Effects of cavities on monopole antenna current distribution and decoupling from mounting structure

The effects of cavities located near the base of a ground plane-mounted coax-driven monopole antenna are investigated. The cavities, are either embedded in the tubular antenna itself or in the region below the conducting surface. Coupled integral equations are formulated and solved to determine the distribution of current on the structure. Two primary functions of the cavities are discussed. 1) At certain frequencies the input impedance of the cavity is infinite and the current at the base of the antenna is forced to zero. A single structure may contain several cavities, each cavity having unique resonant frequencies. It is demonstrated that superposition of these cavities may provide multiple resonant frequencies within a large frequency band. The utility of base cavities in the design of an antenna that is decoupled from the ground plane on which it is mounted is discussed. 2) Regardless of frequency, the cavities affect the distribution of current on the structure and, concomitantly, the standing wave ratio and gain of the antenna. The cavities are shown to be efficient and physically-rigid tools for current-shaping and tuning coax-fed antennas. Results computed from solutions of the coupled integral equations are compared with those from physical measurements.