Optimized helical monopole antennas for portable VHF communication devices

Abstract

The helical monopole antenna has been used extensively as compact antenna for portable VHF communication devices. Conventionally, it has a constant pitch and radius along its body, these being chosen to make the antenna resonant at the frequency of interest. By allowing both radius and pitch to vary along the antenna, it is possible to optimize the antenna in terms of efficiency and bandwidth. The current paper investigates the effect of antenna environment upon such optimizations and, in particular, the situation where the VHF antenna is fed against a small radio casing instead of a perfect ground plane. It is shown that the optimization is little affected by such changes to environment and that the optimized designs are robust. The current paper concentrates on optimizations of pitch since helix radius will need to be constrained in the case of a portable radio.