Microstrip Balun Over High Impedance Wire-Ground

This letter presents a novel idea for the design of coupled line balun, where the operation relies on rejecting the common mode due to the presence of a high impedance wire-ground. This concept is verified by circuit modeling, EM simulations, and practical measurements of the proposed structure. The simple, though accurate, circuit model provides clear understanding of the concept of operation and simplifies the design flow and optimization. The novel design exhibits reliable performance at 2.4 GHz with a 50% bandwidth (1.8-3 GHz), an amplitude imbalance less than 0.5 dB, a return loss better than 15 dB, and a phase imbalance less than 10^deg.