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Saab Ericsson Space and Ericsson Microwave Systems have recently completed the installation of a new efficient test facility. The facility is a fully automated test range designed for high th roughput of measurements. The facility is mainly used for tests of antennas for satellites and for mobile com munication. It is used as a far-field range for small antennas or as a spherical near field range for directive antennas. The frequency range covered is 0.8 - 40 GHz. A design driver for the facility was the logistics of measurements, short test time and easy access to the AUT during measurements. To achieve this, high speed positioners and easy access to the AUT via a drawbridge in the anechoic chamber were introduced. The computer controlled RF system allows the use of automatic mode switching to test the AUT in either receive or transmit mode and to change frequencies and mixers without operator intervention.
This paper will describe the measurement series performed on Global Horn flight antennas to be used on the Intelsat IX satellite series. The work was performed by MEMCO under contract to Space Systems I Loral. The Global Horn antenna system provides highly isolated RHCP and LHCP beams that cover the earth disc, as viewed from synchronous orbit. The corrugated wall horn is designed to maximize the gain at the edge of earth coverage angle, which in this case is defined as plus or minus 9.8 degrees from the beam peak. The horn has near perfect E-and H-plane amplitude and phase equality to achieve low off-axis cross-polarization (-55dB) across the earth disc. A well-matched orthomode transducer (OMT) and low axial ratio polarizer complete the antenna assembly. The paper will describe the anechoic chamber measurement series and techniques used to measure the circularly polarized cross-polarization isolation values in the -50dB (A.R.=0.05 dB) to -60dB (A.R.=0.02dB) region. Bench measurements of the polarizer, which has a measured axial ratio less than 0.02dB, will also be presented. Directive gain measurements of the flight antennas will also be presented and discussed. The techniques presented in this paper are also used by MEMCO to design and measure circularly and linearly polarized probes and source antennas used in Nearfield Scanners and Compact Antenna ranges.
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.
For many years the Navy has been using Anechoic Chambers and RF absorbing material. Recent events have brought into question the safety of RF absorbing material and the chambers which are covered with this material. Little, if any, information has been presented in the past to provide a solid picture of the actual danger that exists in these environments. A series of tests and inspections were conducted by the Navy on RF anechoic chambers and the materials inside. The materials were tested for fire susceptibility and chemically analyzed for salt compounds. Salt compounds have been used to make materials fire-safe. Results will be presented which show the susceptibility of various materials to fire from flames, electrical current and heat. A series of recommendations will be presented for using these materials in chambers to maintain safe working conditions.
This paper presents an efficient three dimensional (3-D) SAR imaging algorithm us ing range migration techniques. The algorithm is used to form 3-D radar reflectivity images of targets measured in anechoic chambers. As an input, the algorithm requires frequency domain backscatter data which have been acquired us ing a stepped frequency system equipped with an antenna that synthesizes a 2-D planar aperture. Resolution in the vertical and horizontal cross-range directions is given by the dimensions of the synthesized aperture, whereas resolution in ground-range is provided by the synthesized frequency bandwidth. The presented formulation has been justified by using the stationary phase method. Results both with syn thetic and measured backscatter data show the high efficiency of the technique. The extension of the algorithm when the antenna synthesizes a 2-D spherical aperture has been addressed. Re sults with this aperture geometry show that the technique is still highly efficient.
Motorola Cellular Subscriber Research Laboratory has developed and installed new hardware and software for measuring the performance of cellular and satellite phones. The hardware facilities consist of twin 16' cubical, near-field, anechoic chambers. Each has a spherical-scan system custom designed for cellular phone testing. The software consists of data collection, data presentation and database management software running under Windows NT 4.0™. Accurate testing of cellular phones requires that the measurements be made with a human phantom consisting of a human-shaped, liquid-filled fiberglass shell. These phantoms are fragile and must remain vertical. This required that an arm be implemented for the theta axis while a typical azimuth only positioner is used for the phi axis. The Theta axis arm is shaped like a "U" and is mechanically driven from both ends allowing the cross-piece of the "U" to be of a lightweight dielectric material so as to have minimum scattering.
Nearfield Systems, Inc. (NSI) has delivered the world's largest vertical near-field measurement system. With a 30m by 16m scan area and a frequency range of 1GHz to 50GHz, the system consists of a robotic scanner, laser optical position correction, computer and microwave subsystems. The scanner and microwave equipment are installed in an anechoic chamber 40m in length by 24m in width by 25m in height. The robotic scanner controls the probe positioning for the 33m by 16m vertical scanner using X, Y, Z and polarization axes. The optical measurement package precisely determines the X and Y axes position, alignment errors along the X and Y axes, and Z-planarity over the XY scan plane.
Compact range reflector edge diffraction can be reduced by placing a well-designed R-card fence in front of the reflector edges. The impact of this fence can be expressed mainly in terms of its ability to attenuate transmitted energy through the R-card. Thus, the resistance of the R-card is synthesized to satisfy a chosen GO aperture taper. A Kaiser-Bessel taper produces an ideal taper transition and hence a large target zone at the lowest operating frequency. Since a proper design requires that the R-card be located near the curved reflector edge, multi flat R-card segments are designed and assembled around the periphery of the reflector. The R-cards then attenuate the transmitted edge diffracted field and direct the reflected signal away from the test zone onto the anechoic chamber walls, which results in a significant improvement in the uniformity of the test zone fields.
