A quad‐band bandpass filter using split‐ring based on T‐shaped stub‐loaded step‐impedance resonators

Abstract

AbstractIn this article, a novel design method of multiband bandpass filters (BPFs) which have flexibly tunable center frequencies is presented by using T‐shaped stub‐loaded step‐impedance resonators (SLSIRs)’ multimode characteristics. By controlling the impedance ratios and the length ratios of the T‐shaped open stubs, the center frequencies of a multiband BPF can be decided. To verify the effectiveness of the proposed method, a 1.34/2.2/3.3/4.5 GHz quad‐band filter using split‐ring based on SLSIRs is designed, fabricated, and measured. The proposed filter is composed of a pair of tri‐section T‐shaped open stubs transmission lines, which form a folded split‐ring. By controlling the impedance ratios, the length ratios of the open stubs and the width of the gap of the folded split resonance loop, the quad‐band center frequencies are flexibly tunable. Owing to the 0° feed structure, seven transmission zeros can be realized near the pass‐band for enhancing the selectivity. The measured results show good agreement with the theory and the EM simulation.