Over-moded operation of waveguide-to-coax transition at 60 GHz

In the design of a 57-63 GHz MMIC amplifier, a hermetic waveguide-to-coax transition was involved. A relatively large-diameter coaxial glass bead had to be employed, allowing the TE/sub 11/-mode to propagate from 52 GHz upwards. Measurements showed transmission blocked around 60 GHz, which could be explained as a resonance of the TE/sub 11/-mode. Conditions for blocking-free operation were analysed and an improved transition was designed, realized and tested which shift the frequency of the TE/sub 11/ resonance upward outside the band.