AMTA Paper Archive

A general theoretical approach is formulated to describe the complex electromagnetic environment of an N-element array. The theory reveals the element-to-element interactions and multiple reflections within the array. To experimentally verify some features of the theory, measurements on experimental array panels in various configurations were made. These array panels consisted of 256 microstrip radiating elements. In each of the configurations both the near-field and portside signals were measured to study the interactions between these panels. In particular, the effects of open-circuited array panels on the radiation pattern of a single panel are observed both in the near field and in the far field. It is found that internal scattering is the main mechanism of interaction between panels, rather than reradiating of signals received from adjacent panels. The effects of scattering are observable at the -50 dB level.

Techniques developed for the design of shaped, off-set reflector antennas have been applied to the design of compact ranges. Shaped optics which map an axially symmetric feed pattern into an elliptical aperture distribution have been designed. Some of the major design considerations for this type of system are examined in this paper. The design has been verified both analytically and experimentally.

When performing antenna pattern measurements on far-field antenna test ranges or in anechoic chambers, one of the main problems concerning the pattern accuracy is range reflections. Previous works dealing with this have been limited to the one-dimensional case.

An antenna radiation pattern measurement technique which allows near real time pattern capture is presented. This technique uses relatively simple field probles and detectors to cover a reasonably broad operating band. The captured pattern data is digitized with a resolution of 1.0 degree and has an angular range of 150 degrees. Many captured patterns or snap-shots could be recorded during a given time interval and later viewed for diagnostic evaluations where rapid changes in the pattern are expected.

This paper will discuss the disadvantages of a conventional Earth Station Antenna (ESA) pattern measurement technique compared with an alternative, time-proven technique. These measurement techniques are used to verify that a particular ESA has been properly constructed, focussed correctly and meets or exceeds a manufacturer's pattern envelope and gain specifications. These tests are performed on-site through a satellite link.

This paper describes a novel technique for acquisition of far-field antenna patterns from a very low side-lobe antenna. The low side-lobe requirement imposes stringent multipath restrictions on the measurement range and to accommodate this requirement a vertical range configuration is employed rather than the more conventional range which is parallel to the earth's surface. To assure accurate measurement of side-lobe levels, multipath levels were specified at minus seventy dB (-70 dB) relative to the direct-path, peak-of-the-beam level. In this novel range configuration, an Antenna Under Test (AUT) is oriented to face skyward and operated in a receive mode with E-Field illumination provided from an airborne source. An optical tracker provides data of airborne source location and time-division multiplexing of both frequency and antenna beam position enable optimization of data acquisition efficiency. Post-acquisition processing provides de-interleaving of the desired beam(s)/frequency(s). This paper will present a discussion of the problems encountered and the techniques employed to overcome them in the design of this range. A description of the range will also be presented.

The Georgia Tech Research Institute is designing a large outdoor compact range for the U. S. Army Electronic Proving Ground at Ft. Huachuca, Arizona. This range will be used to measure patterns of antennas installed on aircraft and vehicles. The goal of full hemispherical coverage with vehicles weighing up to 140,000 pounds has resulted in a unique positioner design, described in this paper. The 5-foot diameter quiet zone is centered 42.5 feet above the ground. The positioner's azimuth over elevation geometry keeps even large systems inside the quiet zone through the full range of positioner motion. The turntable is driven in continuous azimuth rotation by a hydraulic motor. The tilt table is driven through its -1 degree to +91 degree elevation range by two hydraulic cylinders. The tower is designed to carry a 140,000 pound vehicle in a 100 MPH survival wind. The structure consists of two steel frames, joined at the top. Both are enclosed in sheet metal shells to minimize scattering into the quiet zone.

It is well known that the compact range can be and has been used very successfully for scattering measurements. Recently, the compact range at The Ohio State University ElectroScience Laboratory was used to measure the patterns of two 8-foot diameter reflector antennas and their microwave horn feeds. Very good measurements have been achieved. In the paper, the results of the horn antenna measurements are presented while the results of the reflector pattern measurements are discussed in another paper. [1].

This paper describes methods commonly used by anechoic chamber manufacturers to characterize chamber performance. Test procedures depend first on the purpose of the test; second on the purpose of the anechoic chamber and third on the amount of information required. Most anechoic chambers are built for a specific use. In order to prove its design, the test will be done accordingly. In most anechoic chambers one measures the reflectivity level because this is a measure for the accuracy on future measurements when the chamber is in operation. Anechoic chambers can vary from Antenna Pattern Test Chambers to Radar Cross Section Test Chambers, Electronic Warfare Simulation Chambers and Electro Magnetic Compatibility Test Chambers. Each type of chamber will have its specific evaluation technique. Some techniques can be done by the chamber user himself. Other methods need some special equipment that will or can only be used for that particular test method. Some customers want to do their own calibration on a regular basis. They can purchase this special equipment from the chamber manufacturer, if necessary. More complicated methods make use of computer controlled equipment. The data required can be taken in the chamber. This can be done relatively fast. All sorts of information about the chamber characteristics can be obtained in a later stage in a different format by use of the right software. This paper gives possible evaluation methods for different types of anechoic chambers. Detailed information about each method can be obtained from Emerson & Cuming.

