Dispersion Equations of a Rectangular Tape Helix Slow-Wave Structure

A rectangular tape helix slow-wave structure with infinitesimal thickness and finite width in free space is investigated. With the expansion of surface currents in the helix and the applications of the modified Marcatili's method, as well as average power flow matching method at the boundaries, the dispersion properties and the interaction impedance for transverse antisymmetric modes in a rectangular tape helix immersed in free space are obtained. It is shown that, compared with the results of the simplified sheath model by previous researchers, higher accuracy has been obtained between the calculation results of the present theory and the data obtained from HFSS, and the validity of the present theory is further demonstrated by comparison with experiments. The improved characteristic equations hold scientific and practical significance in the design and performance evaluation of such plane slow-wave structure in the application of compact traveling-wave tubes. The distribution characteristics on the cross section of the longitudinal electric field fundamental component are also discussed based on this theory.