Parallel ring-lines type rat-race circuit for loose coupling utilizing composite right-/left-handed transmission lines

This paper describes a rat-race circuit with parallel ring-lines by replacing a part of ring-lines with composite right-/left-handed transmission lines (CRLH-TLs). Although a minimum coupling factor of a conventional rat-race circuit is limited by highest impedance of ring-lines that can be manufactured, a coupling factor of the parallel ring-line type rat-race circuit proposed in this paper is decided by differences of admittances of parallel ring-lines. As a result of designing the rat-race circuit with a coupling factor of -20 dB at an operation frequency of 4 GHz, proposed rat-race circuit realizes wideband characteristics of about 35.5 % for the numerical results. Furthermore, up to 40 dBm input power broadband characteristics including reflection, isolation, and couplings can be maintained for the fabricated rat-race circuit.