An omnidirectional circular antenna array excited parasitically by a central driven element

There is a continuing practical need for antennas which radiate a pattern which is omnidirectional in azimuth, but with a specified shape in the vertical direction. One example is that of a broadcast station, where the radiated power should be concentrated along the ground. Conventional solutions of this problem make use of an array of colinear elements, or of sectionalized elements, or of top-loaded antennas. The height of the antenna is often of the order of a half-wavelength. Page1 proposed that a circular array be used for this purpose, and showed that an array of several radiators distributed about the circumference of a circle, and with an element at the center, could be made to radiate an omnidirectional pattern in azimuth, with a null at any prescribed vertical angle. In the antenna described by Page all of the elements are driven, which entails complicated matching and phasing networks. It is the purpose of this paper to describe an antenna, similar in geometry to that of Page, in which only the center element is driven. The remaining elements are excited by mutual coupling from the center element, and the desired null in the vertical pattern can be controlled either by varying the length of the parasites, or by inserting a tuning reactance at the center of the parasites.