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A Hewlett Packard 8410 Network Analyzer was modified to be used as an automated far-field antenna range receiver. By using external mixers, analog to digital signal conversion, and an external computer/controller, the HP8410 is capable of measuring signals as low as -110 dBm. The modified receiver is an intergral part of an automated far-field range which features computer controlled test antenna positioning, system measurement parameters, and data acquisition, as well as customized measurement file management. The system described was assembled and made operational taking advantage of off-the-shelf hardware available at minimal cost.
A technique for aligning a phased array is described. Array element attenuation and phase commands are derived from far-field patterns measured without calibrations. The technique is based on iteratively forming mulls in the antenna pattern in the directions specified by a uniform array illumination. It may be applied in situations where array elements are not individually accessible, or where an array contains no build-in calibration capacity. The alignment technique was evaluated on a far-field range with a linear, 32-element array operating at L-band. The array containing transmit/receive modules with 12-bit amplitude and phase control. Insertion attenuation and phase measurements were comparable to those obtained by conventional techniques. However, the alignment procedure tends to compensate for the effects of nonuniform element patterns and range multipath. Thus, when used to implement other excitation functions, the array sidelobe performance with adaptive calibrations was substantially better.
Electronically scanned phased array antennas typically have a large number of beam positions. Accurate on-line monitoring of phased array beam positions can be used to ensure proper antenna and total system performance. Bendix has developed and successfully implemented a beam-position monitoring technique designated the “RF Integral Monitor System”. Use of this on-line technique does not interfere with normal system operation and yields results that are comparable to results obtained on an actual far field antenna range. The RF Integral Monitor technique and specific hardware implementations, for both linear and circular electronically scanned phased arrays, will be described in this paper.
To enhance the performance of existing near field techniques the new idea of far field pattern determination from only amplitude distributions of the near field is proposed. In this way the difficulties related to phase measurements are overcome. Some different algorithms are introduced and discussed. In particular, after recalling results for the planar geometry, cylindrical scanning surfaces are considered. The feasibility and the performances of the introduced algorithms are shown through numerical examples.
A method for obtaining the individual element excitations of an array antenna from measured radiation patterns is presented. Applications include element failure diagnosis, phased array antenna calibration, and pattern extrapolation. The measured far-field information is restricted to visible space which does not always contain the entire Fourier domain. A typical example is phased array antennas designed for large scan angles. A similar problem arises during near-field testing of planar antennas in which case the significant far-field domain is restricted by the scanning limitations of the near-field test facility. An iterative procedure is then used which is found to converge to the required solution. The validity of the approach has been checked both using the theoretical radiation patterns and real test cases. Good results have been obtained.
Lockheed’s Advanced Development Company (LADC), located in Burbank, California, has recently completed construction of a state-of-the-art indoor Antenna/RCS test facility. This facility is housed in a dedicated 40,000 square foot building which is a maximum of 80 feet high. This building contains three anechoic chambers providing Antenna/RCS measurement capability from 100 Mhz to 100 Ghz. The largest chamber, with dimensions of 64 feet by 64 feet by 97 feet is configured as a compact range. This chamber utilizes the largest collimating reflector that Scientific-Atlanta has ever constructed. Primary test usage of this chamber is for RCS measurements in the frequency band of 700 Mhz to 100 Ghz. The second chamber is configured as a tapered horn test range. Its dimensions are 155 feet long with a 50 foot by 50 foot by 55 foot volume measurement zone. This chamber is utilized for RCS tests in the VHF, UHF, and L frequency bands and antenna tests from 100 MHz and up. The third chamber, with dimensions 14 foot by 14 foot by 56 foot, is a far field chamber designed to check out and evaluate small items up to 100 GHz. The entire facility has been designed to maximize efficiency, minimize the cost of operation, and produce outstanding quality data from Antenna/RCS measurements. A number of innovative techniques in model handling, model access, and model security were incorporated into the facility design. These features, as well as utilization of unique Lockheed designed and built pylons, allowed achievement of all these goals.
The Dassault Electronique flexible near-field antenna test facility, ARAMIS, has been used for test and calibration of state-of-the-art active phased-array antennas which were designed for military SATCOM operation. The 14-month successful program dramatically emphasized the benefits of a flexible antenna test facility such as ARAMIS. These benefits are the following: • Flexibility o Far-field mode (test of radiating elements and modules) o Planar near-field mode (test of sub-arrays and complete antenna) o High-resolution field mapping mode o Array Element testing • Speed: quick mode switching, “on the fly” multiplexed acquisition • Versatility: calibration of a module, a sub-array and the antenna; radiation patterns; gain; faulty element detection • Productivity: a single indoor facility performing different types of measurements, integrated software Test results gathered during this program and showing the ARAMIS contribution are presented.
