A microstrip diplexer from metamaterial transmission lines

A planar diplexer using a metamaterial directional coupler is presented. It is composed of a conventional microstrip line and a metamaterial transmission line, which is a dual composite right/left-handed (D-CRLH) transmission line. The metamaterial coupler works in dual modes and leads to alternative coupling coefficients. At the right-handed mode, it is under weak coupling condition. It works under tight coupling at the left-handed mode, while a quasi 0-dB coupling is achieved. Then, the input microwave is delivered to the coupled port and the transmitted port with respect to the operating mode of the diplexer, respectively. The proposed diplexer is compact and owns a deep rejection band between two output bands. An insertion loss less than 1.7 dB and isolation higher than 20 dB are achieved in the demonstrated design. Simulated results agree well with measurements.