Helix waveguide design

A theoretical analysis of circular electric wave propagation in slightly curved, and slightly non-circular and non-uniform helix waveguide is the basis for design considerations. A radial wave impedance at the helix interface is used to calculate the effect of composite jacket structures. Three different applications of the helix waveguide for circular electric wave transmission are discussed. As a mode filter the helix waveguide should have a lossy jacket which causes a high transmission loss for all unwanted modes. For sharp intentional bends with tapered curvature the helix waveguide should have a jacket of low-loss dielectric material surrounded by a highly conducting coaxial shield. For an all-helix waveguide, in order to reduce both mode conversion-reconversion effects at imperfections and loss in curvature, the jacket should be medium lossy and also surrounded by a metallic shield. The distance between helix and shield should, in all applications, be about a quarter of the radial wavelength in the material. Measurements have been made on unwanted mode transmission in mode filters, TE01 loss in intentional bends of low-loss shielded helix waveguide and TE01 loss in bends of lossy shielded helix waveguide. Intentional bends of a 2 in-internal-diameter low-loss helix waveguide increase the TE01 loss at 55 Gc/s by about 0.001 dB per degree change in direction. In a lossy helix waveguide of 2 in internal diameter a radius of curvature of about 300 ft doubles the TE01 loss at 55 Gc/s.