Asymmetry in Log-Periodic Dipole Antennas and Hybrid Antennas due to the Infinite Balun

Abstract

A Log-Periodic Dipole Antenna (LPDA) employing an infinite balun feed mechanism does not typically exhibit pronounced radiation pattern asymmetry. Similarly, a hybrid LPDA-broadband-dipole antenna does not exhibit significant asymmetry in the frequency range in which the constituent LPDA acts alone in producing radiation. Nevertheless, a slight pattern asymmetry exists in such antennas due to the unbalanced excitation of the exposed center conductor of the internal coaxial line at the feed point where is emerges from the boom. That is, this small portion of the center conductor is driven against the remainder of the LPDA. The small asymmetry can be discerned in the primary lobe of the principal plane patterns but manifests itself in a less subtle manner in the antenna symmetry tests called for in several contemporary EMC standards. Perhaps surprisingly, the effect is observable at frequencies well below those at which the active region is located near the feed point. It is worthwhile to note that this behavior can exist with no common-mode (CM) current on the feed transmission line, rather only on the LPDA booms. We examine this behavior in some detail using full-wave analysis of a typical LPDA antenna including a detailed feed model. We also provide measured data showing clearly similar behavior.