Design and Fabrication of a Planar Helix Slow-Wave Structure for C/X-Band TWT

This paper presents the design and fabrication of a planar helix slow-wave structure (SWS) with straight-edge connections at C/X-band frequencies. Besides RF performance, various issues such as vacuum compatibility of materials as well as ease and accuracy of assembly techniques are specially emphasized. The basic SWS is fabricated on a pair of alumina substrates using printed-circuit techniques and is fixed in a nonmagnetic stainless steel housing. The SWS is fed with coaxial couplers. Simulated and measured cold-test parameters of the fabricated structure are presented, showing a wide -10-dB bandwidth in the 6-9-GHz frequency range. The measured phase velocity and circuit loss match the simulated values well. The coupling impedance and hot-test parameters are also studied using simulations. It is shown that the proposed structure is able to provide a saturation output power of 100 W in the frequency range of 6-8 GHz. Such a structure is an attractive candidate for low-cost C-and X-band traveling-wave tubes fabricated using printed-circuit techniques.