AMTA Paper Archive

In an industrial park, a clear area sufficient for antenna measurements is very hard to find and even more difficult to justify. The solution to this problem was to use the roof of a large industrial building. To avoid the reflections from industrial stacks used for air conditioning and spray booths and to provide a flat surface sufficient for good ground reflection operation, an elevated ground reflection antenna range was constructed. The range consists of a ground screen 40 feet by 110 feet mounted on a metal framework 14 feet high. A telescopic source antenna tower is located at one end of the range, and an azimuth-over-elevation antenna positioner with a model tower is located on a raised platform at the other end of the range. The range was evaluated using a lightweight field probe and the experimental data compared to calculated data derived from the NEC-BSC2 computer code. An analysis was made of the probe data defining the sources of extraneous energy and their possible reduction. Pattern comparison data is given to illustrate the correlation between the field probe data and the actual uncertainty experienced in making UHF antenna pattern measurements on the elevated ground reflection range. Finally, planned physical improvements to the range are discussed.

At Southwest Research Institute, an automated antenna performance evaluation system has been developed for evaluation of mast-mounted direction finder antennas. This system utilizes a dual-channel receiving system and IF processor with off-line antenna pattern analysis software. Antennas are mounted on a test range which includes computer-controlled antenna positioners, test frequency transmitters, and a data acquisition equipment group. Amplitude and phase data is digitized and recorded for automated off-line antenna performance evaluation. The evaluation software provides a Fourier analysis of the antenna patterns which characterize distortion, alignment, relative phase relationships, amplitude mismatch, and bearing deviations (from theoretical values) for each antenna array.

The Vulnerability Assessment Laboratory (VAL) anechoic chamber at White Sands Missile Range, New Mexico was reconfigured and refurbished during the last part of 1988. This paper will be a facility description of the state-of-the-art Special Electromagnetic Interference (SEMI) investigation facility. Electromagnetic susceptibility and vulnerability investigations of US and, in some cases, foreign weapon systems are conducted by the EW experts in the Technology and Advanced Concepts (TAC) Division of VAL. EMI investigations have recently been completed on both the UH-50A BLACKHAWK and AH-64A Apache helicopters in the chamber. The paper will cover the facility's three anechoic chambers, shielded RF instrumentation bay, computer facilities for EM coupling analyses, and the myriad of antenna, antenna pattern measurement, amplifier, electronic, and support instrumentation equipment for the chambers. A radar cross section measurement and an off-line RCS data processing station are also included in the facility.

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.

The purpose of this paper is to discuss the accuracy requirement of a generic measurement system for in-flight antenna pattern evaluations. Elements of the measurement technique will be described. An attempt is made to distinguish the measurement requirement for a narrow beam radar antenna in contrast to that for broad beam communication antennas. Major elements of the measurement technique discussed include the flight path geometry, the multipath propagation problem, and the measurement errors. Instrumentation requirements consist of the ground segment, the receive and the tracking subsystems, and the airborne equipment, the radar components and the navigation and attitude sensors. Considering the in-flight antenna pattern testing as a generalized antenna range measurement problem, various sources of measurement errors are identified. An error budget assumption is made on each error component to estimate the overall expected accuracy of the in-flight antenna pattern measurement.

A 400 element phased array antenna has been constructed at the GEC-Marconi Research Centre. Each radiating element is fed from its own phase shifter. The radiation patterns of this array have been measured using a recently constructed Cylindrical Near Field Test Facility. The radiation pattern is obtained on a two dimensional grid and contains both amplitude and phase information. It is therefore possible to transform these data back to the array aperture to obtain the array excitation amplitudes and phases. The spatial resolution obtained in the aperture is a function of the angular coverage of the radiation pattern used. The effect of deliberately introduced phase errors on the calculated aperture data is shown.

