This paper proposes parallel-coupled stepped impedance resonators (SIRs) used for wideband bandpass filters with reduced size and improved spurious response suppression in out-of-band. The filter design concept is demonstrated by both two- and four-parallel coupled line resonator structures with the same fundamental frequency (f0) of 6.5 GHz. The first bandpass filter has been designed using two resonators with λg/4 slot and embedded slot feeds for spurious response suppression at 2f0 and 3f0, respectively. The measured insertion loss is around 0.3 dB and the operation band is from 4.4 to 9.2 GHz (FBW = 73.8%). For the second bandpass filter, the embedded slot feeds and additional loaded open-stubs have been utilized to improve the wide spurious response suppression and sharp skirt characteristic in the passband. The measured passband of frequency response is from 4.2 to 8.6 GHz (FBW = 67.7%). The insertion loss of passband is lesser than 0.25 dB and the return loss is better than 15 dB. The filter designs are described in details. The simulated and experimental results are demonstrated and discussed.
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