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This paper describes specially constructed instrumentation and positioning systems used in evaluating RF absorber, discusses measurement techniques, and presents data and conclusions from current programs. The selected absorbers which were evaluated are typical of those used in anechoic chambers and terminated ranges for antenna, radome and RCS testing.
Review of a computational technique used in the design of anechoic chambers is presented. Details of an interactive computer program to predict fields inside anechoic chambers are discussed. Use of the program in (a) computing change in field distribution due to reflections from the walls of the chamber and (b) optimizing cost/performance ratio of the chambers is illusterated.
A great deal of effort has gone into the optimum design of anechoic chambers over the years, however little attention is generally given to the choice of the source antenna used to illuminate these chambers. Typically, any antenna which operates at the desired frequency and that happens to be available in the antenna laboratory is commandeered for us as a source.
The Compact Range RCS Measurement System is comprised of the Harris Shaped Compact Range and the Hughes Short Pulse Coherent RCS Measurement System. The range offers a 10 foot spherical quiet zone with less than ±0.25 dB amplitude ripple, 0.2 dB amplitude taper, and ±2 degrees phase ripple. The short pulse system offers a pulse width as small as 5 nsec with range gate increments of 100 psec minimum. The system has a sensitivity of –70 dBsm without integration and –120 dBsm with 50 dB of coherent integration. System linearity is better that ±0.5 dB over the 70 dB instantaneous dynamic range. The Shaped Compact Range offers nearly 98 percent illumination efficiency with negligible spillover which minimizes the required anechoic chamber size and the amount of absorbing material necessary. The block diagram of the system is shown in Figure 1.
This paper describes the equipment, mechanics and methods of one of the outdoor ranges at Teledyne Micronetics. A computer controlled microwave transceiver uses pulsed CW over a frequency range of 2-18 GHz to measure the amplitude, phase and polarization of the signal reflected off the target. The range geometry, calibration and analysis techniques are used to optimize measurement accuracy and characterize the target as a set of subscatterers.
The near-field antenna testing technique is now an established testing approach. It is based on the work done over a twenty-year period by the National Bureau of Standards (Boulder, Colorado), The Georgia Institute of Technology and others. The near-field technique is used for large aperture, high frequency antennas where the antenna to probe separation necessary to test in the far-field of the antenna is prohibitively large.
The purpose of an instrumentation radar is to characterize the Radar Cross Section (RCS) of a target as a function of target aspect and radar frequency. In addition, an instrumentation radar may be used to produce a high resolution radar image of a target which is useful in target identification work and as a diagnostic tool in radar cross section reduction. These purposes differ from those of a conventional radar, in which the objective is to detect the presence of a target and to measure the range to the target. Several different radars are currently used to perform radar cross section measurements. Common instrumentation radars may be classified as CW, Pulsed CW (Low-Bandwidth IF), Linear FM (FM-CW), Pulsed (High-Bandwidth IF) and Short Pulse (Very High-Bandwidth IF). These radars accomplish the measurement task in distinct manners, and it is sometimes difficult to determine where the strength or weakness of each radar lies. In this paper, a set of performance criteria is proposed for RCS measurements. The proposed criteria can be applied uniformly to any instrumentation radar independent of the type of radar design employed. The criteria are chosen to emphasize those performance characteristics that relate directly to RCS measurements and thus are most important to the user. Two instrumentation radars which have been designed at Scientific Atlanta, namely the Series 2084 (Linear FM) and the Series 1790 (Pulse), are used to illustrate the application of the performance criteria.
In recent years a new class of reflector antennas utilizing array feeds has been receiving attention. An example of this type of antenna is a reflector utilizing a moveable array feed for beam steering. [1]-[3]. Due to the circuitry required to adjust the weights for the various feed array elements, an appreciable amount of loss can be introduced into the antenna system. One technique to overcome this possible deficiency is to place low noise amplifiers with sufficient gain to overcome the weighting function losses just after each of the feed elements. In the evaluation of signal processing antennas that employ amplifiers the standard antenna gain measurement will not be indicative of the antenna system’s performance. In fact, by only making a signal measurement, the antenna gain can be made any arbitrary value by changing the gains of the amplifiers used. In addition, the IEEE Standard Test Procedures for Antennas [4] does not cover the class of antennas where the amplifier becomes part of the antenna system. There exists a need to establish a standard of merit or worth for multi-element antenna systems that involve the use of amplifiers. This communication presents a proposed figure of merit for evaluating such antenna systems.
This presentation will focus on the recently revised ANSI C95 RF Radiation Exposure Standard. Some of the research background for the new standard will be given, and its impact will be explained. Instrumentation guidelines for measuring potentially hazardous fields will be presented. The possible damaging effects of non-ionizing RF radiation is receiving increased attention in the public eye, and it behooves the practicing antenna engineer to be aware of the potential dangers to health and safety from exposure of RF energy.
