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.