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An investigation of the use of array mutual coupling measurements, to evaluate displaced phase center antenna (DPCA) performance, is made. The details of a subscale space based radar (SBR) DPCA phased array and the array mutual coupling technique are discussed. DPCA results are quantified experimentally under a number of test conditions. It is shown that the test array beam decorrelation computed from array mutual coupling data, is in good agreement with both theoretical predictions, planar near field measurements and direct far field measurements.
The matrix of scattering coefficients which describes the transfer of excitation between the port of an antenna and free space forms a fundamental description of that antenna. In carrying out the spherical near-field to far-field transforms for a probe-corrected measurement one is required to utilize the scattering coefficients of the probe antenna. An essential feature of any software system which supports probe-corrected measurements is the capability of analyzing and storing these coefficients.
Two alternative sampling techniques for planar near-field measurements are discussed. The first technique reduces the number of data points taken by 50% by measuring the field and its differential in one direction at each point. The second technique samples the field on a hexagonal lattice and allows reduction in the number of samples taken by up to 25%. Far-field patterns for an X-band antenna calculated from these alternative near-field sampling schemes are presented and compared with the far-field patterns calculated using conventional planar near-field techniques.
A novel technique for improved accuracy of sidelobe measurement by planar near field probing has been developed and tested on the modified near field scanner at the National Bureau of Standards. The new technique relies on a scanning probe which radiates an azimuth plane null along the test antenna’s mainbeam steering direction. In this way, the probe acts as a mainbeam filter during probe correction processing, and allows the sidelobe space wavenumbers to establish the dynamic range of the near field measurement. In this way, measurement errors which usually increase with decreasing near field signal strength are minimized. The probe also discriminates against error field which have propagation components in the direction of mainbeam steering, such errors may be due to multipath or scanner Z-position tolerances. Near field probing tests will be described which demonstrate measurement accuracies from tests with two slotted waveguide arrays—the Ultralow Sidelobe Array (ULSA) and the Airborne Warning and Control System (AWACS) array. Results show that induced near field measurement error will generate detectable far field sidelobe errors, within established bounds, at the –60dB level. The utility of te probe to detect low level radar target scattering will also be described.
When performing far-field testing on large-aperture antennas, the range length 2D2/? (that is needed to achieve a ‘flat’ phase front at the test plane) is sometimes inconviniently long. In these instances, the compact range of Figure 1 may be used as an alternate. In this range, the spherical wave radiated by the range source antenna is converted to an approximately plane wave by a large parabolic reflector. The antenna to be tested is immersed in this plane wave, at a location that is well within the near-field of the reflector. Also, for many antennas of interest, the reflector is likewise in the near-field of the test antenna, although this is not a requirement. (For those cases where the reflector is in the far field of the test antenna, there is little motivation to use a compact range, since a conventional far-field range of the same length would suffice.)
When performing far-field testing on large-aperture antennas, the range length 2D2/? (that is needed to achieve a ‘flat’ phase front at the test plane) is sometimes inconviniently long. In these instances, the compact range of Figure 1 may be used as an alternate. In this range, the spherical wave radiated by the range source antenna is converted to an approximately plane wave by a large parabolic reflector. The antenna to be tested is immersed in this plane wave, at a location that is well within the near-field of the reflector. Also, for many antennas of interest, the reflector is likewise in the near-field of the test antenna, although this is not a requirement. (For those cases where the reflector is in the far field of the test antenna, there is little motivation to use a compact range, since a conventional far-field range of the same length would suffice.)
Antennas are designed to operate with planar phase fronts, but are usually tested on finite length ranges that produce curved phase fronts. The result is a pattern error near the main beam. For conventional antennas the accepted range length requirement is R>2D2/? which produced a spherical phase error of 22.5 at the perimeter of a diameter D at wavelength ?. This, in turn, causes a 35 dB shoulder. For ultra low sidelobe antennas (ULSA) even longer ranges have been suggested. Such range sizes may be unavailable as well as undesirable, since the larger the range the more difficult it is to eliminate reflections.
