The Complete Surface-Current Distribution in a Normal-Mode Helical Antenna

Abstract

An investigation of the surface-current distribution in a normal-mode helical antenna (NMHA) is reported. This enables precise prediction of the performance of normal-mode helical antennas, since traditional wire-antenna simulations ignore important details. A Moment-Method formulation was developed, using two geometrically orthogonal basis functions to represent the total nonuniform surface-current distribution over the wire of the helix. Extended basis functions were used to reliably treat the discontinuity of the current at the free ends. A surface kernel was used all over the antenna's structure. The surface-current distribution was computed for different antenna geometries, such as dipoles, loops, and helices. For helices, the currents were investigated for different pitch distances and numbers of turns. It was found that the axially-directed component of the current distribution around the surface of the wire was highly nonuniform, and that there was also a significant circumferential current flow due to inter-turn capacitance, both effects that are overlooked by standard filamentary current representations using an extended kernel. The impedance characteristic showed good agreement with the predictions of a standard filamentary-current code, in the case of applied uniform excitation along the local axis of the wire. However, the power-loss computations of the present technique produce significantly different results compared to those well-established methods when the wires are closely spaced.