Compact quadruple-mode resonator for wideband bandpass filter design

This study presents the realisation of a wideband bandpass filter using a novel compact quadruple-mode resonator. The quadruple-mode resonator, which is evolved from the square open-loop resonator, is an interesting structure for a compact design. The resonator has two odd and two even mode resonant frequencies whose approximate analytical expressions are derived. For a wide bandwidth filter, three short branch-lines are introduced to the resonator in order to widen the separation between the resonant frequencies. A filter with a 3 dB fractional bandwidth of 96.0% is demonstrated. Simulated and measured results are in good agreement.