Separation of the Antenna from the remainder of the system is not possible with a fully active phased array such as MESAR, since each array element has an associated electronic module which contains amplifiers (separate for transmit and receive), phase shifters, switches, etc. The "antenna" is therefore not reciprocal and it also requires a control system. As a result, the system used for pattern acquisition is considerably more complex than that used for testing conventional antennas and some of the traditional parameters are either not obtainable or require redefining. The methods used for testing the MESAR antenna are given together with details of the range equipment involved.

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.

For transforming a Fresnel region pattern to a far-field pattern, we present here two methods, the "discrete beam sampling" method (DBSM) and the "displaced beam" method (DBM), which allow an accurate characterization for both linear as well as circular antenna apertures. Both methods assume a simple Fourier transform relationship between the aperture field distribution and the far-field of the antenna. The Fresnel region field is then essentially perturbed by an aperture quadratic phase error assumed to exist because of the finite distance at which the field pattern is characterized. Numerical simulation and its results are presented to show the accuracy of the reconstructed far-field data. Finally, an error analysis is performed to show the sensitivity of the above two methods.

The nonuniform sampling technique utilizes measured (or simulated) amplitude and phase far-field data at nonuniformly sampled data points and constructs the pattern from these limited number of measured data. The technique relies on the fact that the antenna far-field pattern is proportional to the Fourier transform of a function which is related to the induced current on the antenna. The application of nonuniform sampling technique becomes important in the situation for which it will be difficult (or impossible) to measure the far field at regular intervals. In this paper, the application of the nonuniform sampling technique is demonstrated for antenna pattern measurements. The foundation of the technique is first reviewed and the required mathematical steps for the implementation of the technique is summarized. Both one dimensional and two dimensional cases are reviewed with attention given to the applicability of closed form expressions for the determination of the sampling coefficients. Numerical results are presented and comparison to measurements are shown. In particular, the application of this technique to a recently proposed space-station based antenna experiment is presented.

In recent years there has been an increasing demand for antenna calibrations at millimeter wave frequencies. Because of this the National Institute of Standards and Technology (NIST) has been developing measurement capabilities at millimeter wave frequencies. The development of gain and polarization measurement capabilities have been previously reported. This paper reports on the development of the capability to measure an antenna pattern which has been achieved during the last year. Measurement accuracies of better than 4 dB have been achieved for sidelobes which are 40 dB below the mainbeam peak. NIST is now providing a new measurement service for antenna patterns in the 30-50 GHz frequency range.

To perform antenna measurements, it is necessary that the entire antenna structure is illuminated by a uniform plane wave. Since almost all sources radiate spherical waves, plane wave field configurations can be achieved locally only at very large distances from the source. The proliferation of compact range designs have reduced the distance required to achieve nearly plane wave field configurations to distances which can be satisfied by indoor facilities. While most compact ranges have been designed to create a nearly plane wave field configuration, at Arizona State University an operational compact range exists which creates a nearly cylindrical wave field structure. The pattern measured under cylindrical wave illumination is transformed, using analytical and numerical methods, to obtain the plane wave response of the antenna system. Measurements have been performed, using the cylindrical wave compact range, of a 15 GHz axial waveguide antenna on a 1/10 scale Advanced Attack Helicopter model. The measurements were then transformed and compared with those made of the same antenna system in a plane wave compact range facility.

Implementing an antenna test range has traditionally been viewed as a major and costly undertaking, requiring significant long term facility planning, computer hardware interfacing, and software development. This paper describes a complete low cost, yet high accuracy portable near-field measurement system that was privately built for less than $2,000 and interfaced to a PC compatible computer. The design and operation of this system, including the scanner, microwave hardware, and computer system will be described. This system has since been extended into a commercial product capable of providing rapid and accurate measurements of small to medium size feeds and antennas within a small office or lab space at significantly lower cost than standard antenna test techniques. The system has demonstrated an equivalent sidelobe noise level of less than -50 dB, includes a probe corrected far-field transform and holographic back projections, and can output pattern cuts, contour plots, 3D plots, and grey scale images of antenna performance.

