AMTA Paper Archive

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.

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 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.

RCA-Astro Electronics in Princeton, N.J. designs, develops and tests multiple-beam offset reflector antenna systems in the C and Ku frequency bands for satellite communications. Antenna measurements are performed at the antenna subsystem and the system level and on the complete spacecraft to demonstrate that alignment and performance meet their specification. This paper discussed the antenna range designs and test techniques involved in data acquisitions for contour patterns, cross-polarization isolation and antenna gain characterization. A description of the software required to obtain, analyze and present the data will be included in addition to typical test results.

Wideband RCS instrumentation systems can provide a high degree of range resolution. By combining wideband RCS data with a synthetic-aperture or Doppler processing, the spatial distribution of radar reflectivity can be determined. These systems provide diagnostic capabilities which are useful for locating scattering sources on complex objects and for assessing the effectiveness of modifications. The Proceedings of the 1983 meeting included a paper which described a linear-FM system operating over a 3 GHz bandwidth capable of measuring RCS vs range, cross range, and frequency using a single measurement set-up. This paper analytically demonstrates a procedure for extracting CW RCS patterns from the wideband data obtained using the linear-FM system. By combining the latter and the former processing, it is possible to obtain from a single data array both wideband responses showing the spatial distribution of scatterers and narrowband responses which are the traditional CW RCS patterns. The paper includes experimental verifications of these assertions by comparing results of CW measured data with data extracted from wideband RCS measurements.

The following features have been added to the spherical near-field software set which is available for the Scientific-Atlanta 2022A Antenna Analyzer. Gain Comparison Measurement Probe Pattern Measurement and Correction Thermal Drift Correction Spherical Modal Coefficient Analysis Far-Field, Radiation Intensity, and Polarization Display The addition of the probe pattern correction permits antenna measurements to be made at range lengths down to within several wavelengths of touching. The addition of probe polarization measurement permits three antenna polarization measurements to be made and analyzed as well as two antenna polarization transfer measurements. Correction for phase and amplitude errors attributable to thermal drift is accomplished by the return-to-peak method. Reduction of antenna patterns to spherical modal coefficients is an essential feature of spherical near-field to far-field transforms and is offered as an augmentation to antenna design. Far field display features permit the far fields of antennas to be presented in both component and radiation intensity formats, in circular, linear and canted linear polarization components.

In order to accurately determine the far-field of an antenna from near-field measurements the receiving pattern of the probe must be known so that the probe correction can be performed. When the antenna to be tested is circularly polarized, the measurements are more accurate and efficient if circularly polarized probes are used. Further efficiency is obtained if one probe is dual polarized to allow for simultaneous measurements of both components. A procedure used by the National Bureau of Standards for determining the plane-wave receiving parameters of a dual-mode, circularly polarized probe is described herein. First, the on-axis gain of the probe is determined using the three antenna extrapolation technique. Second, the on-axis axial ratios and port-to-port comparison ratios are determined for both the probe and source antenna using a rotating linear horn. Far-field pattern measurements of both amplitude and phase are then made for both the main and cross components. In the computer processing of the data, the on-axis results are used to correct for the non-ideal source antenna polarization, scale the receiving coefficients, and correct for some measurement errors. The plane wave receiving parameters are determined at equally spaced intervals in k-space by interpolation of the corrected pattern data.

Recent construction of RCS ranges has involved paraboloidal reflectors ranging from a few feet to sixty feet in diameter. These reflectors have required broad band feeds because the typical radar illuminator-receiver is capable of operating over an octave in frequency. This paper will describe a series of feeds which cover any octave in frequency from 100 mHz to 8 gHz, with coaxial line inputs. In addition waveguide-port feeds will be described which cover all of the standard waveguide bands up to 18 gHz. The four basic requirements for all of these feeds are: a) capable of handling the radar power, b) VSWR less than 2 to 1, c) orthomode operation with a 30 db isolation between the two linear polarizations and d) a radiation pattern which is constant with frequency. A fifth problem, for the reflectors which are truncated, is that of providing an elliptical cross section beam over the frequency band.

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.

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 Electromagnetic Engineering Section of the National Research Council of Canada maintains a variety of pattern ranges and associated instrumentation to serve the needs of Canadian industry, government departments and universities. An extensive review of the facilities in 1983 revealed the need for significant modifications to maintain the current state-of-the-art level in antenna measurement technology.

