Wide‐stopband and high selectivity step impedance resonator bandpass filter using T‐network and antiparallel coupled line

Abstract

A wide-stopband attenuation with high-selectivity step impedance resonator (SIR) bandpass filter (BPF) is presented in this study. The T-network stubs are used to produce transmission zeros (TZs) close to the centre frequency ( f 0 ) with a high-selectivity. Moreover, the antiparallel coupled lines can produce a TZ at spurious frequency of conventional SIR BPF for all step impedance ratio ( K ). For the validation, a half-wavelength ( λ /2) SIR BPF with T-network stubs at the input/output ports was designed at f 0 of 2.6 GHz. From the experiment, the insertion and return losses at f 0 were determined to be 0.91 and 21.25 dB, respectively. The insertion and return losses were better than 1.1 and 16.5 dB, respectively, of the 180 MHz bandwidth. The TZ frequencies ( fZ 1 , fZ 2 , and fZ 3 ) were located at 1.69 GHz (0.63 f 0 ), 2.98 GHz (1.12 f 0 ), and 3.17 GHz (1.19 f 0 ), respectively, with a high selectivity. The stopband attenuation was better than 25 dB from DC to 2.23 GHz and from 2.89 to 9.82 GHz.