Theory of the Balanced Helical Wire Antenna

Abstract

A finite, balanced helical wire antenna excited by a delta‐function generator is investigated. The geometry of a helical wire is examined and an exact integral equation for the current distribution along the antenna is formulated. For a thin helical wire, the exact integral equation can be approximated by a one‐dimensional approximation, which can be solved by a Fourier‐series method. In the solution, the logarithmic singularity at the driving point is taken into account and is subsequently subtracted out. Thus, the input admittance is obtained. The theoretical results are compared with the experimental result. It is found that the current amplitude decays exponentially. This is contradictory to the usual assumption that the current along a helical antenna is a traveling wave with a constant amplitude.