Transient radiation from thin, half‐wave, orthogonal dipoles

Abstract

The intent of this work is to ascertain through analytical means whether a circularly polarized radiated field is possible for some antenna configuration with an ultrawideband (UWB) source. Consequently, the behavior of the radiated electric field from identical, thin, half‐wave, electromagnetically uncoupled, orthogonal dipoles in free space that are excited by a finite‐duration UWB voltage is studied. The zero‐order approximate solution for the current distribution along a dipole is used to permit easy calculation and interpretation of the time domain current and radiated far field. The far field is derived, and this result is applied to a finite‐cycle‐sine input (L cycles) to each dipole, where the carrier frequency ƒ0 corresponds to the dipoles' common length and the inputs are separated in time by 0.25/ƒ0. When the dipoles and the feed network are perfectly matched and the observation direction is broadside, the time domain field is extended in time to (L + 0.75)/ƒ0 with respect to the duration of the source L/ƒ0, and circular polarization occurs for (L ‐ 0.75)/ƒ0. For the UWB case (L = 1), the radiated field's duration is 1.75 times the duration of the source, and circular polarization occurs only for one quarter of the period of the source. When the dipoles and the feed network are not matched exactly, the field's duration is infinite, and circular polarization never occurs in the strict sense. However, if the feed network and the dipoles are approximately matched, the field is negligible after a time, and nearly circular polarization occurs.