Radiation patterns of unsymmetrically fed prolate spheroidal antennas

Abstract

This paper describes the radiation pattern of the un-symmetrically-fed prolate spheroidal transmitting antenna. Maxwell's equations are solved in prolate spheroidal coordinates subject to the boundary conditions. The prolate spheroidal functions are expressed in the form of power series and Laurent series. Radiation patterns have been obtained for antennas of three different lengths up to about one wavelength long, for length/thickness ratios of about 5/1, 10/1,22/1, and 316/1, and for nine unsymmetrical gap locations as well as for the symmetrically-fed cases. It was found that the two most important factors affecting the radiation pattern of a fairly thin antenna were the location of the gap and the electrical length. For antennas less than a half wavelength long, the pattern was the usual symmetrical figure eight and was essentially independent of the location of the gap (except for magnitude changes due to the different gap impedances). For antennas two-thirds to three-quarters of a wavelength long the figure eight patterns could be "bent" in the direction of the longer element, and for antennas one wavelength long or longer minor lobes began to appear.