In order to meet the requirements for the suppression of mirror frequencies in the 5G RF front end, this paper proposes a novel miniaturized image rejection bandpass filter by loading Stepped-Impedance Resonators (SIR). By analyzing the relationship between the impedance ratio of a half-wavelength SIR and its electrical length, we have designed an improved second-order bandpass filter, which reduces the size by 34.3% compared to traditional five-order hairpin filters. In order to further enhance the performance of the filter, the use of a radial stub, as opposed to the traditional rectangular open stub, allows for the generation of a wider band transmission zero, which can be analyzed using lumped equivalent circuits. This integration improves the stopband rejection of the filter. The results show that the passband range is 5.35 GHz-6.64 GHz; the rejection in the stopband range 8.10 GHz-11.98 GHz is over 45 dB; and the size is only 0.385λg×0.295λg.
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