal antennas are fre quently evaluated in the presence of a conducting surface. Antenna designers usually predict the an tenna scattering and radiation performance over an infinitely conducting ground plane. To bridge the gap between a (possibly curved) antenna host surface and the designer's infinite ground plane model, an antenna testbody is required. This testbody must possess a variety of demanding attributes, such as a very close approximation to an infinite ground plane, low testbody signature, ability to provide positional accuracy (in both azimuth and elevation), physical stability (for repeatability and background subtraction), just to name a few. The most widely regarded testbody has been the "almond" testbody [1, 2], which boasts a very low signature, and excellent fidelity when compared to an infinite ground plane. This paper addresses a new variation from the traditional "almond" testbody, in which a unique positioning design provides a phase-stationary antenna aperture center under rotation of both azimuth and elevation. This testbody will be used for a variety of antenna tests at Wright Laboratory's Radiation and Scattering Compact Antenna Laboratory (RASCAL).
Evaluation of serrated edge and blended rolled edge compact range reflectors is presented in this paper. An interactive approach is used to design the serrated reflectors. Several issues associated with the serrated reflectors are also discussed in this paper. Quiet zone fields for various serrated edge with an optimally designed blended rolled reflector are presented for comparison. In addition, simulations of a low sidelobe phased array measurement using serrated and blended rolled edge reflectors are shown to investigate their impact on the measurement accuracy.
A novel computer simulation methodology to properly characterize the role of probe directivity/pattern compensation in cylindrical near field scanning geometry is presented. The methodology is applied to a linear test array antenna and the JPIJNASA scatterometer (NSCA1) radar antenna. In addition, error analysis techniques of computer simulation and measured have been developed to determine the achievable accuracy in pattern measurements of the NSCAT antenna in cylindrical near field.
Large scanners used for near-field antenna measurements require careful attention to the design and fabrication process to maintain probe position accuracy. This paper discusses the design, implementation, and results of a novel optical probe position tracking system used by NSI on a number of large near-field scanners. This system provides measurement of the probe X, Y and Z position errors, and real-time on-the-fly position correction. The use of this correction can significantly enhance measurement accuracy, and can reduce the cost of building large near-field scanners.
The Compensated Compact Range (CCR) has been proven to be a high performance test facility for payload and large satellite antenna measurements. To efficiently measure complete antenna farms with various types of antennas on the spacecraft in the same test campaign led to the growing demand for testing e.g. Global Horn antennas on the spacecraft in the CCR. As a matter of fact, medium gain antennas feature a small aperture simultaneously requiring larger test angles. Therefor, main interferer like "direct leakage" between the CCR feed and the antenna under test have to be quantified and their impact on the measurement accuracy have to be reconsidered. The presented paper will investigate theoretically the feasibility of testing high performance widebeam antennas in the Top-Fed-Cassegrain double reflector system. Qualified measurement results of INTELSAT Global Horn Antennas featuring medium gain and extreme crosspolarization purity will be presented.
Test zone field (TZF) compensation increases antenna pattern measurement accuracy by compensating for non-plane wave TZFs. The TZF is measured over a spherical surface encompassing the test zone using a low gain probe. The measured TZF is used in the compensation of subsequent pattern measurements. TZF compensation is demonstrated using measurements taken in an anechoic chamber, far-field range. Extraneous fields produced by reflection and scattering of the range antenna field in the chamber causes the TZF to be non-planar. The effect of these extraneous fields on pattern measurements is shown. Measured TZFs are also shown. TZF compensation results for pattern measurements using a high-gain, X-band slotted waveguide array are presented.
The Three-Antenna gain method is commonly used on far-field ranges to determine an antenna's absolute gain. This is especially true when no other calibrated antenna is available. This method has been used for years by calibration laboratories such as NIST to calibrate probes and gain standards for far and near-field ranges. In some cases, the calibration is too costly or does not meet the schedule requirements of the near-field test range. An alternative is to calibrate the probe or gain standards directly on the near-field range. In this paper we present the results of a study done to show the accuracy of the Three-Antenna gain method when used on a near-field range. An extensive error analysis is presented validating the utility of this method.
The advantages of mesh antennas include good storability and low mass for large on-board antennas over 10M in diameter. Their weak point is that surface adjustment is necessary to attain high accurate surface. Surface adjustment traditionally involves the repeated measurement of surface node position with a theodolite system and subsequent cable adjustment. These steps take much time. This paper describes a surface adjustment scheme that uses near field measurement for a modular mesh antenna composed of mesh, cable network and supporting structure. The node positions of the antenna are obtained by back projection of the far field pattern generated from the near field pattern. The cable network has low sensitivity to changes in local node position. The results of tests show that the surface accuracy needed to achieve the required RF performance can be obtained quickly without theodolite systems.
This paper describes a 550 GHz planar near-field measurement system developed for flight qualification of the radio telescope carried onboard the NASA submillimeter wave astronomy satellite (SWAS). The very high operating frequency required a new look at many near-field measurement issues. For example, the short wavelength mandated a very high precision scanner mechanism with the accuracy of a few microns. A new thermal compensation technique was developed to minimize errors caused by thermally induced motion between the scanner and spacecraft antenna.
