Abstract: A bandpass filter has a plurality of resonant cavities. The plurality of resonant cavities are arranged into a sequence of adjacent resonant cavities. Each resonant cavity is configured to define the same fundamental resonant frequency. The filter includes a plurality of coupling irises, with one of the coupling irises positioned between each pair of adjacent resonant cavities. Each resonant cavity includes a plurality of cavity sections. Each resonant cavity includes a capacitive iris positioned coupling the cavity sections to one another. The frequency of secondary resonance modes varies amongst the resonant cavities in the plurality of resonant cavities.