The design of a multipurpose Antenna/RCS range at GTRI is described. A novel approach to design of the far-field antenna range utilizes the bottom 40-foot section of a 130-foot windmill tower. The top 90-foot section is used as the main support for a slant RCS measurement range offering a maximum depression angle of 32º. A 100-tom capacity turntable, capable of rotating an M1 Tank, is located 150 feet from the 90-foot tower. The rigidity and stability of the tower should allow accurate phase measurement at 95 GHz for wind speeds up to 10 mph. In addition, a 500-foot scale-model range uses the ground plane effect to enhance target signal-to-noise and is designed to be useful at frequencies up to 18 GHz. Initially, the radar instrumentation to be utilized with the ranges includes several modular instrumentation systems and associated digital data acquisition equipment at frequency bands including C, X, Ku, Ka, and 95 GHz. The properties of these systems, which include coherence, frequency agility, and dual polarization, are discussed.
This paper describes a receiver and exciter built by Texas Instruments for automated testing of electronic-scan antennas. The equipment is suitable for both near-field and far-field testing, and is programmable through a General-Purpose Interface Bus (GPIB) conforming to IEEE Standard 488. A two-channel design is described, but the technology is equally applicable to receivers from one to three (or more) channels. The receiver outputs are digitized as 10-bit I and Q (In-phase and Quadrature) components.
A technique for improving the accuracy of G/T measurements of highly directive antennas is introduced. The technique presents was developed to overcome uncertainties in ephemeral information, antenna positioning, system gain stability, and other random and nonrandom phenomena. The particular application discussed uses Casseiopeia-A as a noise source but the technique can be adapted for use with other extraterrestrial noise sources.
This paper describes a horizontal, cylindrical surface, near-field measurement facility which was designed and constructed in 1984 and is used for the determination of far field patterns from near field measurement of UHF television transmitting antennas. The facility is also used in antenna production as a diagnostic and alignment tool.
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 ability to gather real-time data from a remote site is of significant value in the far-field test of large scale non-reciprocal antenna arrays. With the advent of microprocessors, digitally controlled test equipment, and high speed data links, what was once impossible has not only become feasible but also economically realizable. This paper discusses the design of a remote data-gathering capability currently on-line at the Westinghouse Ridge Road Antenna Range. The system described is a computer-controlled phase and amplitude measuring technique remoted over a 1/3 mile range with a 56K baud fiber-optics data link. Considerations of system configuration, timing, protocol, error-detection and self-diagnostics are discussed.
A monostatic radar measurement system at the U.S. Navy Pacific Missile Test Center (PACMISTESTCEN) located at Pt. Mugu, California was utilized to obtain incidence angle performance of radar absorbing structure (RAS) panels. The traditional methods of obtaining reflectivity data for absorptive materials over a range of incidence angles is a technique known as the NRL arch. Developed over 30 years ago by the U.S. Naval Research Laboratory, the technique utilizes moveable bistatic antennas on an arch equidistant from the test material panel in order to obtain incidence angle data.
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.
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 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.
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 classical problem encountered when measuring the far-field radiation pattern of an antenna in a medium-distance range is the degradation that occurs when undesirable reflections (from the ground or nearby objects) are present. To reduce this problem, the source and test antennas are often installed on towers to remove them from the reflective objects, RF absorptive materials are used to reduce the magnitude of the reflected signals, and often the reflective objects in the range are adjusted in order to null out the reflections and “clean up” the range. These solutions are often limited in their effectiveness and can be prohibitively expensive to implement.
The ability to gather real-time data from a remote site is of significant value in the far-field test of large scale non-reciprocal antenna arrays. With the advent of microprocessors, digitally controlled test equipment, and high speed data links, what was once impossible has not only become feasible but also economically realizable. This paper discusses the design of a remote data-gathering capability currently on-line at the Westinghouse Ridge Road Antenna Range. The system described is a computer-controlled phase and amplitude measuring technique remoted over a 1/3 mile range with a 56K baud fiber-optics data link. Considerations of system configuration, timing, protocol, error-detection and self-diagnostics are discussed.