In this paper we present a three-antenna measurement procedure which yields the polarization of an unknown antenna to an accuracy comparable to that of the improved method of Newell. The complete method is based on step-scan motion of the two polarization axes on which the antenna pairs are mounted. As a special case this step-scan procedure includes the usual single axis polarization pattern method of polarization measurement. This three antenna polarization measurement method can be readily automated and is carried out straightforwardly with the assistance of a minicomputer for data acquisition and data reduction. The data reduction method is based on conventional digital Fourier transform techniques and has the advantage of inherent noise rejection. It utilizes a large number of sample points which greatly overdetermine the parameters to be measured. The method has been verified experimentally with measurements made on multiple overlapping sets of three antennas, as is conventional for this kind of procedure. The data are presented for broad-beam antennas of the type used as near field probe horns.

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.

The compact range has been used for many years to measure directive antenna patterns. More recently, however, there has been increased interest to use the compact range for scattering measurements. In order to provide the proper field illumination for such measurements, the traditional designs must be improved in terms of the stray signals coming from the reflector termination. One attempt to improve the field quality in the measurement zone was to use a rolled edge structure added to the basic parabolic reflector. This improved the system performance but required excessively large structures to meet the system requirements. Thus, a novel blended surface was developed which satisfies the measurement requirements without adding large structures. This new design can provide ripple levels no larger that 1/10th of a dB across the target zone as will be shown in the oral presentation.

This paper reports on a study conducted by the Georgia Institute of Technology for the U.S. Army Electronic Proving Ground, Fort Huachuca, Arizona to determine the feasibility of a large (50-foot quiet zone) outdoor compact range located at Fort Huachuca. The range is to be operated over the frequency range from 5 to 100 GHz. The main function of the range would be to measure patterns of low gain antennas mounted on military vehicles and aircraft, to determine whether antenna/vehicle interactions were degrading system performance. The paper presents both the electromagnetic and mechanical rational used as a basis for feasibility. The feasibility study considered many possible compact range configurations including the center fed paraboloidal reflector, the offset fed paraboloidal reflector (both prime feed and subreflector feed) and the dual crossed parabolic cylinder (DCPC) reflectors.

Compact range reflector systems have been previously used for far zone measurements in which case the feed is located at the reflector focus. It has been determined that near zone antenna pattern and backscatter measurements are feasible if the feed is appropriately located. Feed location information has been determined as a function of the radius of curvature of the near zone incident wavefront at the center of the measurement volume. Furthermore, numerous field quality data have been calculated. Field quality is defined as the closeness of the near zone field distribution in the measurement volume to the desired uniform spherical wavefront. The capability to measure near zone backscatter data was demonstrated with a 4-inch diameter cylinder, 4 feet in length. These measurements were made at 10 GHz, for a near zone range radius of 50 feet in the Ohio State University compact range facility. The near zone backscatter response for this cylinder was also calculated using a GTD analysis. A comparison of the calculations and measurements demonstrate the feasibility of the compact range for near zone backscatter measurements. This development leads to the consideration of compact range reflector systems for more general electromagnetic field simulations. For example, by employing an array feed, instead of a single feed element, the incident field in the measurement volume can be controlled in a rather flexible way. It is the purpose of this paper to explore some possible simulations.

System-2000 instruments were created for pattern range applications. The SD-2000 Synchro Monitor was developed in 1983, the MC-2000 Motor Controller in 1984, and System-2000 Host Processor in 1985. Dual black and white video monitors are being used both for graphics and closed circuit television. A rigid body motion application written in FORTH includes graphic primitives to simulate range components. In this paper, a simple aircraft model is installed on a model tower. A square hole in the vertical stabilizer simulates where a probe or antenna is to be located. The hole is offset from the inter-section of model and tower rotation axes, for discussion. Raster and spiral scanning are examined. Spiral scanning required simultaneous control o two drive motors. Emphasis is placed on using System-2000 dual axis features for motor control and graphic imaging of successive model positions.

In this paper we describe the implementation of an antenna pattern recorder using a desktop digital computer to replace the conventional analog electro-mechanical element. This means that all pattern recorder front-panel controls and charts are displayed on and accessed via the computer’s CRT, keyboard and peripherals. It has all the regular features, e.g. choice of scales, pen up/pen down etc., plus a multitude of additional features, obtained owing to the use of a digital computer, which will later be outlined in detail. In spite of the numerous options available, the instrument is very easy to master, requires no preliminary knowledge of computer operation and programming. It is entirely menu-driven and designed to trap most operator errors while maintaining a user-friendly environment suitable for technician-level operation.

This paper is a discussion of the upgrade of an automated antenna pattern data acquisition and analysis system located at the U.S. Army Electronic Proving Ground (USAEPG), Ft. Huachuca, Arizona. The upgrade was necessary as the existing facility was inadequate with respect to frequency coverage, data processing, and measurement speed and accuracy. The upgrade was also necessary in view of USAEPG long range plans to automate a proposed large compact range.