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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 method for implementing and/or improving background subtraction performance in wideband outdoor RCS range measurements is described. The method estimates and corrects for systematic changes that have occurred between a test and back ground measurement. Results from the application of a phase-only version of the techniques to background measurements from the RATSCAT RAMS facility are presented. Background subtraction performance improvements of as much as 20 dB are demonstrated.
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.
NPL has been providing antenna gain standards since the late 1970's, initially to service internal needs for microwave field strength standards. To meet the increasing industrial demand for the calibration of microwave antennas in areas such as satellite communications and radar, NPL has developed an antenna extrapolation range. The current facility, which is due to be replaced by the end of the year, is used to measure the gain of microwave antennas in the frequency range 1 to 60 GHz, often with a gain uncertainty as low as ± 0.04 dB. Axial ratio, tilt, sense of polarisation and pattern measurements can also be made in the same facility, while for larger antennas a planar near-field scanner is used. Of the many measurement techniques for determining the gain of an antenna, the most accurate is the three antenna extrapolation technique [1,2] which was developed at the National Institute of Standards and Technology (NIST) at Boulder, Colorado, and is the method used at NPL. This is an absolute method as it does not require a prior knowledge of the gain of any of the antennas used. Since calibration data is often required across a wide frequency band, the measurement techniques and software have been developed to allow measurements to be performed at a large number of frequencies simultaneously. This reduces the turn round time, the cost and the need for interpolation between measurement points.
The European Space Agency (ESA) began serious investigations into the implementation and exploitation of near field antenna testing techniques already in the early 1970s where all three near field measurement geometries were considered (1). Spherical near field scanning was selected by ESA as being the most promising alternative to even larger conventional outdoor ranges. In the meantime, work was underway at the Technical University of Denmark (TUD) on spherical wave theory and its application to near field antenna measurements (2,3). As work began under ESA contract to demonstrate the technique, the most important aspect, the transformation algorithm and software was developed allowing dual polarized probe pattern and polarization corrected spherical near field measurements to be implemented (4).
A variety of unique instrumentation radars have been developed by the RF & MMW Systems Division at Eglin Air Force Base in order to support both static and dynam ic Radar Cross Section (RCS) measurements for Smart Weapons Applications. These systems include an airborne multispectral instrumentation suite that was used to collect target signatures in various terrain and environmental conditions (95 GHz Radar Mapping System - 95RMS), a look-down tower based radar designed to perform RCS measurements on ground vehicles (MMW Instrumentation, High Resolution Imaging Radar System MIHRIRS), two high power (35 & 95 GHz) systems capable of mapping/measuring both attenuation and backscatter properties of Obscurants and Chaff (MMW Radar Obscurant Characterization System MROCS: 1&2), and a Materials Measurement System (MMS) which provides complex free space, bistatic attenuation and reflectivity data on Radar Absorbing Materials (RAM), paints, nets and specialized coatings/materials. This paper will describe the instrumentation systems, calibration procedures and measurement techniques used for data collection as well as several applications which support modelina and simulation activities in the Smart Weapon community.
Antennas characteristics can be measured in two ways. lfrequency Domain Method (FDM) is more widely known. The main measuring instruments: Microwave Generator and Receiver. In Time Domain Method (TDM) measurements are fulfilled by using superwide band pulses. The main measuring instruments: Pulse Generator and Sampling Oscilloscope. TDM shows a number of advantages but for narrow-band antennas TDM is difficult to apply and FDM is required. At the testing polygons aimed to measure various antennas we set equipment allowing to use both measurement methods. For TDM we used a two channel sampling converter SD200 of Geozondas production with bandwidth 0-18 GHz. To unify measurements we developed a 3-channel sampling converter SD303 allowing besides pulse to measure sine wave amplitude and phase difference in dynamic range 100 dB. The third channel is used for synchronization. Thus the same instrument assures antenna measurements both in TDM and FDM. At 100 m distance the following characteristics are obtained in Time and Frequency Domains Measurements: Bandwidth 1- 18 GHz. Antenna pattern dynamic range 60 dB Gain measurement accuracy 0.5 dB Phase difference between 2 antennas error 0.5 - 3° (depends on frequency). Hardware, software and digital signal processing algorithms are considered.
High-speed receivers for near-field antenna and RCS measurements have traditionally been one-of-a-kind, expensive, difficult to interface and lacking in software support. Advances in digital signal processing, computer technology and software development now provide the means to economically solve these problems. NSI offers a high speed receiver subsystem, the Panther 6000 series, that allows multiplexed beam and frequency measurements at a rate of 80,000 independent amplitude and phase measurement points per second. The Panther 6000 receiver directly digitizes the 20 MHz IF test and reference input channels, and includes a high speed beam controller (HSBC) to sequence the measurement process. The HSBC receives an input trigger to initiate a measurement sequence of user-defined frequencies and beam or pol states. NSI also offers a multi-channel all-digital receiver subsystem, the Panther 6500, to interface directly with Digital Beam Forming (DBF) antennas. The Panther 6500 allows up to 16 channels of l and Q digital input (16 bits each) with 90 dB dynamic range per channel. The all-digital DBF receiver reduces the cost, complexity and performance limitations associated with conventional instrumentation in DBF antenna measurement applications. All Panther series receivers are fully integrated with the NSI97 antenna measurement software and operate with existing microwave sources, mixers and IF distribution equipment.
The development and implementation of novel measurement systems for rapid electromagnetic (EM) field testing by using linear arrays of modulated scattering elements is presented and discussed. The measurement systems employ the Advanced Modulated Scattering Technique (A-MST) to accomplish rapid sampling of the incident electromagnetic field along the length of the linear probe array at rates that are faster than conventional mechanical scanning of a single probe by a factor of 10 to 1000 or more. The A-MST probe array may be located in the nea r-field (NF) or far-field (FF) of the EM sources.
For multi-frequency measurements, improving source frequency agility will greatly reduce measurement times. This paper introduces a new microwave source with a frequency agility 50 times faster than traditional sources. A description of the source, its specifications, and the unique features that make it well suited to antenna measurements is provided, as well as information on integrating the source with an antenna range. Actual antenna test scenarios are provided, and the corresponding measurement times are compared to the test times achieved with traditional sources.
We explore new methods for the evaluation of absorber-lined chambers in the 30-1000 MHz frequency range. Current domestic and international standards recommend chamber evaluations using CW site attenuation measurements. While these satisfy regulatory requirements, they are of little value in actually understanding a chamber environment. A technology, that circumvents the limitations of CW test methods, is currently under development at NIST. In order to explore the potential of this new methodology, we have developed a two-dimensional finite-difference time-domain model for a fully anechoic ferrite tile chamber. The results obtained are quite promising and demonstrate the potential of time-domain measu rements in chamber evaluations. A new chamber figure of merit is proposed that permits a more direct evaluation of the installed absorber system.
This paper examines candidate configurations for a smart radar absorber or reflector which is capable of self-tuning and perform while scan operation. The discussion is supported by both modelled and measurement data.