The compact range is an electromagnetic measurement system used to simulate a plane wave illuminating an antenna or scattering body. The plane wave is necessary to represent the actual use of the antenna or scattering from a target in a real world situation. Traditionally, a compact range has been designed as an off-set fed parabolic reflector with a knife edge or serrated edge termination. It has been known for many years that the termination of the parabolic surface has limited the extent of the plane wave region or, more significantly, the antenna or scattering body size that can be measured in the compact range. For example, the Scientific Atlanta (SA) Compact Range is specified to be limited to four foot long antennas or scattering bodies as shown in their specifications. Note that the SA compact range uses a serrated edge treatment as shown in Figure 1. This system uses a parabolic reflector surface which is approximately 12 square feet so that most of the reflector surface is not usable based on the 4 foot square plane wave sector. As a result, the compact range has had limited use as well as accuracy which will be shown later. In fact, the compact range concept has not been applied to larger systems because of the large discrepancy between target and reflector size. In summary, the target or antenna sizes that can be measured in the presently available compact range systems are directly related to the edge treatment used to terminate the reflector surface.
The complete sequence of RF tests required to evaluate the electrical performance of a broad band UHF helix antenna to be used in the zero gravity environment of space is described. The development of an adequate structure which would support the antenna and yet cause no pattern perturbation is mentioned. The test range configuration used, with the UHF antenna inside and anechoic chamber and the source antenna illuminating it through a polyfoam window in one side, is discussed. The problems encountered in taking radiation pattern plots and in making gain measurements using a gain standard near the low frequency limit, 250 MHz, of the antenna test range and the methods utilized to minimize their effect are given in some detail.
When low crosspolar pattern measurements are required, as in the case of the L-SAT Propagation Package Antennas (PPA) with less than -36 dB linear crosspolarization inside the coverage zone, the use of good polarization standards is mandatory (1). Those are usually electroformed pyramidal horns that produce crosspolar levels over the test zone well below the -60 dB level typically produced by the reflectivity of anechoic chambers. In this case the alignment errors (elevation, azimuth and roll as shown in fig. 1) can become important and its efects on measured patterns need to be well understood.
This paper describes the new antenna test facility under construction at General Electric Space Systems Division in Valley Forge, PA. The facility consists of a shielded anechoic chamber containing both a Compact Range and a Spherical Near-Field Range. In addition, it provides for a 700’ boresight range through an RF transparent window. The facility will be capable of testing antenna systems over a wide frequency range and will also accommodate an entire spacecraft for both system compatibility and antenna performance tests.
As today’s electronic support measures (ESM) systems become more complex so must the test equipment required for qualification and final acceptance tests. Tests and test facilities have become more complex, costly and massive when these ESM systems are integrated into vehicle size structures which must be tested as a unit. This paper describes an automated anechoic chamber which was built to solve some of the special problems associated with the testing of a physically large, electronically sophisticated ESM system. Some features of the automation thought to be unique are the methods used to position the test antennas without any required operator interaction. Other unique features of the design include methods of aligning the test article to the source antennas and the technique used for chamber qualification.
This paper deals with the design, calibration and performance of a new antenna test range facility at the David Florida Laboratory in Ottawa, making use of an existing 40 foot cube anechoic chamber and a Scientific-Atlanta 2020 system. The main purpose is to use the same test range for the calibration of a nominal seven foot by five foot Standard Gain Horn and ultimately for gain and pattern testing of an eight foot space qualified axial mode helix, which must be maintained inside the anechoic chamber. This rules out a completely outdoor test range.
The USAF Rome Air Development Center has recently constructed a laboratory building which has recently constructed a laboratory building which has been designed to implement the measurement of microwave antennas and electromagnetic systems. The new facility consists of dual elevated open-ended chambers with retractable doors, a 2700 foot outdoor range, a variable short range and a 40 x 20 x 18 foot anechoic chamber. Wide frequency band instrumentation is installed to provide efficient high speed data collection and analysis required to support the center’s technology development mission in C3I. A presentation of the facility’s capability and design will be given as well as a brief historic overview of significant antenna measurements of the past.
Of the wide variety of antenna parameters and system parameters that are measured in anechoic chambers, not all are compatible with all chamber designs. This paper has been primarily designed to answer the following question; “Knowing the type of measurements one intends to make in a chamber, how does one choose and carry out the chamber design so that the chamber will be well suited?” Secondarily, this paper is designed to answer the corollary question; “Can an existing chamber of a particular design be well-suited to particular measurements?”
A review will be made of advances in anechoic chamber technology from the precursors of World War II to the huge complex chamber of today. A glimpse of the technology of the 80’s will be offered.