Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
In early 1999 a number of broadband absorbers were tested to evaluate their high power performance. In this paper the procedure for the tests is described and some results are presented. Later this year some additional tests were done to see effects of cooling by forced air. A conclusion is drawn that a standard procedure for verification of the high power capabilities of absorbers is needed in industry and that existing high power absorber does not necessarily meet specification as stated.
Microwave Instrumentation Technologies (MI Technologies) in cooperation with Hollandse Signaalapparaten B.V. (Signaal) and the Royal Netherlands Navy has designed and produced a compact antenna test range to specifically address the unique testing requirements imposed in the testing of active phased array antennas. The compact range was built specifically to test Signaal's new Active Phased Array Radar (APAR) prior to introduction into various naval fleets throughout the world. This reversible Compact Antenna Test Range (CATR) allows antenna testing in both transmit and receive modes. The measurement hardware is capable of testing both CW and pulsed waveforms with high dynamic range. In addition to conventional antenna pattern measurements the system is capable of measuring EIRP, Gff and G/NF, as well as providing analysis software to provide aperture reconstruction. A special Antenna Interface Unit (AIU) was designed and built to communicate with the Beam Steering Computer which controls the thousands of T/R modules which make up the APAR antenna system. A special high power absorber fence and other safeguards were installed to handle the transmit energy capable of being delivered from the APAR antenna system.
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.
The tapered chamber has been used for more than 35 years for mainly lower-frequency antenna measurements. The basic design of the tapered chamber has not changed significantly since its inception. Tapered chambers provide better quiet zone fields by placing the feed antenna's phase center at the vertex of the tapered walls, virtually eliminating reflections from the side walls. Recent innovations that better chamber versatility include an ultra-wideband SBH feed antenna [1,2], a less visible rotating support structure for the AUT and a new Chebyshev-based absorber treatment [3]. Utilizing these new features, a tapered chamber has been designed to have a large bandwidth, yet have an overall structure that is small enough to allow portability. This paper describes a chamber that operates from 400MHz to 40GHz and has an overall length less than 30' long. Structure, components, and field results are presented in this paper.
The Boeing Near Field Test Facility (NFTF) in St. Louis, MO was constructed in 1991 to conduct near field RCS measurements of production parts, models, and full-scale operational aircraft. Facility upgrades were identified in 1997 to support operational aircraft testing, such as the F/A-18 E/F. Target rotation mechanization, measurement antennas, and the test radar were identified as requiring upgrades. The target rotation hardware was upgraded to a 40-foot diameter turntable capable of handling production fighter aircraft. Antennas were mounted in an elevation box, which also contains the radar and an absorber aperture. The elevation box translates vertically, and pitches in elevation for different view angles. A new Lintek Elan radar, with a frequency range of 2ml8 GHz, 200 Watt Traveling Wave Tube (TWT) amplifiers, and Programmable Multi-Axis Controller cards (PMAC), controls all motion in the facility. In addition, modifications to the facility were completed to improve efficiency and ergonomics.
The antennas used in an anechoic chamber illuminate not only the target but also the walls, thus generating spurious signals. This problem is particularly significant at low frequencies. This study describes improvements to a standard, rectangular horn. Several solutions are tested, such as lenses, prolongation of the horn face, metal boxes with absorbers surrounding the horn, etc. The best solution appears to be the prolongation of the horn faces, with the fitting of a metal plate with absorbers at the rear. However, as the dimension of the horn increases with the ogival plates, the horn/target interaction also has to be taken into account.
This contribution will show how active frequency selective surfaces (AFSS) loaded with PIN and varactor diodes can be used concurrently to dynamically control the reflectivity characteristics of a microwave absorbers. The PIN and varactor diodes are used respectively to effect abrupt and gradual changes in an absorber's reflectivity characteristic.
In 1997 ORBIT/FR and Dornier Satellitensysteme (DSS), a corporate unit of Daimler-Benz Aerospace, agreed on strategic cooperation in the area of Compact Range products. This includes a licence agreement which allows ORBIT/FR to use the DSS developed reflector manufacturing technology utilizing steel castings to produce the highest precision reflectors machined for quiet zone sizes up to 8 x 12 ft. The standard product line includes both single reflector ORBIT/FR designs and the cross-polar compensated double reflector DSS design. Chelton (Electrostatics) Ltd. In Marlow, UK, is the first ORBIT/FR customer to receive a compact range using DSS technology. This radome measurement system uses a single reflector compact range with quiet zone of 4 x 6 ft. Other components include antenna, radome, and feed positioners, an HP 8530 based RF system, FR959 software and absorbers. Special software was developed to fully automate the entire radome acceptance test (up to 30 hours of acquisition and data evaluation) with a single command.