Microwave holographic metrology is considered to be a key technique for achieving improved performance from large reflector antennas, especially at the shorter wavelengths. An important benefit of microwave holography is that the mathematically transformed data yields precise information on panel alignments on a local scale [1-5]. Since the usage of the holographic technique requires both the amplitude and phase data of the measured far-field patterns, one must carefully assess the impact of systematic and random errors that could corrupt the data due to a variety of measurement error sources.

The holographic antenna measurement system developed for the COMSAT Labs far-field range was tested with various antennas including axis-symmetric reflector antennas, offset single and dual reflector antennas, and phased-array antennas. Numerous examples which demonstrate the value of holographic measurement as an antenna diagnostic tool are presented. Microwave holography utilizes the Fourier transform relation between the antenna radiation pattern and the antenna aperture electromagnetic field distribution. Complex far-field date are collected at sample points and a Fourier transform is performed to give amplitude and phase contours in the antenna aperture plane. These contours facilitate reflector antenna diagnosis. The feed illumination and blockage pattern are provided by the amplitude distribution. The aperture phase distribution allows simple determination of deviations in the reflector surface and feed focusing. For phased-array antennas, the contours provide a measure of the complex element excitation. Measurement system parameters including pointing accuracy, phase stability, and measurement dynamic range were studied and refinements implemented to increase speed, accuracy, and resolution of the contour plots. To prevent aliasing errors, sampling criteria were explored to determine the optimum parameter ranges. For most antenna positioners, the antenna center is displaced from the rotation center. The importance of properly accounting for this displacement is discussed in the final section.

The techniques of near-field antenna pattern measurement can be extended to near-field RCS measurement. The motivation for doing so is precisely the same as that for near-field antenna measurements; i.e., the convenience of an indoor antenna range, and an improvement in accuracy. Although the near-field measurement problem is solvable in principle in a manner analogous to the near-field antenna problem, it requires a significantly larger amount of time to take the necessary data, and to subsequently process the data to obtain useful quantities. BDM is currently involved in an on-going program to evaluate the feasibility of near-field bistatic RCS measurements. At the time of this writing, a complete set of mathematics has been formulated to handle the probe correction and data processing. The hardware has been built, software development is near completion, and the analysis of canonical scattering objects has been completed. Experimental data soon to be taken for these objects will be presented. It is hoped that the technique will prove to be a practical approach to RCS measurements.

For over a decade the National Bureau of Standards has utilized the Planar Near-field Method to accurately determine antenna gain, polarization and antenna patterns. Measurements of near-field amplitudes and phases over a planar surface are routinely obtained and processed to calculate these parameters. The measurement system includes using a cw source connected to an accessible antenna port and a two channel receiver to obtain both amplitude and phase of the measurement signal with respect to a fixed reference signal. Many radar systems operate in a pulsed-cw mode and it is very difficult if not impossible to inject a cw signal at a desired antenna port in order to calibrate the antenna. As a result it is highly desirable to obtain accurate near-field amplitude and phase data for an antenna in the pulsed-cw mode so that the antenna far-field parameters can be determined. Whether operating in the cw or pulsed-cw modes, one must be concerned with calibrating the measurement system by determining its linearity and phase measurement accuracy over a wide dynamic range. Tests were recently conducted at NBS for these purposes using a precision rotary vane attenuator and calibrated phase shifter. Such tests would apply not only to measurement systems for determining antenna parameters but also to systems for radar cross section (RCS) measurements. The measurement setup will be discussed and results will be presented.

A 15-meter diameter self-deployable antenna has been developed which utilizes the hoop-column structural concept with a gold-plated molybdenum mesh reflector. This antenna was developed to determine if a system could be designed and built with the dimensional tolerances necessary for in-space operational performance and for use as a test article in a ground based technology development program. One feature of the design is the provision for reflector surface shape control by cable adjustment. The antenna was deployed and tested at the Martin Marietta Denver Aerospace Near-Field Test Laboratory to measure its surface shape and its electromagnetic performance. RF test results show very good agreement between predicted and measured radiation patterns. The antenna is currently undergoing modifications which will allow automated surface adjustments and adaptive feeds to be utilized for further improvement in the electromagnetic performance. Controls, structural, and simulated thermal deformation tests will be integrated with future electromagnetic tests.

