A waveguide bandpass filter having multiple waveguide sections including alternating ridge waveguide sections and evanescent waveguide sections. The ridge waveguide sections define a resonant cavity and the evanescent waveguide sections reactively load the resonant cavity thus intercoupling the ridge...http://www.google.de/patents/US4675631?utm_source=gb-gplus-sharePatent US4675631 - Waveguide bandpass filter
