The helically wrapped circular waveguide

This paper contains a theoretical investigation into a form of helical waveguide intended for use as a slow-wave structure within a microwave amplifier. The primary advantage of a helical waveguide over a tape helix is its greater heat capacity; unfortunately, a closed structure like the helical waveguide has a low-frequency cutoff, making it more dispersive than the open-structured tape helix. Previous analyses have shown the helically wrapped rectangular waveguide to be quite dispersive, limiting its usefulness as a slow-wave structure. This paper considers the possibility of reducing the dispersion of the helical waveguide by using a circular waveguide helix instead of a rectangular waveguide helix. Switching to a circular boundary changes the loading conditions over that of a flat boundary, straightening out the dispersion curves and reducing the low-frequency cutoff of the helical waveguide. Two properties of the helically wrapped circular waveguide are calculated: the dispersion and interaction impedance characteristics. It is found that the helical waveguide is more dispersive than the tape helix at low wavenumbers, but has a higher interaction impedance. The higher interaction impedance would permit operation at a higher wavenumber, where low dispersion makes moderately wide-bandwidth amplification possible. The high heat capacity of the structure promises to make it suitable for high-power microwave amplification.