A bicone telemetry and command antenna is a stack of two physical antennas with toroidal patterns which have radiation patterns which are omnidirectional in the azimuth plane, perpendicular to the transfer orbit spin axis of the satellite, but are directional in the elevation plane. Each of the two physical antennas,, which operate at different frequencies and polarizations to avoid feedback, has two independent RF inputs (for redundancy) making it actually a four antenna configuration. Each physical antenna consists of three components, which are the feed input section with dual RF inputs, a circular polarizer and a radiation structure comprised of slots, in the circumference waveguide structure, which feed the circumferential conical horn necessary to obtain the required directivity in the elevation plane. The procedures and the problems encountered in constructing and testing each of these parts, as well as the components necessary to permit their testing as independent units is discussed. Because of the broad radiation patterns which characterized these omnidirectional C-Band and K-Band antennas, special consideration had to be given to the measurement of the antenna patterns. These problems and their solutions are highlighted in the paper.

The Boeing V-22 Osprey tilt rotor aircraft is a candidate platform for use as an airborne surveillance radar system. The impact of radar RF energy scattering from the aircraft's large propellers is a concern due to the potential for interference with an airborne pulse doppler radar where frequency changes are used to discriminate moving targets from ground clutter. In order to ascertain the effects of the scattering, a unique measurement system was devised for recording the time modulated antenna pattern of an array antenna.

Many experimental and analytic studies on travelling waves have been performed in relation to their radiation properties for antenna applications. One common structure that has supported a fast travelling wave is a slotted waveguide. Such structures can also support travelling waves from a scattering viewpoint. This aspect was verified by incorporating a trough in an almond test body to observe its scattering characteristics using aspect angle patterns, frequency spectra and transient signatures from compact range measurements at the ElectroScience Laboratory, OSU. The travelling wave behavior is also correlated to the calculated travelling wave propagation constant for this structure with good agreement.

Prior to 1965, there were no geostationary communications satellites. But since the success of Early Bird (INTELSAT I), in 1965, an explosion has occurred in the number of communication satellite (communication channels) in the geostationary orbit (GSO). Because of this increase of satellites in the fixed satellite service (FSS), coupled with the even greater demand for the more desirable positions above land masses and additional channels in the 4/6 GHz band, the FCC and CCIR are attempting by every possible means to increase the number of satellites and channels available. Interchannel interference, of course, must not be increased. Besides enlarging the frequency spectrum available, the use of orthogonal polarizations and closer inter-satellite spacings are under consideration as a means of increasing channel capacity. In 1981, the FCC proposed a decrease from 4 degrees to 2 degrees spacing for satellites operating at 4/6 GHz in the FSS. While many current users prefer larger "close" spacings (2.5 to 3 degrees), 2 degrees will probably become the required inter-satellite spacing for the FSS and is the currently accepted antenna design requirement. When satellites are more closely spaced, their ground terminal antennas must not only have narrow main beams but must also have very low side lobe levels to avoid interference with adjacent satellites. The CCIR has established a new reference pattern, 29-25*log(phi) (ref. CCIR Recommendation 465-1), shown as the overlay in Figure 5. Ground terminal antennas with a diameter to wavelength ratio greater than 100 must comply. This study set out to determine whether this specification could be applied to a carefully designed antenna with a diameter to wavelength ratio as low as 50, 3.5m for the 4/6 GHz band. The future market for such an antenna would be in a low cost earth terminal (LCET) intended for the 'smaller users', who require rapid, reliable communications. The small users may lease channels until the time when their increase in data transmission requirements and the decrease in the cost of earth terminals justifies acquiring their own small earth terminal.

This paper discusses the configuration and performance of the HP 8510B Network Analyzer for antenna range measurements. Both manual and automated pattern measurements can be performed. The HP 8510B, introduced in May of 1987, adds several key new features to the HP 8510A which improves its performance in antenna test applications. The HP 8510B now supports an external mixer configuration which provides significant improvements in measurement sensitivity. An analog output has been added to the HP 8510 which allows manual antenna measurements to be made conveniently and quickly. A "FAST CW" mode has also been implemented in the HP 8510 which provides automated data acquisition rates in less than one millisecond per measured point.

The AM Broadcast service area depends on the radiation pattern of the antenna employed. The approach used here to compute the service area requires the radiation pattern of the monopole antenna mounted on a perfectly conducting plane earth. The effects of the ionosphere and the finitely conducting earth can then be calculated and the service area determined. The use of the theoretical thin monopole radiation pattern for the determination of the actual service area is not very accurate. The best solution is to use the measured radiation pattern. But due to the large dimensions of the antenna it is more practical to use a scale model for the measurement.