A Novel W-band Substrate Integrated Microstrip to Ultra-thin Cavity Filter Transition

A novel substrate integrated microstrip to ultra-thin cavity filter transition operating in the W-band is proposed in this letter. The filter includes a conventional tapered microstrip transition structure, which guides the electromagnetic field from the microstrip line to the reduced-height dielectric-filled waveguide, and an air-filled matching cavity which is placed between the dielectric-filled waveguide and the ultra-thin cavity filter. The height of the microstrip line, dielectric-filled waveguide and the ultra-thin cavity filter are the same, enabling seamless integration within a planar radio-frequency (RF) circuit. To facilitate testing, mature finline transition structures are integrated at both ends of the microstrip line during fabrication. The simulation results of the fabricated ultra-thin cavity filter with the finline transition structure, with a passband of 91.5–96.5 GHz, has an insertion loss of less than 1.9 dB and a return loss better than − 20 dB. And the whole structure has also been measured which achieves an insertion loss better than 2.6 dB within the filter's passband, including the additional insertion loss introduced by the finline transitions.