Ellipsoid distribution of antenna array elements for obtaining hemispheric coverage

#The present invention is directed to antenna array configurations and, in particular, to an improved element distribution that employs an ellipsoid (an ellipse of revolution) for obtaining substantially uniform hemispherical coverage.

#Phased array antennas are a commonly employed expedient for obtaining a prescribed antenna profile that may provide communication coverage over a defined surface area in space. For satellite communications, in particular, where coverage of the entire upper hemisphere is required, previously proposed arrays have involved multiple planar arrays configured as a tetrahedral arrangement or as a spherical or hemispherical dome. Exemplary illustrations of such hemispherical and spherical approaches are presented in the October 1981 issue of "Microwaves" and in a technical brief of the Aerospace Systems Division of Ball Brothers, entitled "Electrically Steerable Spherical Array, Executive Summary", October 1981.

#Unfortunately, investigation has shown that a hemispherical array provides illumination of only one-half of the surface area at the horizon, while the entire surface area is illuminated at zenith. This results in an excess gain as the elevation angle increases, which is inherently inefficient. Tetrahedral arrays have similar problems at the intersections of the planes.

#FIG. 1 shows a three dimensional view of a hemi-ellipsoid configured array of antenna elements; and

#FIG. 2 shows a hemi-ellipse in the (x-z) plane for facilitating an understanding of the selected geometry, size and spacing of an antenna array in accordance with the present invention.

#Referring to FIG. 1, there is shown a perspective view of the antenna surface configuration of the present invention, in which an array of antenna elements 11 are distributed over a surface 10 having the shape of an ellipse of revolution. In FIG. 1, the axis about which the ellipse is rotated is denoted as the z-axis 12, with only a portion of the elements 11 being shown for purposes of clarity. It will be realized that for complete hemispherical coverage elements 11 are distributed over the entirety of surface 10. Preferably the spacing between adjacent elements is substantially uniform over the entire hemi-ellipsoid surface 10; however, if the spacing is too close, this may result in mutual overlap in the coverage profiles of individual elements. In the latter instance the spacing between elements is adjusted to prevent mutual interference as will be explained below.

#In order to provide a complete understanding of the mechanism of the ellipsoid geometry of the array surface and the procedure for choosing element locations and spacing in the surface of a hemi-ellipsoid, as shown in FIG. 1, a description of the selection of ellipsoid characteristics for a practical example of the application of the present invention to satellite communications will be described with reference to the ellipse profile depicted in FIG. 2.

#In the present example, the array of antenna elements 11 is oo provide complete satellite communication coverage for the hemisphere above the antenna (upper hemisphere) for sinnals at 2.0 GHz. The required G/T is selected to be such that the antenna gain must be a minimum of 10 dB. In general, the selection of an individual array element will be based on required bandwidth, efficiency, ease of fabrication, physical size, etc. The procedure described here applies generally to the class of elements whose transfer characteristic is predominately determined by the quantity of energy passing through some arbitrary surface (e.g. microstrip patch, spiral, horn feed, etc.)