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.
Probe arrays based on the Advanced Modulated Scattering Technique (A-MST) permit rapid measurements of electromagnetic fields at microwave and millimeter wave frequencies. Applications of A MST probe arrays to diagnostic testing of a) large anechoic chamber environments and b) microwave antennas are summarized in this paper.
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).
One of the key problems in SAR interferometry is the determination of the absolute phase of a scatterer by unwrapping the calculated phase difference of two SAR images. Since the phase difference is a function modulo 2p software algorithm are used to perform a phase unwrapping to obtain an unambiguous phase and thus the corresponding height information. This paper presents a procedure using two different transmit frequencies to enlarge the unambiguous range for height determination. Measurements performed in an anechoic chamber are used to test the processing. Using this procedure it is not only possible to resolve the height of a surface but also of single scatterers in space or in urban areas with steep slopes.
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.
The development* of a real time electronic system to accurately measure the pattern of high gain, ultralow sidelobe level antennas in the presence of multipath scatterers is described. Antenna test ranges and anechoic chambers contain objects that scatter the signal from the transmitting antenna into the main beam of a receiving antenna under test (AUT), thereby creating a multipath channel. Large measurement errors of low sidelobes can result. The fabrication of a feasibility demonstration model Antimultipath System (AMPS) is complete. This AMPS uses a 10 MHz wide phase-shift-keyed spread spectrum modulated signal to illuminate the rotating AUT to tag each multipath by its delay. The novel receive section of the AMPS sorts out each multipath component by its delay and adaptively synthesizes a composite cancellation waveform (using delay, amplitude, and phase estimates of the scattered components) which is subtracted from the total signal received by the AUT. After subtraction the resultant is the desired direct path signal which produced the free space pattern of the AUT. Laboratory and antenna range test results are presented and show the promise of measuring sidelobe levels 60 dB below the main beam.
For indoor antenna measurements, compact ranges or near-field/far-field techniques are most frequently used. One of the major problems is the handling of physically large antennas. Compact ranges will in general provide test-zone sizes up to approximately 5 meters in diameter. Applying the planar NF/FF technique, even larger test-zone sizes can be realized for certain applications. On the other hand, requirement of real-time capability, for instance in production testing, will exclude NF/FF techniques. It has been shown previously that single-plane collimators have a pseudo real-time capability which makes these devices comparable to compact ranges. Furthermore, the physical test-zone sizes which can be realized when compared to compact ranges are approximately 2-3 times larger for the same size of the anechoic chamber. Finally, it will be shown that the accuracy in sidelobe level determination, gain and cross polarization is considerable higher than with other indoor techniques, even at frequencies below 1 GHz.
In recent years there has been much interest in developing low frequency radar cross section (RCS) measurement capability indoors. Some of the principal reasons for an indoor environment are high security, all-weather 24-hour operation, and low cost. This paper describes recent efforts to implement VHF/UHF RCS measurement capability down to 100 MHz using the large compact-range collimator in the Bistatic Anechoic Chamber (BAC) at Point Mugu. The process leading to this capability has given rise to a number of technical insights that govern successful test results. An emphasis is placed on calibration and processing methodology and on measurement validation using long cylindrical targets and comparing the results with method-of-moment computer predictions and with measurements made at other facilities.
This paper discusses the test capabilities of the Benefield Anechoic Facility (BAF) and its mission to support avionics and electronic warfare (EW) test and evaluation (T&E) of current and future generation manned and unmanned aerospace vehicles. Testing at the BAF can provide the dense, complex, and realistic signal environment necessary to evaluate integrated systems/subsystems to meet both Development Test and Evaluation (DT&E) objectives. The BAF, located at the Air Force Flight Test Center (AFFTC), Edwards Air Force Base, California, USA, is part of the Avionics Test and Integration Complex (ATIC). The BAF provides a quiet, secure, and controlled electromagnetic environment to test installed/integrated systems, their associated weapons, avionics and EW systems. This testing is accomplished within a very large anechoic chamber, providing a realistic free-space and controllable radio frequency (RF) environment.
New windows which allow the user to select the level of sidelobe suppression near the DFT resolution limit are reported. By a parametric study, we identify the truncated Lorentzian and Gaussian functions as better choices compared with the popular Hann windows.
At high frequencies, the scattered fields from a radar target can be modeled as a sum of contri butions from a finite number of scattering centers. We use a parametric model based on the Geometric Theory of Diffraction (GTD) to estimate the location and type of scattering centers present in a frequency domain data set. The parameters of the model are estimated using a modified MUSIC algorithm that incorporates the GTD model. A new spatial smoothing algorithm is also introduced.
RCS data measured under near-field conditions is corrected to the far-field. The algorithm uses the HUYGEN's principle approach. The processing technique is describes and validates using anechoic chamber data and simulations taken on flat plate target at a distance from the radar R << 2D2/A, where D is the target cross range extend and A the wavelength. Good agreement with the theoretically predicted far-field RCS patterns is obtained.