Wideband bandpass filter with wide stopband based on stub-loaded resonator

In this letter, a wideband bandpass filter (BPF) with wide stopband is designed based on a novel stub-loaded resonator (SLR). The SLR features its harmonic shifting characteristic which can improve the passband bandwidth and upper stopband performances. The resonator is composed of a T-type open stub embedded with a circular patch centrally loaded in a half wavelength resonator. The circular patch is introduced to lower the first and second spurious responses of the loaded stub to extend the bandwidth and stopband. Finally, a BPF prototype with a fractional bandwidth of 50% is designed and fabricated. The expected performance is observed in the measured results. The measured stopband with the rejcetion greater than 18 dB can be extended to 4.8 f0.