Airborne or spaceborne radar systems often require adaptive suppression of interference and clutter. Before the deployment of this adaptive radar, tests must verify how well the system detects targets and suppresses clutter and jammer signals. This paper discusses a recently developed focused near-field testing technique that is suitable for implementation in an anechoic chamber. With this technique, phased-array near-field focusing provides far-field equivalent performance at a range distance of one aperture diameter from the adaptive antenna under test. The performance of a sidelobe-canceller adaptive phased array antenna operating in the presence of near-field clutter and jamming is theoretically investigated. Numerical simulations indicate that near-field and far-field testing can be equivalent.
The near field technique has grown from experimental systems of the early 1960s to sophisticated accepted means of testing antennas. Several schemes have been employed, namely planar, cylindrical and spherical scanning. The spherical scanning system chosen for one of the near field ranges at GE Aerospace is different from most near field systems in that the test antenna remains stationary while the probe is made to scan over a surface of an imaginary sphere surrounding it. The sampled field is corrected for positional, phase and amplitude errors and transformed to the far field. Radiation patterns, gain, EIRP, group delay and amplitude response were measured for a shaped beam communications antenna.
Near-Field antenna measurements were made using a Hewlett Packard 8510 automated network analyzer. This system features measurement sensitivity better than -90 dBm at measurement speeds of one data point per millisecond in the fast data acquisition mode. The system was configured using external, even harmonic mixers and a fiber optic distributed local oscillator signal. Additionally, the time domain capability of the HP 8510, made it possible to generate far-field diagnostic results immediately after data acquisition without the use of an external computer.
We have developed planar near-field codes, written in Fortran 77, to serve as a research tool in antenna metrology. This new package has a highly modular structure and can be used to address a wide variety of problems in antenna metrology. We describe some of the inner workings of the codes, the data management schemes, and the structure of the input/output sections to enable scientists and programmers to use these codes effectively. The structure of the code is open, so that a new application can be incorporated into the package for future use with relative ease. A new module can rely on the large number of reusable subroutines currently in existence, and new routines are easily integrated into the existing library. Examples of applications of the codes to basic research problems, such as transformation of a near field to the far field and probe position error correction, are used to illustrate the effectiveness of these codes. Sample outputs are shown. The advantage of a high degree of modularization is demonstrated by the use of DOS batch files to execute Fortran modules in a desired sequence.
A spherical range probing technique for the location of secondary sources in far-field compact and spherical near-field antenna measurement ranges are presented. Techniques currently used for source location use measurements of the range field on a line or plane to locate sources. A linear motion unit and possibly a polarization rotator are necessary to measure the range field in this manner. The spherical range probing technique uses measurements of the range field made on a spherical surface allowing the range positioners to be used for the range field measurement. The plane wave spectrum of the measured range field is used for source location in the spherical probing technique. Source locations in the range correspond to the locations of amplitude peaks in this spectrum. Source resolution limits of this technique is illustrated using simulated range measurements. Obtaining a plane wave spectrum from measured data is discussed.
An overview of results are presented for far field pattern, antenna gain and antenna temperature measurements of reflector antennas in several frequency bands. The pattern and gain measurements were taken in the compact range at The Ohio State University. The dynamic range available, which gives the ability to take a full 360 degree pattern, and the relatively high speed at which data is collected, are major advantages for pattern and gain measurements in the compact range. In a series of related measurements an 8-foot diameter Cassegrain reflector was used for antenna temperature measurements under clear weather conditions in an outdoor environment.
This paper describes the results of electrical boresight measurement comparisons between one far-field and two near-field ranges. Details are given about the near-field alignment procedures and the near-field error analysis. Details of the far-field measurements and its associated errors are not described here, since the near-field technique is of primary interest. The coordinate systems of the antenna under test and the measurement ranges were carefully defined, and extreme care was taken in the angular alignment of each. The electrical boresight direction of the main beam was determined at a number of frequencies for two antenna ports with orthogonal polarizations. Results demonstrated a maximum uncertainty between the different ranges of 0.018 deg. An analytical error analysis that predicted a similar level of uncertainty was also performed. This error analysis can serve as the basis for estimating uncertainty in other near-field measurements of antenna boresight.
