Feedline Interference with Dipole Performance Theory and Experiment

The characteristics of thin-wire, finite-gap λ/2 and λ/4 dipoles fed with a length of two-wire transmission line have been studied using matrix methods [1]-[4]. Transmission line length and gap size have been varied obtaining the complete solution (current distributions, impedance and far field beam patterns) in each case. Significant departures from the ideal dipole characteristics are found even for electrically small gap sizes and transmission line lengths. When the departures are small, they can be described adequately by a cross polarization ratio and by null fill-in. These effects are minimized by choosing transmission line length It equal to nλ, where n is an integer. Departures from ideal dipole performance are significantly increased as dipole length l decreases, gap length d increases, transmission line length lt, increases and current imbalance increases. Some of the effects noted have been verified experimentally.