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Conventional pattern measurements are difficult to apply when the aperture is very large (250 lambda or more), particularly in the case of a relatively fragile antenna structure intended for a space application. Near field techniques can offer a solution, but may need a relatively large R.F. enclosure and custom instrumentation. This paper examines various alternative approaches in the case of the 15 m planar array under development at CAL for Radarsat. Specifically, the techniques under consideration include planar probing, cylindrical probing, planar cylindrical probing, intermediate range spherical probing, and some special variants. It is shown that the fact that the Radarsat antenna generates shaped beams as opposed to pencil beams impacts the relative accuracies achieved by these techniques to a very significant extent. The data collection and processing time, the size of the anechoic chamber needed, and the instrumentation requirement are also important considerations.
Spar Aerospace, along with other aerospace companies, have experienced an evolution in the development of spacecraft antennas over the past 20 years. Spacecraft antennas originated as either simple antennas providing figure of revolution patterns for spin stabilized communication satellites or simple monopoles for telemetry and command purposes. Communication satellite antennas later evolved to shape beam reflector type configurations. Spaceborne antennas are now moving to even larger reflector antennas and to planar arrays for radar applications. This evolution in spaceborne antennas has been followed by a parallel evolution in antenna test facilities and facilities requirements.
Near-field measurement techniques are widely used today for antenna far-field pattern characterization. Since the 60's, much has been done concerning accuracy. The three main coordinate systems, planar, cylindrical, and spherical have been investigated. probe corrections have been introduced [1] - [6].
Near field ranges have been used extensively to measure antenna parameters. These ranges have been shown to be very accurate for measuring absolute gain, polarization, and gain patterns. Most antennas are intended to be used with a receiver, a transmitter, or both. In many cases, it is important to characterize the antenna and active electronics as a system.
Measurements of the Electromagnetic Field in the quiet zone of Scientific-Atlanta's Model 5753 Compact Range are presented. The Model 5753 is believed to be the largest high frequency compact range yet built and measurements demonstrate a quiet zone exceeding 8 ft. high by 12 ft. wide. Both field probe measurements and pattern comparison measurements are presented in the operating frequency range of 1-94 GHz.
The planar scanning system is commonly used in the near field testing of high gain antennas, where the rectangular measurement grids are used. The polar grids are also used, which are more convenient when the antenna aperture is circular. In the planar scanners the measurements are carried out in the x-y plane in increments of both x and y. The result of the measurement is an mxn matrix of the near field data consisting of m cuts with n data points per each cut. The far field patterns may then be calculated, using the near field data, by the aperture field integration or the modal expansion methods [1]. In this paper the aperture field integration method is studied, where the far field components can be calculated from [1] - [2].
The geometry of a dual parabolic cylindrical reflector system and its projection on the plane of symmetry are shown in Fig. 1. It consists of a point source f and two parabolic cylindrical reflectors S1 and S2 with curvatures in two orthogonal planes and of focal lengths F1 and F2, respectively. Alpha is the angle between the generator of the sub-reflector and the main beam direction. It is considered positive if the generator of S1 rotates towards the main reflector and negative if it rotates in the other direction. The feed orientation is specified by the angle gamma which is the angle between the feed axis and the normal from the feed point f to S1. The feed angle is 2f , which is the angle subtended by the sub-reflector in the principal planes. The sub-reflector geometry is selected such that it subtends equal angles from the feed in two orthogonal planes. The main reflector geometry is selected to intercept all reflected rays from the sub-reflector. The projected aperture of the main reflector is rectangular in shape, the sides of which are denoted as A and B. The ratio between these aperture sides is given by [1]. The separation between the two reflectors may be increased by any value delta which results in reducing the aperture dimensions. The feed radiation pattern is assumed to be rotationally symmetric and its electric field distribution in the feed coordinates is represented by cos??. If the feed is vertically polarized in the asymmetric plane (along y-axis), the y and x-components of the aperture field are the co-polar and cross-polar components, respectively. The feed may also be horizontally polarized along the unit vector [sin (?+a) i + cos (?+a) k] in the symmetric plane. In this case the co-polar and cross-polar components of the aperture field are the opposite of the above case.
The advent of improved compact ranges has promoted the development of a test body, named the almond, to facilitate the measurement of scattered fields from surface mounted structures. A test body should at least have the following three features: (1) provide a very small return itself over a large angular sector, (2) provide an uncorrupted and uniform field in the vicinity of the mounted structure and (3) have the capability to be connected to a low cross-section mount. The almond satisfies the first two requirements by shaping a smooth surface which is continuous in curvature except at its tip. The name almond is derived from its surface similarity to the almond nut. The surface shaping provides an angular sector where there is no specular component. Hence, only low level tip and creeping wave scattering mechanisms are present resulting in a large angular quiet zone. The third requirement is accomplished by properly mounting the almond to a low cross-section ogival pedestal. The mount entails a metal column between the almond and the pedestal covered with shaped absorbing foam. These contoured pieces hide the column and form a blended transition from the almond to the pedestal and yet allow an unobstructed rotation of the almond. Backscatter pattern and swept frequency measurements performed in our compact range illustrate the scattering performance of the almond as a test body. The almond body alone has a backscatter level of -55 dB/m(squared) in its quiet zone. Comparisons of measured hemisphere backscattered returns on the almond are made with those calculated of a hemisphere over an finite ground plane for both principal polarizations for a verification performance test. * This work was supported in part by the National Aeronautics and Space Administration Langley Research Center, Hampton, Virginia under Grant NSG 1613 with the Ohio State University Research Foundation.
It is sometimes necessary to tolerate a narrow vertical obstacle in proximity to an antenna system. This study quantifies the effects on the antenna horizontal pattern of an obstruction with vertical dimension large compared to the antenna and horizontal dimension small compared to the antenna. Both dimensions are large compared to wavelength.
Electronic scanning phased arrays are being used more and more in radar, EW and communication systems. The development of such an array can be divided into two separate parts: development of the radiating elements and development of the beam forming network. The development of these two parts is often done in parallel and the radiating elements should always be developed taking into consideration the whole array and not only single elements.
This paper presents data from facility evaluation tasks on current projects. The data were obtained on outdoor free-space pattern test facilities, and in anechoic chamber RCS test facilities.
This paper describes the mechanical as well as electrical measurement of doubly curved reflector antennas. The techniques developed for measurement of the new Canadian RAMP Primary Surveillance Radar antenna are described. Instead of a conventional full size template fixture to measure the antenna contour accuracy, an optical twin-theodolite method is used. The problems of the method are discussed and a new simplified analysis for calculating reflector error of doubly curved antennas is presented. Reflector errors are calculated and displayed concurrent with the actual measurements. The measurement of primary and secondary patterns for such antennas are described. Included are brief descriptions of the improved Andrew pattern test range and anechoic chamber facilities.
The paper describes a simple feed horn designed to illuminate an antenna test range used to measure broad bandwidth antenna patterns with rotating linear polarization. Principal requirements of the feed are equal E and H plane beamwidths with minimal sidelobes in all planes. These characteristics are required to avoid undesirable pattern modulation caused by varying specular scatter and unequal beamwidth vs rotation angle. A survey of pyramidal, conical, and diagonal feed horn patterns revealed that each configuration has high sidelobes in at least one plane making it undesirable for the intended application. Both the pyramidal and conical horns have high side lobes in the E plane. The diagonal horn has very good sidelobe characteristics in the principal planes, but has 13 to 16 dB sidelobes in the diagonal plane.
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.