A novel miniaturized bandpass filter (BPF) is proposed, which is based on a stepped-impedance resonator (SIR) and cross-coupling theory. This filter has the characteristics of small size and high out-of-band rejection. The filter consists of four 1/2 wavelength stepped-impedance resonators and two 1/4 wavelength short-circuit microstrip resonators. By designing a new kind of structure, the cross coupling is realized between the second and the fifth resonators, and two transmission zeros are introduced out of band. Zero-degree feeding is realized due to the symmetry of the structure and feeding position, which adds two other transmission zeros outside the band. Four transmission zeros are introduced outside the passband of the filter, which greatly increase the out-of-band rejection of the filter. The passband of the filter is 3.2 GHz~4.2 GHz, and the out-of-band rejection at 2.6 GHz and 4.8 GHz reaches -60 dB. The size of the filter is only 7.2 mm * 8 mm (0.21λg*0.24λg), which realizes the miniaturization of the filter.
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