In the last few years, the interest in millimeter wave systems, like radars, seekers and radiometers has increased rapidly. Though the size of narrow-beamwidth antennas in the 60-200 GHz range is limited to some 20 inches, an accurate far-field antenna test range would need to be very long. The achievement of precision antenna pattern measurements with a 70' or even longer transmission length requires the use of some power that is hardly available and expensive. A cost-effective and more accurate solution is to use a lab-sized compact range that presents several advantages over the classical so-called far-field anechoic chamber: - Small anechoic enclosure (2.5 x 1.2 x 1.2 meters) meaning low cost structure and very low investissement in absorbing material. No special air-conditioning is needed. This enclosure can be installed in the antenna laboratory or office. Due to the small size of the test range and antennas under test, installation, handling and operation are very easy. For spaceborne applications, where clean environment is requested, a small chamber is easier to keep free of dust than a large one. - The compact range is of the single, front fed, paraboloid reflector type, with serrated edges. The size and shape of the reflector and serrations have been determined by scaling a large compact range of ESD design, with several units of different size in operation. The focal length of 0.8 meter only accounts in the transmission path losses and the standard very low power millimeterwave signal generators are usable to perform precision measurements. The largest dimension of the reflector is 1 meter and this small size allows the use of an accurate machining process, leading to a very high surface accuracy at a reasonable cost. The aluminum alloy foundry used for the reflector is highly temperature stable. - Feeds are standard products, available from several millimeter wave components manufacturers. They are corrugated horns, with low sidelobes, constant and broad beamwidth over the full waveguide band and symmetrical patterns in E and H planes. - The compact range reflector, feeds and test positioner are installed on a single granite slab for mechanical and thermal stability, to avoid defocusing of the compact range. - A micro-positioner or a precision X Y phase probe can be installed at the center of the quiet zone. Due to their small size, these devices can be very accurate and stable. Due to the compactness of this test range, all the test instrumentation can be installed under the rigid floor of the enclosure and the length of the lossy RF (waveguide) connections never exceeds 1 meter.
Performance verification of an adaptive array requires direct, real-time sampling of the antenna pattern. For a space-qualified array, measurements on a far-field range are impractical. A compact range offers a protected environment, but lacks a sufficiently wide field of view. Conventional near-field measurements can provide antenna patterns only indirectly. This paper shows how far-field antenna patterns can be obtained in a relatively small anechoic chamber by focusing a phased array in the near-field. The focusing technique is based on matching the nulls of far-field and near-field antenna patterns, and is applicable to conformal or nonuniform phased arrays containing active radiating elements with independent amplitude and phase control. The focusing technique was experimentally verified using a 32-element, linear, L-band array. Conventionally measured far-field and near-field patterns were compared with focused near-field patterns. Very good agreement in sidelobe levels and beamwidths was achieved.
A special purpose 80 element linear phased array antenna was aligned using an iterative phase cycling method. First, the array was aligned to yield maximum main-beam power in the reactive near-field zone and then in the far-field zone. A record of the phase-shifters settings achieved for each zone was kept for use as look-up table during operation. In situ electronic main-beam steering was performed to compare sidelobe performance for the two cases. This report describes the measured results obtained using the phased cycling alignment procedure and compares the measured one-way radiation pattern for the two distance conditions.
Large arrays require large separations between the transmit antenna and the antenna under test (AUT) to measure pattern parameters in the far field. For the subject AUT, a range of 6 miles with a spurious signal level of -58 dB was necessary to obtain the required accuracy. Measurements have been performed on a significantly shorter range without serious degradation. The antenna was focused for the angle of electronic scan and the resulting pattern measured. The theoretical far field patterns were compared with the calculated focused patterns for the short range. The maximum sidelobe error of 1/2 dB occurred at 60 degrees scan. There was no noticeable degradation in beamwidth, gain, or foresight at any scan angle. A 6-mile range would have produced a 2-dB sidelobe error. The measured range reflection level was -50 dB. The transmit dish with sidelobes of 22 dB was replaced with an array that had 40 dB sidelobes. This change reduced the reflections to below the required -58 dB. The antenna was focused using a range calibration technique and the measurements substantiated the theory.
Under many circumstances it is necessary to experimentally estimate the radar cross section of targets in a cluttered environment. A significant reduction in the clutter can be obtained when cross range filtering can be done. In this experimental RC measurement concept, scattering measurements are performed using a moving radar antenna. Thus scattering as a function of target plus clutter versus aspect angle in the near field can be measured. Next, a back projection algorithm can be used to estimate the scattering as a function of position in the neighborhood of the target. The known range to which the signal is to be focussed is used to project back to the target area. An estimate of the RCS at points along a line in the plane of the target is computed. The clutter responses can then be removed from the data, and the remaining target-only values projected forward again (possibly to the far field) to estimate the RCS of the target alone.
The hardware and software developments undertaken to upgrade two far-field measurement facilities - a 12-m anechoic chamber and a 35-m outside range - are described. A method (termed quasi-far-field, QFF) for deriving antenna far-field patterns from a single plane scan at a distance less than the traditional distance of 2D2/? is described. The QFF technique involves pattern sample and subsequent pattern transform and reconstruction, from the easement distance to the far-field distance. A discussion of the limitations inherent in the QFF transform, including range length, is given. Experimental results for measurements made on circular-aperture antennas with both symmetric and asymmetric illumination, and on antennas with elliptical apertures, are described.