Novel design of output waveguide coupler for high-power Ka-band helix traveling wave tube

A novel design of waveguide output coupler for helix traveling wave tubes operating in the Ka-band satellite downlink frequency range has been proposed. To achieve effective RF coupling between the slow-wave structure and the external waveguide, a multi-section impedance transformer approach has been adopted. Quarter-wavelength matching sections, based on transmission-line theory, gradually convert the source impedance to the load characteristic impedance, ensuring maximum power transfer. In contrast to the traditional designs, here, the center conductor dimension has been maintained uniform and the outer conductor dimensions have been modified in each quarter-wavelength coaxial section for smooth impedance transition and effective power coupling. The geometry of the coupler has been systematically analyzed using a 3-D electromagnetic simulator. It has been demonstrated that the designed coaxial coupler to the waveguide coupler, which employs the standard WR-42 waveguide, acts as a high power ∼300 W and low-loss guiding structure. It also helps in avoiding dielectric breakdown and thermal melting because waveguides have no inner conductor as in coaxial couplers. In order to guarantee consistent RF performance under practical operating conditions, extensive geometric sensitivity analysis has been performed on all the geometric parameters, which enables setting the dimensional tolerances during fabrication at acceptable structural deformation levels during the tube operation.