We are developing a new indoor bistatic measurement technique for scale model targets. This procedure will collect far-field data at bistatic angles from 60° to nearly 180° and near-field data over a 10' high, 10' radius cylinder surrounding the target. A stationary parabolic reflector illuminates the target while a duplicate parabolic reflector, rotated to its bistatic position, acquires far-field data. The independent, concentrically mounted near-field scanner gathers comparison data. Most compact range reflectors employ shaped edges to avoid edge diffracted signals entering the measurement volume. We report results of using shaped absorber material over otherwise unmodified reflector edges to reduce diffraction. High-resolution 3D images of sample structures demonstrate the practicality of this approach.
Due to the high cost of constructing anechoic chamber, the multi-usage of a chamber in various applications is very effective in terms of cost as well as space. In this paper, we describe an anechoic chamber, currently used at SK Telecom in Korea. This is designed for the measurements of both far/near field antenna and EMC/EMI in the identical chamber. This anechoic chamber and measurement system support antenna test in the frequency range of 150 MHz to 40 GHz and satisfy the requirement of ANSI C63.4 and CISPR16.1for EMC/EMI. The near field measurement system supports planar, cylindrical and spherical methods to test various types of antennas. For the far field and EMC/EMI measurement, the planner near field scanner is hidden by movable absorber wall. The AUT positioner is foldable and can be stored under the chamber floor. Brief description of the chamber and the measurement system with measured results are also provided.
The Wright Laboratory at WPAFB, OH, operates an advanced compact range facility (ACRF) for RCS measurements. The ACRF employs a dual chamber compact range system to generate a plane wave in the target zone. The main reflector, which is a blended rolled edge paraboloid, is housed in the main chamber; whereas, the feed assembly and the subreflector, which is a serrated edge ellipsoid, is housed in the sub chamber. The two chambers are electromagnetically coupled through a small opening near the focal point of the main reflector. The compact range system was originally designed to perform RCS measurements at frequencies above 1 GHz. Recently, there has been some interest in us ing the ACRF to perform RCS measurements at lower frequencies, from 100-1000 MHz. In fact, the ACRF facility has been successfully used to measure small targets at these lower frequencies, but one would like the target zone to be as large as possible. In order to accommodate a larger target zone, the first step was to evaluate the performance of the ACRF at lower frequencies. The performance evaluation revealed that the subreflector edge diffraction was leaking through the coupling aperture into the target zone. Some feed spillover was also observed in the target zone. To control these stray signals in the target zone, an absorber fence was designed for the ACRF. The absorber fence sits near the focal point of the main reflector. A prototype absorber fence has been built and installed in the ACRF. The performance of this absorber fence is discussed in terms of the improvement in the target zone fields.
Recently, we designed two doubly periodic curved pyramidal absorbers using Rantec absorber material. One of the pyramidal absorbers is 4011 high and is designed to operate at frequencies as low as 300 MHz; whereas the second pyramidal absorber is 6011 high and is designed to operate at frequencies as low as 200 MHz. The design goal was to achieve at least 45 dB attenuation for normal incidence. Based on our design, Rantec built the new pyramidal absorbers. The back-scattered fields of the new pyramidal ab sorbers were measured in the Wright Laboratories' (WL) advanced compact range facility (ACRF) us ing a 12' x 12' panel. In this paper, the measured data is presented and compared with the theoretical predictions. For reference, the scattered fields of a 72" pyramidal absorber are also included. The 72" pyramidal absorber was built by Ray Proof.
The new EMC-standards in Europe have strengthened the requirements for test facilities. In this paper examinations are concentrated on anechoic chambers, which are mostly used for measuring radionoise emissions. To become accredited a chamber have to own excellent performance, which is only possible by excellent absorbers and a careful choice of the measurement axis. A program for the evaluation of anechoic chambers has been developed and recently extended to permeable materials. This allows the calculation of chambers equipped with ferrite tiles or even a combination of ferrite and foam absorbers. Furthermore the numerical code is a very helpful tool during the planning phase of a chamber and offers the possibility to find the best way to improve the performance of older chambers. To estimate the performance the results are compared to the field distribution in an ideal Open Area Test Site (OATS).
