Propagation of an electromagnetic wave through a helical waveguide

This paper is concerned with the closed helical waveguide and deals mainly with the rotation of the planes of polarisation of waves passing through it. A theory is presented for electromagnetic propagation in a helical waveguide. The waveguide is divided into two regions: the groove space and the centre space. The regions are considered separately, and the dispersion equation obtained by matching the fields at the boundary between them. The characteristics of propagation in helical waveguides are discussed and propagation curves given for two helixes. Measurements have been made of wavelengths in the helix, and it is found that measured values are slightly smaller than calculated values. Investigation of the rotation of the plane of polarisation of a wave passing through the helix shows that, at frequencies approaching cutoff, the rotation is linear and between 60° and 80° per wavelength. At higher frequencies the rotation is 360° per pitch and is independent of wavelength. The former effect is shown to be caused by two modes in the helix interacting; the latter effect is caused by the interaction between space harmonics of a single mode.