Analysis of the loop-coupled log-periodic dipole array

A simple theoretical model is described to analyze the loop-coupled log-periodic dipole antenna (LPDA). Computed and experimental data are presented, comprising an investigation into both the far field and the dipole and feeder currents, for a wide range of antenna parameters. Particular attention is paid to a class of parasitic resonances characterized in the swept-frequency radiation pattern by a narrow-band reduction in front-to-back ratio. The calculations show that these resonances involve high out-of-phase currents in adjacent dipole elements, and also predict correctly their dependence on the nature and location of the feedline termination. Design data are presented showing how gain and front-to-back ratio can be optimized and parasitic resonance effects minimized.