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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.
An innovative technique has been developed for accurately measuring very low Sidelobe Antenna patterns by the method of planar near field probing. The technique relies on a new probe design which has a pattern null in the direction of the test antenna’s steered bean direction. Simulations of the near field measurement process using such a probe show that -60dB peak side-lobes will be accurately measured (within established bounds) when the calibrated near field dynamic range does not exceed 40 dB. The desireable property of the new probe is its ability to “spatially filter” the test antenna’s spectrum by reduced sensitivity to main beam ray paths. In this way, measurement errors which usually increase with decreasing near field signal level are minimized. The new probe is also theorized to have improved immunity to probe/array multipath and to probe-positioning errors. Plans to use the new probe on a modified planar scanner during tests with the AWACS array at the National Bureau of Standards will be outlined.
The development of advanced spacecraft communication antenna systems is an essential part of NASA’s satellite communications base research and technology program. The direction of future antenna technology will be toward antennas which are large, both physically and electrically; which will operate at frequencies of 60 GHz and above; and which are nonreciprocal and complex, implementing multiple beam and scanning beam concepts that use monolithic semiconductor device technology. The acquisition of accurate antenna performance measurements is a critical part of the advanced antenna research program and represents a substantial antenna measurement technology challenge, considering the special characteristics of future spacecraft communications antennas.
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 measurement of microwave antennas indoors began with the advent of commercial absorbing material. The use of absorbers can be traced back to a 2 gHz material developed by the Dutch in the Thirties. During the Forties, considerable progress was made on absorbing materials, but even after World War II, security considerations limited the application. Some materials found use as indoor shields for antenna tests, but limited bandwidth limited the utility of these materials. When a broad band absorber was developed the antenna experts did not believe that this material would be made commercially because they presumed a limited market.
A planar surface, near-field measurement technique is presented for the near-field measurement of monostatic radar cross-section. The theory, system configuration and measurement procedure for this technique are presented. It is shown that the far field radar cross-section can be determined from the near field measurements. An associate near-field radar cross-section measurement technique is presented for the measurement of bistatic near field radar cross-section. The bistatic technique requires a plane wave illuminator in addition to the planar surface near field measurement system. A small compact range is used as the bistatic illuminator. Bistatic near-field measurements are presented for a simple target.
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.
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.
Test techniques and test results will be presented on Compact Range testing of a 1.9m offset reflector at 20/30 GHz. The antenna is part of a demonstration model for an intersatellite link antenna system.
A set of near-field measurements has been performed by combining the methods of non-probe-corrected spherical near-field scanning and gain standard substitution. In this paper we describe the technique used and report on the results obtained for a particular 24 inch 13 GHz paraboloidal dish. We demonstrate that the gain comparison measurement used with spherical near-field scanning give results in excellent agreement with gain comparison used with compact range measurement. Lastly we demonstrate a novel utilization of near-field scanning which permits a gain comparison measurement with a single spherical scan.
Conventional antenna test techniques – both far field “slant ranges” and near field – pose limitations for radiative RF testing of satellite antennas and payload systems, of increasing complexity in terms of size, operating frequencies, configurations and technology, particularly when such systems need to be evaluated in their “in-situ” locations on typical satellite platforms, in their flight configurations. Often, combination of tests and simulation has been the only recourse for evaluating system performance. In this paper, a methodology is proposed to achieve these test objectives via the use of a suitable configures, wideband, large (Quiet zone 7m x 5m x 5m), compact range for evaluation od system parameters like E.I.R.P., G/T, C/I, BER, and RF sensing performances. The test plan and evaluation schemes appropriate for these tests are elaborated to demonstrate the validity and usefulness of the approach. For some specific parameters like C/I (for a multibeam payload system) and the radar parameters (for a satellite borne radar system), it turns out that the proposed test methodologies offer the only realistic and complete tool for evaluating such system at satellite level.
Test results will be presented for a four foot S-Band reflector antenna together with the compact range modification and test verification at 2.2 GHz. Similarly compact range test results will be presented for an eight foot K-band reflector antenna at 23 GHz.
Just as with far-field or compact ranges, it is important to evaluate spherical near-field ranges with electromagnetic field-probe measurements. Recall that the fundamental motion for utilizing the spherical near-field measurement technique is to permit antenna measurements to be made at short range lengths, relieved from the constraint of the far-field criterion. Just as the illumination function in the test zone of an ideal far-field range is a uniform planar wavefront, the ideal illumination function for a near-field range is a spherical wavefront from an elemental dipole. The field probe measurements provide a quantitative and qualitative assessment of the deviation of either a near-field or far-field range from ideal conditions.
