Design of a compact rat-race coupler using dual transmission lines

In this paper, a new guided-wave structure, the dual transmission line, is exploited to develop a compact rat-race coupler based on the reduced-length coupler circuit configuration. A conventional transmission line can be equivalent to a set of dual transmission lines which are realized by two parallel high-impedance transmission lines. The circuit configuration of the reduced-length rat-race coupler can approximately provide a 50% circuit size reduction. As the transmission lines of this coupler are replaced by high-impedance-based dual transmission lines, the folding technique can be flexibly applied to earn a further circuit size reduction. The proposed rat-race coupler only occupies a 21.87% circuit size of the conventional one. The measured results of the developed rat-race coupler are in a good agreement with simulated results by full-wave electromagnetic simulator.