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Compact Antenna Test Ranges are eminently suitable for obtaining the far-field patterns of various types of antennas provided that the frequency is not too low. Typically, a low frequency limit of 1 or 2 GHz is realizable. There are, however a number of important applications between 500 and 1000 MHz for antenna diameters between 1 and 3 meters. The far field distance R = 2D2/l is just too large for an indoor far field range. It is generally accepted at present that a good solution for an indoor range for these kind of measurements is very difficult to realize. In this paper the low frequency performance of single and dual-reflector Compact Antenna Test Ranges will be investigated. It will be shown that with carefully designed serrations and feeds, excellent antenna measurements can be carried out at frequencies below 1.0 GHz for a large number of applications. For purposes of comparison, low frequency performance of a compact range with so called blended rolled edges will be presented as well.
The objective of this study is to develop a new techniq ue to compensate the instrumentation errors of an antenna near-field test range. The methodology presented demonstrates that it is feasible to calculate the far-field radiation from near-field measurement with one deconvolution that will include all the errors introduced by the instrmentation. Measrements were performed on a standard gain horn as a reference and the analysis includes a theoretical comparison with a computer model of the standard gain horn, simulated using WIPL. Furthermore, four scenarios of error in the system flatness were analyzed, to verify that the technique is capable of correcting planarity errors.
Proper operation of a planar NFR (near field range) includes probe correction as part of the processing of the measured data to result in accurate far field angle patterns, particularly for low cross polarized patterns. The far field transform of the near field data produces the angular spectrum which is the product of the plane wave transmission coefficient pattern of the AUT (antenna under test) with the plane wave receiving coefficient pattern of the probe. Probe correction consists of dividing the angular spectrum by the complex probe angle pattern resulting in the pure far field pattern of the AUT [1]. For best accuracy of co and cross polarized AUT patterns one needs to use accurately measured probe complex co and cross polarized patterns in probe correction for each NFR test frequency. The most accurate probe measurements are usually obtained from specialized test laboratories. However, if the number of frequencies is large, this may create problems due to cost or schedule. Because of this it is typical to procure probe calibration at only a few frequencies spanning the test band for each AUT even though pattern measurements are needed at several additional frequencies falling between the calibration frequencies. A typical strategy at any given test frequency is to perform probe correction using the nearest-neighbor-frequency probe calibration data. This strategy produces some unknown error in the processed probe corrected far field patterns of the AUT at each non-calibrated frequency. Inthis paper we will show a method for estimating the non-calibrated frequency probe correction error for co and cross polarized patterns with examples.
A bias error in planar near-field measurement data comes from receiver crosstalk or leakage effects [1, 2, 3]. The bias error is a complex constant added to every near-field data sample. After transformation from the near-field to the far-field, the bias error becomes an easily identifiable spike located at the center of k-space. If one is measuring a horn, then the bias error produces a small bump or spike at the center of the far-zone pattern (i.e. at the center of k-space). If one is measuring a highgain antenna with the antenna beam pointed away from the center of k-space, then the bias error causes an erroneous sidelobe spike at the center of k-space. The bias constant is difficult to estimate be cause it may be more than 60 dB below the peak near-field level. Nevertheless, if the effect of the bias error can be seen in the far zone pattern of the test antenna, then it can be estimated and removed from the measured data. An algorithm is presented that is used to estimate the bias constant directly from the near-field data, then the bias constant is simply subtracted from the data. Examples using measured data are used to illustrate how the algorithm works and to show its effectiveness.
Papers were presented at the last two AMTA meetings reporting on the effect of rotator system alignment on the results of spherical near-field measurements. When quantifying the effect of non-intersection errors on the AUT directivity, these two papers presented very different results. One AMTA paper 1 and an earlier study at The Technical University of Denmark 2 found that the directivity error was extremely sensitive to non-intersection errors while the other AMTA paper3 found a very small sensitivity. During the past year, scientists at the Technical University of Denmark, The National Institute of Standards and Technology, and Nearfield Systems Inc. have been working together to determine the reasons for these differences. It now appears that the major reason for the difference is due to the method used to acquire data on the sphere. Theta scans that pass through the pole, or equivalently, phi spans of 180 degrees, produce both plus and minus phase errors that tend to cancel in the on-axis direction. Theta scans that do not pass through the pole, or equivalently phi spans of 360 degrees, produce phase errors of the same sign over the sphere which are concentrated in the on-axis direction. Examples of measurements and recommendations for using this information in spherical measurements will be presented.
The NIST 18 term error budget is used to estimate the magnitude of each individual source of error and then combine them to the total uncertainty for the planar nearlield range designed for antenna characterization of the ERIEYE Airborne Early Warning System. The ERIEYE AEW System consists of two large phased array antennas, one at each side of the Dorsal Unit which is located on the top of the airplane fuselage. T/R-modules are connected to the antenna waveguides to control the beamsteering and the very low sidelobe level. The sidelobe level is supervised by a calibration during operation, using a table of calibration data. The table of calibration data is produced by iterative computer runs of programs performing the two transformations Near-field-to-Far-field and Far-field-to-Waveguide Excitation - the characterization. Characterization to very low sidelobe level in the calculated farfield is possible when using for instance planar nearfield technique to measure an active antenna. The errors at the planar nearfield range are misleadingly compensated for by the characterization. Therefore a minimization together with a continuous control of the noise level is necessary.