ORBIT/FR has recently delivered an integrated facility capable of being used for Antenna, Radar Cross Section (RCS), and EMI measurements to the Naval Underwater Warfare Center in Newport, RI. The facility includes a shielded anechoic chamber, a compact range system capable of producing a 6 foot diameter quiet zone, multi-axis positioning equipment, and a complete complement of Antenna, RCS, and EMI measurement instrumentation and data collection hardware/software. The facility is capable of operation over a frequency range of 100 MHz to 50 GHz, with compact range operation feasible above 2 GHz. The facility can be reconfigured to go between antenna and RCS measurements in any band using both frequency band and antenna/RCS mode switching. In addition, automatic positioning of the appropriate compact range feed to the reflector focal point is available. EMI measurements require minimal relocation of absorber in an isolated area of the chamber floor. Performance of the system is optimized by location of critical RF equipment on the compact range feed carousel or on the positioning system rail carriage. This system offers a unique combination of performance and convenience for making all three types of measurements.
Use of a compact range for testing high power antennas is generally limited to testing the antennas at low power levels. In most cases, this is adequate, but for antennas where the management and dissipation of power is a key test parameter, the antenna and transmitter must be tested at the design power level. If this testing is to be performed in a compact range, it is important that the energy be captured and safely dissipated because allowing the energy to be incident on the absorber could result in destruction of the facility. The chamber under construction for Hollandse Signaalapparaten in Hengelo, Netherlands is designed to receive this energy in a specific region of air cooled absorber and to dissipate the heat into the chamber as an added load on the HVAC system.
In this paper, we present a new technique for measuring the input impedance of balanced antenna systems. The process uses standard two-port scattering parameters for balanced antennas, feeding each of the balanced input ports as the port of a two-port. The scattering-parameters will be related to the designed input impedance which may be obtained by post-processing the data. In addition, the scattering-parameters may be used to check for the assumed balance of the system. Both experimental and simulated results will be presented to validate the technique.
The Naval Command, Control and Ocean Surveillance Center RDT&E Division (NRaD) has been using a 500 MHz Linear Frequency Modulated (LFM) radar to collect measurements of flying aircraft. These data have been used to generate high resolution Inverse Synthetic Aperture Radar (ISAR) images of the targets [l]. Digital Signal Processing (DSP) hardware had been added to the radar and algorithms have been implemented to perform ISAR processing on the data in real time. A VME bus architecture has been developed to provide a scaleable, flexible platform to test and develop real-time processing software. Algorithms have been developed from a system model, and processing software has been implemented to perform pulse compression, motion compensation, polar reformatting, image formation, and target motion estimation.
Measurement of active array high-power transmit patterns in an indoor near-field facility raises significant issues concerning safe microwave power levels and absorber power-handling capability. An extension of the planar near-field measurement technique for the safe and accurate measurement of active array high power transmit patterns is considered to address these issues. This new technique involves sequentially turning on groups of elements around each probe position while making measurements for each group of activated elements. Simulation results indicate that this technique is potentially feasible for safely and accurately measuring low sidelobe active array transmit patterns.
In order to measure the performance of microwave absorbing materials a broadband free- space measurement system constructed in INTA. The is a kind of N RL Arch that gives us the possibility of measurements in d ifferent configurations. It comprises a set of dielect ric loaded rectangular waveguide antennas, coaxial vector analyzer, sample support and a computer. A TRL calibration technique in the plate near field is developed taking advantage of the calibration functions implemented in the network analyzer and the time domain gating. We introduce the use of typical RCS calibration standards as the calibration reflect standards. It gives us the possibility of performing the near field free space calibration in the same way that it is usually done in waveguide, but for directions di fferent to the normal. This calibration allows us to check the edge diffraction behaviour of the samples in the measurement and is thought to be adecuated for thin materials.
Absorber is mounted in an anechoic chamber to attenuate stray signals. In this application the stray signals impinge on a whole continuous absorber wall. Consequently, to evaluate chamber performance, one must determine the reflection properties associated with an absorber wall instead of a finite absorber panel. Unfortunately, absorber is normally evaluated experimentally using a finite absorber sample. As a result, absorber measurements are corrupted by edge (end) effect errors. These errors have been observed in measured data using ISAR image techniques, especially for high performance absorbers. One can isolate these error terms by using image filtering. The corrected image is then transformed back to the frequency and angle domains, such that the resulting data will much better represent the true absorber performance. Measured and calculated results will be shown to validate this new method for high performance absorbers.