Evaluation methods for analyzing the performance of anechoic chambers have typically been limited to field probing, free space VSWR and pattern comparison techniques. These methods usually allow the users of such chambers to qualify or determine the amount of measurement accuracy achievable for a given test configuration. However, these methods in general do not allow the user to easily identify the reasons for limited or degraded performance. This paper presents a method based on synthetic aperture imagery which has been found usable for finding and identifying anechoic chamber performance problems. Photographs and illustrations of a working SAR imaging/mapping system are shown. Discussions are also given regarding the method's advantages and disadvantages, system requirements and limitations, focusing processing requirements, calibration techniques, and hardware setups. Both monostatic and bistatic configurations are considered and both RCS and antenna applications are discussed. The SAR system constructed to date makes use of a portable HP-8510 based radar placed on a hydraulic manlift for easy system maneuverability and flexibility. The radar antenna is mounted on an 8 foot mechanical scanner directed toward the area to be mapped. An image is processed after each scan of the receive antenna. Measured data and example results obtained using the mapping system are presented which demonstrate the system's capabilities.

Due to the limited size of a compact range, an antenna with low sidelobes, broad bandwidth, broad beam, small physical signature, low scattering level and reasonably high power handling are required. Historically, slot line antennas are circuit board type antennas noted for their thin cross-section, low cost of fabrication, scalability and high package density in array applications. A broadband version, fed by a microstrip line (and therefore easily connected to microstrip transceiver circuits etched on the same circuit board) is described in this paper. Test models with different shapes and using different dielectric materials were built and tested. The measured VSWR, radiation and scattering patterns of the various antenna designs are presented.

The properties of a focused scalar horn-lens antenna are presented. The behavior of the field from the lens to the far field is determined from electromagnetic principles and measured antenna patterns at the focal distance are shown.

Recently an antenna pattern measurement technique has been developed which eliminates the effects of the finite ground plane on which the test antenna is mounted. The scattered fields from the edge of the ground plane can often cause perturbations in the total fields, and thus, result in significant differences in the measured patterns as compared the theoretical predictions. This technique consists of the measurement of the edge diffracted fields and their subsequent subtraction from the original pattern. A simple theoretical model is developed to introduce the subtraction technique, and comparisons are made which show the excellent agreement between theoretical (obtained assuming an infinite ground plane) and “corrected” experimental antenna patterns. Experimental results are given from an open-ended waveguide opening into both circular and square ground planes.

A novel differential phase measurement method is developed. No flexible cables or rotary joints are needed in this method. Phase center positions and phase patterns of two corrugated horns are measured at 105-115 GHz and 176-190 GHz by using this method. Good agreement between the measured values and theoretical values, calculated with the modal matching technique, is obtained. Also a new phase error correction method is introduced. This method makes possible to measure the phase error in the cable and then to remove the error numerically from the results. The accuracy of the phase error correction is limited by the phase measurement device in the system. Experimentally this method is verified at 10 GHz.

The aperture opening design of the subreflector chamber for a dual-chamber Gregorian compact range system is presented in this paper. The subreflector is a serrated edge ellipsoidal reflector. The performance of the subreflector chamber and absorber aperture opening has been evaluated in terms of pattern measurements and by cross-range diagnostic techniques. The results of this evaluation have been used to further improve the design of the aperture opening of the subreflector chamber.

A new approach has been developed to achieve an octave bandwidth, reduced size feed fot compact range reflectors. It can provide highly isolate, orthogonal polarizations with a minimal size, suitable for operation at frequencies down to 500 MHz and below. Its construction is relatively simple, with only a few specific dimensions. The beam-width is compatible with compact range reflector feed requirements. The method uses crossed dipoles over a small circular ground plane, with a rim to equalize the E- and H- plane patterns. Parasitic elements are employed to extend the bandwidth with matching provided via a section built into the feed line. The design was optimized using the Numerical Electromagnetics Code (NEC) computer program.