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

In 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 degrees 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.