An S-band telemetry antenna system was designed and fabricated using a 30-inch diameter lightweight Luneberg lens as the aperture. It is equipped with four feeds in the azimuth plane to achieve single beam patterns or multiple beam patterns. Initial measurements with the lens without a radome were made with various feeds and feed combinations in the compact range of the Georgia Tech Engineering Experiment Station. The final design also done by Georgia Tech to Kentron Specifications, uses a custom designed quad ridged circular feed with orthogonal linear polarization outputs which are converted to left- and right-hand circular polarization using 90o hybrid couplers. A control panel permits the operator to manually select a single beam coverage of 11o x 11o, twobeams combined for 22o x 11o sector coverage, or four beams combined for 44o azimuth x 11o elevation sector coverage. A automatic mode permits the full gain of a single beam, about 22 dB, to be attained and switched automatically to the RF feed containing the greatest signal power as sensed by eight total power radiometer receivers; one for each orthogonal polarization for each of the four antenna feeds. Selectable integration time constants are 0.1, 1, 10, and 100 milliseconds. Dependable switching is obtained for signals of -99 dBm or greater. The RF switching is achieved by PIN-diode switches in 10 nanoseconds. The system employs eight state-of-the-art gain and phase matches GaAs FET low-noise preamps which have a noise figure of 1.1 dB and gain of 51 dB. External limiters at the input of each LNA protect the devices from accidental RF inputs up to six watts average power. The system was designed as a removable package to be flown aboard the U.S. Army’s C-7A Caribou aircraft with an opened rear cargo ramp to collect terminal TM data from missile reentry vehicles (RV’s) impacting near the Kwajalein Missile Range. Flight testing of the system against target of opportunity missions began the third week of June 1982. The system is expected to be declared an operational system in support of ballistic missile testing by December 1982.
A strong emphasis is now being placed on techniques for reduction of radar cross-section. A missile or aircraft which is invisible to radar has an important strategic advantage. With this fact in mind, the user of a weapons system may place an upper limit on the radar cross-section that he will permit his missile or aircraft to have. The designer must then make use of “stealth technology” to reduce the cross-section to an acceptable level. In order to verify the design, radar cross-section measurements must be made. Thus the current emphasis on cross-section reduction leads to an important need for accurate and reliable methods of measuring radar cross-section.
Over the past year an extensive set of verification tests has been made on a particular test antenna in order to confirm the design and operation of a spherical near-field algorithm. The measurement checks included data taken at two frequencies at three range lengths, with two coordinate orientations and with two types of probe horns. Comparisons were made against the compact range and among the various spherical near-field tests. In this talk we show examples from this program of measurements and summarize the results which demonstrate the operation of the spherical near-field scanning technique.
Compact Antenna Ranges (C.R.) proved to suitable for indoor measurements of antennas of moderate size (up to about 4 feet) in the frequency ranges from 4-18 GHz. Where less accurate measurements are allowed, the upper frequency limit can be as high as 60 GHz in current C.R. design. Dimensions of such a range are approximately 4 times larger (in linear dimension) than those of the test antenna. This is due to the face that there is a considerable taper in the amplitude over the aperture of the C.R. Considerable improvements in the electrical performance may be expected for ranges in which two crossed parabolic cylindrical reflectors are used. Due to the increased focal length the uniformity of the amplitude distribution across the final aperture is increased considerably compared to conventional design. Furthermore, an asymmetrical plane-wave zone can be created which makes it possible to measure the patterns of asymmetrical antennas or devices including the direct environment (antennas on aircraft or spacecraft). A compact range which consists of a main reflector with overall dimensions of 2x2 metres has been used for experimental investigation in the 8-70 GHz frequency band. At 10 GHz the plane-wave zone has a slightly elliptical shape (100x90cm). The amplitude variations are in this case less than 0.3dB; the corresponding phase errors are less than 4 degrees. It has been shown that the reflectivity level can be kept below –60dB. Only a minor degradation in performance was found at 70 GHz. In conclusion, the performance of this new compact range is as good as, or better than that of most outdoor ranges. The upper frequency limit is about 100 GHz for ranges of moderate size (up to 3 metres). Summarizing, the main advantages compared to other compact ranges are: -Larger test zone area (up to 2x) for the same C.R. reflector size -better crosspolar performance -considerably higher upper frequency limit The last-named is due to the cylindrical reflector surfaces, which are easier and cheaper to manufacture than double-curved surfaces.
The compact antenna range has been recognized as an effective means of testing microwave antennas. Antennas which normally require long outdoor ranges for testing can be tested under far field conditions at an indoor facility, using the compact range. The compact range operates on the principal that a parabolic reflector will transform an incident spherical wave into a collimated plane wave in its near zone. The plane wave produced is suitable for testing antennas, thus simulating far field electromagnetic criteria in the near zone. The typical compact range is housed in a room approximately 20 feet wide, 40 feet long and 20 feet high. The performance of the compact range has been well documented and specified over a frequency range of 3.95 GHz to 18.0 GHz. Now, through recent testing performed at Scientific-Atlanta, the compact range can be specified for operation up through 60.0 GHz. This paper describes the tests that were performed, discussed the results of these tests and establishes performance specifications for operation at these millimeter frequency bands.