A Compact Log-Periodic Branch-Line Balun with an Octave Bandwidth

Abstract

Conventional half-wavelength branch-line baluns are bulky and suffer from narrow operating bandwidth. The use of log-periodic type can extend the bandwidth greatly at the expense of the circuit area. In this article, a planar branch-line balun, characterizing a wide operating bandwidth and a compact size, is developed. The analysis of such a kind of three-port network starts from its symmetric four-port network. Even- and odd-mode analyzing methods are adopted to determine the line impedance of the branches. The size reduction of the developed balun is achieved by periodically loading a slow-wave structure on the transmission line itself. Also, the slow-wave factor is studied to give design guides in practical designs. With five-section topology, an octave bandwidth log-periodic balun is developed, where its circuit area is approximately halved as compared with the traditional log-periodic balun for the same operating bandwidth. To validate the study, a demonstration balun, operating from 2 to 4 GHz, is optimally designed, built and tested. Results from measurements confirm the predicted performance with good consistency.