Traditional measurement methods assume that very accurate antenna to range alignment of the antenna under test (AUT) is convenient or possible. It has recently been shown that the use of non-rectilinear co-ordinate systems are of particular use for the purpose of correcting antenna to range misalignment. Additionally, this misalignment correction can be used to construct an extended composite measurement plane from a series of mis-aligned scans that themselves can be considered as constituting a polyhedral measurement surface. This paper describes the additional processing that is required to yield corrected near and far field data from an acquisition of a mis-aligned AUT. This technique is then illustrated with example results. The agreement of the corrected results is determined via the application of image classification techniques which correlate antenna patterns in a reduced vector pattern space in terms of their overall global features.
The theoretical study of scattering by various objects inside a circular hollow dielectric waveguide (HDW) is important to analyze the overall accuracy of the method in which this guiding structure plays a role of the main component of a micro-compact compact range. Here, we propose an theoretical approach to the solution of the problem of electromagnetic scattering from a spherical object inside a circular HDW based on the well-known method of separation of variables and the concept of recursive T-Matrix algorithm. Owing to the approach, we studied electromagnetic properties of a spherical scatterer inside a circular HDW as well as obtained basis to develop an approach for calculations of scattering by objects of other shapes. The results calculated for metallic spherical scatterers inside circular HDW were compared with corresponding measurements data of backward and forward scattering characteristics at 4-mm wave band.
A helicopter-based radar cross section (RCS) measurement system was designed and demonstrated during the past year. The system was a novel combination of modified and un-modified commercial off the shelf (COTS) equipment and software, a minor amount of new hardware, and extensive prior experience. Validation was accomplished using known calibration standards and existing test practices relevant to this type of system, and data were collected and processed for a number of targets of opportunity. The primary subsystems include the measurement radar, the helicopter, antennas and associated mount, boresighted video and recorder, and the calibration tools. The SCI1000 radar was employed because of the combination of its excellent performance at the desired test target range and its minimal physical and power demands. The Bell 500 helicopter was chosen for its size and its wide availability on the world market. Data products were RCS vs. aspect, downrange profile history, and two-dimensional imaging following pre-processing by a robust motion compensation algorithm.
RCS measurements at in service aircraft often require fast RCS - analysis capabilities. DaimlerChrysler Aerospace therefore extended its RaSigma facilities with a turntable and elevation system especially designed for RCS measurements at aircrafts. The designer and supplier of the turntable and elevation system was the German company HD GmbH. Aircraft with a maximum weight of 75 t can be raised to a height of approximately 13 m. The aircraft is supported by three girders at its landing gears or other hard points. The test range ist 300m long (today) and can increase up to 3000m . RCS measurement are performed in the gated CW mode. The RaSigma outdoor range operates in elevation range mode, with a special antenna design for a homogeneous field distribution over object height and frequency.
Compact range collimating reflectors provide far-field conditions for radar signature measurements. Traditionally, the quiet zone is presented uniformly about the collimator boresight and depends upon both the size of the reflector and the beamwidth of the illuminating antenna, with a maximum determined by the reflector dimensions. Targets are placed in the center of the quiet zone and rotated about the center of gravity (cg) during measurement. Limitations on target size are defined by the quiet zone bounds. For large targets with a non-central cg location, a portion of the target may extend beyond the quiet zone boundary. A technique for synthesizing a larger quiet zone uses displacement of the collimator beam by means of feed point offset to allow far-field measurement of an asymmetrically-mounted extended target. Simultaneous measurements for each offset are then combined to produce the complete measurement. This technique was implemented for measurements of an ARIES ballistic missile target.
The paper presents an example of the design process undertaken to determine the RCS response of LO features mounted on a test body. Although not unique, the example considers the various aspects which determine the accuracy of the final data in the design of the experiment and signal processing. The high quality of experimental results illustrate the potential of using an integrated approach in which the designs of the test body, the measurement process, the signal processing techniques, and validation of results are optimally applied to meet the objective not achievable by conventional means.
Operationally active hangers are not well suited for making wholebody RCS measurements for aircraft signature diagnostics. While it is much more feasible to make localized regional or zone measurements in a hanger, the utility of such data for determining overall signature growth has significant limitations. The most obvious limitation is not having all the information necessary to re-assemble the wholebody signature. In this paper we present some discussion and experimental results which explore the limiting factors associated with estimating an entire aircraft signature from localized regional (zone) measurements. An example will be shown where zonal measurement data is inserted into a reference image and then reconstructed to form two-dimensional frequency vs aspect angle RCS. It is shown that a precise coherent alignment of the zone image with the reference wholebody image is not necessary and that only a coarse incoherent alignment is needed if only RCS statistics are desired. This is an important finding which leads to conclusion that it is logistically feasible to make zonal measurements and reconstruct a wholebody RCS estimate for impact analysis.
