Compact wideband bandpass filter with sharp selectivity using short-ended parallel-coupled-line-stub resonator

A compact wideband bandpass filter is presented in this paper. By loading the short-ended parallel-coupled-line-stub (PCLS) at the center of the uniform-impedance half-wavelength line, the tri-mode resonator is realized. Two transmission zeros, due to the PCLS, are generated at the lower and upper cut-off frequencies and sharp selectivity can be obtained. The short-ended PCLS resonator is theoretically analyzed and applied to design a bandpass filter with fractional bandwidths 45%, good in-band return loss (>12.2dB) and low insertion loss (0.9dB at the center frequency 4.0GHz).