Metallic E-plane filter with cavities of different cutoff frequency

A planar integrated circuit filter with multiple all-metal inserts and multiple abrupt waveguide step-wall discontinuities is introduced which achieves broadband high attenuation in the second stopband. The design is based on field expansion into suitably normalised eigenmodes which yield directly the modal S-matrix. This theory allows the immediate inclusion of both the higher-order mode interaction at all discontinuities and the finite thickness of the inserts. Computer-optimised design data for a five-resonator Ka-band prototype filter at 27 GHz midband frequency provide a minimum stopband attenuation of 50 dB between 27.5 and 44.5 GHz.