Field-level maintenance of radar signature treatment requires that non-specialist military personnel properly identify needed repairs. To simplify this task, an automated method is required that can compare radar signature data to baseline data, measure the differences, and identify the source of serious defects. Significant work has been done using artificial intelligence (AI) techniques to simplify this diagnostic task. A portable measurement radar was used to gather signature data on a small MQM-107D target drone. One set of data was collected of a baseline vehicle. Then data was collected after several anomalies were introduced, such as an uncovered pitot tube, wing joint untaped, or fastener screw not tightened. The data was processed as global downrange plots, and then baseline data was subtracted from anomaly data and the difference was compared to signature specifications as a function of angle. AI was used to identify signature defects that require repair. The results showed that an AI-aided diagnostic tool could help identify places where signature treatment repair was needed. This tool can be adapted to a variety of user and target needs.
A compact cassegrain antenna has been designed for dual-band satellite communications, operating at 20GHz and 30GHz. The antenna consists of a parabolic reflector, a hyperbolic sub-reflector, and a dual-band choke feed. The cassegrain structure has been optimized considering theoretical and measured feed patterns using different software packages, for maximum antenna efficiency with minimum sidelobe levels for a compact design objective. Experimental studies have been carried out in the near-field chamber of the University of Manitoba. The knowledgenof the near-field is helpful in order to adjust different components of the cassegrain antenna. After adjustment, results in terms of gain and radiation patterns are computed by Fourier transform using near-field data, and compared to the measurements realized in the compact range of the University of Manitoba. Comparisons are also made with the results obtained by simulation.
A near-field antenna range will often utilize a flexible microwave cable assembly as a means to transport the sampled signal from the moving sample antenna to a receiver as part of the measurement system. The performance of that cable directly impacts the quality of the final far-field pattern. It has been observed that the cable had been exhibiting a flex life much shorter than anticipated. Analysis of a failed cable revealed that the problem was the result of non-uniformities in the extruded jacket, which produced sites of high stress. These sites ultimately caused the cable conductors to work harden and fracture. A cable which utiized a woven expanded Polytetrafluoroethylene (ePTFE) fiber as an outer jacket was substituted, resulting in a threefold improvement in flex life to date, with the cable still in operation at this writing.
This paper presents an accurate and portable method for RF testing of AN/SPY-1 Antenna Arrays on Navy ships. With four antennas per ship, the usual methods for RF testing are time consuming and very costly. Currently, the most thorough and accurate method of testing is to remove an array and ship it to the original equipment manufacturer's near-field facility. A Portable Planar Near-Field Scanner System (PPNFSS) was developed by Nearfield Systems Inc. for the Naval Surface Warfare Center-PHD to perform RF testing without removing the array from the ship. The system consists of a portable robotic scanner, optics, microwave subsystem, environmental/anechoic enclosure and active thermal control system. The system was designed to mount to various array/ship configurations with severe envelope and environment constraints. The design is modular to allow packaging in ruggedized transit cases and a 48 ft. shipping container.
Two types of cellular handset testing are presented. The first studies models of a cellular handset near the human head. A comparative analysis is done between simulation and measurement of an inexpensive head mockup compared to a more expensive head mockup. Peak gain values have good agreement within about 1 dB. The second type of cellular handset testing is for a PCS band PIFA antenna integrated to a cellular handset. This paper describes the design and experimental study of the radiation patterns of a PCS band (1850-1990MHz) cellular handset with an internal PIFA. The PIFA described in this paper has good gain, impedance matching, and reduced sensitivity to human body interaction. This PIFA is a good cellular internal antenna.
SATIMO has recently installed a spherical near-field antenna measurement system for ALLGON MOBILE COMMUNICATIONS, the market leader in the field of antennas for mobile telephones. This spherical near-field system, as shown in Figure 1, allows for real-time measurements of antennas and will among other be used for the measurements of the radiation characteristics of mobile telephones and satellite terminals in the presence of the human operator. The system consists of a circular of 4m diameter containing 64 dual polarized measurement which are electronically scanned giving a real-time near-field pattern cut over 310° in elevation. A full sphere measurement including near-field to far-field transformation is available in seconds with a single +/- 90° azimuth rotation. The paper will present the measurement system and the results of the final acceptance tests. The acceptance tests are based on both range inter comparisons and also by measurement of key terms in the overall error budget.
This paper describes the extrapolation measurement method for determining gain of directive antennas at quasi-near-field distances. It is based on a generalized threeantenna approach and therefore does not require a priori knowledge of the antennas. It has been used at the National Institute of Standards and Technology (NIST) for over twenty years to calibrate antenna gain standards within 0.1 dB. The basic theory, measurement procedure, data analysis, and sources of uncertainty for the extrapolation gain measurement will be presented.