Improved SCRLH ZOR-based compact microstrip bandpass filter with wide upper stopband

A novel compact microstrip bandpass filter (BPF) based on an improved simplified composite right-/left-handed (SCRLH) zeroth-order resonator (ZOR) is presented in this paper. The improved SCRLH ZOR (ISZOR) can substantially simplify the filter configuration and shrink the filter size as well as ensure a wide upper stopband. A certain orientation of two ISZORs allows a larger gap to produce the specified interstage capacitive coupling, easing the requirement for fabrication precision. Starting from the purely lumped circuit model, the microstrip ISZOR is efficiently generated by utilizing the implicit space mapping technique, with which the microstrip BPF is then constructed and optimized by the Nelder-Mead simplex algorithm. A three-pole filter with a center frequency of 3.1 GHz and a relative bandwidth of 5% is exemplified to illustrate the design process. The measurement shows that the filter is of good bandpass selectivity, with a transversal size reduction of more than 64% compared to a former counterpart and a rejection level better than 30 dB up to 11.76 GHz (close to 4f(0)).