A low-noise travelling-wave tube for 26.000 -40.000 MC/S

The mechanical problems associated with the design of travel 1 ing-wave tubes becom.e more acute as the frequency increases. This is because the small clearance between the beam and slow-wave structure implies that the mechanical straightness of both the travelling-wave tube and the focusing magnetic field must be unusually good at high frequencies, and also because where dielectric materials are associated with transmission lines they must be reduced to very fragile dimensions, in particular the relatively simple glass envelope construction of lower frequency helix travel1ing-wave tubes becomes difficult in Q-band because the glass envelope is either so thin that it will not maintain straightness and resist air pressure, or so thick that wave-guide modes are able to occur either in the interaction region or near the waveguide-to-helix couplings.