Novel synthesized microstrip line with controllable transmission zeros for harmonic suppressions

By utilizing two quasi-lumped LC tanks, a novel synthesized microstrip line is proposed in this paper. Owing to its good slow-wave properties, the proposed synthesized line is capable of significantly reducing the circuit size. Two transmission zeros, contributed by the LC tanks in the synthesized microstrip line, are introduced to suppress the unwanted harmonic signals. An equivalent lumped circuit model, providing accurate prediction on the transmission zeros, is built to simplify the analysis. Based on the new synthesized microstrip lines, a novel compact rat-race coupler with harmonic suppression capabilities is designed as a demonstration. The size of the rat-race coupler is merely 19.8% of that of a conventional ring design. The second and third harmonic suppression levels are better than -18 and -27 dB, respectively.