High rejection quadruple-mode broadband BPF with source-load coupling

A compact and high rejection broadband bandpass filter (BPF) with source-load coupling is proposed using the quadruple-mode stub-loaded resonator. Two identical folded open stubs near the centre plane can adjust high resonant frequencies f_m3 and f_m4 into the desired passband and create two transmission zeros to sharpen the upper skirt and suppress the resonant frequency f_m5. Owing to the source-load coupling, two transmission zeros are created in the upper-stopband and near the lower cutoff frequency not only to deepen the upper-stopband, but also to sharpen the lower skirt. A filter with the fractional bandwidth 50% is simulated, fabricated and measured. Measured results agree well with EM simulations.