A linearly polarized input coupler for slotted waveguide interaction structures

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Abstract
A harmonic gyro-amplifier utilizing a slotted waveguide structure requires a TE/sub n,1/ mode coupler. A 35 GHz input coupler, based on a hybrid structure, has been designed to produce a linearly polarized TE/sub 3,1/ mode in cylindrical smooth-wall waveguide. A nonlinearly tapered transition waveguide to slotted waveguide, having a return loss of -20 dB, has also been designed. X-band versions of the coupler and transition have been fabricated for cold test. Measurements on the coupler indicate a return loss of -9 dB and 3% bandwidth in good agreement with the HP HFSS code. A design study has shown that the bandwidth can be extended by more than 5% by reducing the coupling hole thickness. Preliminary results of a back-to-back coupling test at X-band shows better than 1 dB coupling at the center frequency, again in good agreement with the HFSS code. RF probe measurements have been performed on the X-band coupler. The linear polarization of the launched TE/sub 3,1/ mode has been confirmed.

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