This paper proposes a novel miniaturized bandpass filter by loading a stepped-impedance resonator (SIR). Owing to the intrinsic characteristic of SIR, a third-order bandpass filter with SIR is presented, which has a size reduction of 38% compared with the conventional hairpin-line filter. On account of the electrical tape gap effect of a defected ground structure (DGS), further miniaturization is realized by introducing a pair of complementary split-ring resonator (CSRR) DGSs. Besides, frequency selectivity and out-of-band rejection can be improved by adding CSRR DGS and source-load (S-L) coupling structures, which produce two transmission zeros at two side band of passband respectively. The results show that the passband range is 3.4-3.6 GHz, and the final size is reduced by 50.3%.
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