Multi-band bandpass filter based on direct-connected T-shaped stub-loaded resonator

In this paper, we proposed an optimized structure, named direct-connected T-shaped stub-loaded resonator (DC-TSSLR), which generates nine high-performance resonant modes (RMs) while the conventional works only produces two RMs (odd- and even-mode) with the similar structure. Based on this method, a triple-band nine-RM bandpass filter (BPF) is realized with fractional bandwidths (FBW) of 65%, 42%, and 31%, the insertion losses (IL) of 0.11 dB, 0.21 dB, and 0.37 dB and the return losses (RL) of 42.9 dB, 44.5 dB, and 42.9 dB, respectively. In addition, this scheme was further developed into a modularized design for multi-mode filter design. As example, by cascading another DC-TSSLR, a dual-wideband BPF with eight RMs is realized with the FBW of 55% and 38%, the IL of 0.15 dB and 0.4 dB and the RLs of 16.2 dB and 21.7 dB, respectively. The proposed method offers a modularized-design scheme for multiband BPF design as well as the reference for RMs modulation.