As mobile and satellite phased array antennas move from to concept production the demands on test station throughput increases dramatically. Completely characterizing a Transmit/Receive (TIR) module may require thousands of S-parameter measurements under CW and high-power pulsed conditions, as well as, harmonics, spurious, and noise figure measurements. The measurement throughput of instrumentation used in characterizing the prototype TIR modules simply may not be capable of handling the added volume of a production environment. The volume of measurements, the multiport nature of the device, and the integrated TIR module control make it necessary to reexamine the traditional approaches of separate network analyzers, noise figure meters, and spectrum analyzers. The result is a high-speed modular test ystem that completely characterizes the device in a single connection. The system contains a single receiver and a dedicated controller that utilizes the instrumentation in the most efficient method while maintaining or increasing the accuracy of traditional approaches. This paper describes the high-speed test stations that have been designed and built and are currently in use in several production facilities. Test system architecture is discussed and measurement throughput numbers are given and compared to conventional approaches.
Present day accuracy requirements on high-performance antenna measurements are difficult to meet on any type of compact range. Numerical correction techniques can offer a good solution. An easy and effective method is the Advanced APC-technique. This method requires patterns to be measured on different locations in the test zone so that disturbances of the plane wave can be distinguished. In case of suitable distances, the "true" pattern can be derived from measured amplitude and phase data. Usually, scanning is performed in longitudinal direction. The advantage is that mutual coupling can be distinguished well, but the field ripple in this direction due to extraneous fields varies much slower than in transversal direction. Consequently, first sidelobes can be corrected more efficiently when transversal scanning is performed. Therefore, in this paper a new and flexible way of positioning is proposed depending on the location of extraneous field sources.
The Sacramento Air Logistics Center at McClellan AFB has developed near-field (NF) antemia test capability over the past three years. With assistance from the National Institute of Standards and Teclmology (NIST), McClellan has assembled a modem planar near-field antenna range using components from various vendors. Although the LH( division of McClellan AFB) team's current range has been operating for over a year, it is being continuously improved for measurement accuracy, user-friendliness, and safety. This paper will briefly discuss the evolution of McClellan's near-field program, and then focus on the building of the LH near-field antenna range. Radio-frequency (RF) issues, such as RF design and electromagnetic shielding will be covered. Precision measurement teclmiques such as positioning accuracy and temperature control are discussed. Finally, relevant safety and constrnction issues affecting the McClellan facility will be examined.
One of the world's most sophisticated antenna test ranges is now fully operational. This was designed by the Deutsche Aerospace (DASA) and is operated by Siemens Plessey Systems (SPS). The presented paper will describe the pioneering design philosophy adopted to ensure the stringent performance features. Although this facility is located outside, it allows extremely high precision probing of cylindrical near field of large and very complex antenna systems, with turning diameters up to 16 meters and up to 20 GHz. Besides the RCS optimized 36 m large scanner tower the significant highlights of this facility consist of a comprehensive air-conditioning system for all accuracy dependent components, a permanent autoalignment system, which ensures high precision cylindrical measurements and an interleaved high speed data collection system, which delivers a maximum of data performance within a minimum time frame. Test results including a pattern comparison of the Ref erence Antenna between measurements in DASA facilities and the SPS Cylindrical Near-Field Test Facility show good range performance. The evaluation of the range performance data demonstrates the measurement integrity of the facility and proves to be qualified to characterize a wide range of antennas.
New results on very precise, computer controlled manipulation of string supported targets suspended from an upper turntable (UTT) in the Boeing 9-77 compact radar range are presented. A computer program was developed that uses the precision optical measurement system (POMS) information for feedback to automatically control the conic_pitch and conic_roll of arbitrary radar targets to within ± 0.066° (RMS) of the desired pitch and roll. The system provides quick and accurate maneuvering of targets to any desired static position with accuracy in the static yaw, pitch, and roll of ± 0.01°. Automatic volumetric field probes are also possible using a sphere suspended from computer operated strings. Sphere movement can be continuous or stepped along any desired path and is controlled to within ±0.05 inches anywhere within the quiet zone (± 14 ft high by ± 20 ft wide by ± 25 ft).
Applications that require tight tolerances on dielectric thickness control need accurate sensors. A technique has been developed that will allow for the measurement of thickness without requiring surface contact. High resolution radar imaging, commonly used in RCS measurements , is now being used to measure thickness. Electromagnetic fields reflected from the front and rear surface are detected and the time response delta is converted into thickness. A major advantage of this method is that it is not affected by varying sensor offset height.
This presentation offers some examples of performance in accomplishing high volume testing under the rigorous technical constraints imposed by the satellite industry. As an example of a high speed system, the Scientific-Atlanta Model 2095 will be used to illustrate the capability offered by today's technology. This system has found applicatio0n in the facilities of five satellite manufacturers constructed within the past three years and is proven by its demonstrated application in satellite programs.