Propagation of an electromagnetic wave through a helical waveguide with special reference to the rotation of the planes of polarisation.

The helix has achieved great importance, mainly through its applications as a slow-wave structure. A review is made of some of the relevant published work indicating the main methods of analysing the problem of the propagation of electromagnetic energy through the helix. This thesis is concerned with closed helical waveguides and a theory is presented for electromagnetic propagation in such a waveguide. The characteristics of propagation in helical waveguides are discussed and propagation curves given for the two experimental helices. Expressions are given for the power flow and the coupling impedance, and values calculated for the experimental helices. The effect of small changes in the dimensions of the helices has also been studied. Methods of manufacturing lengths of helical wav- guide are discussed and details given of other equipment which was made for the experimental work. 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 cut-off, the rotation is linear and between 60° and 80° per wavelength. At higher frequencies the rotation is 360° per pitch, independent of wavelength. The former effect is shown to be due to two modes in the helix interacting, the latter, to the interaction between space harmonics of a single mode. A study of the effect of the orientation of the incident mode on the input impedance of the helix revealed that at frequencies approaching cut-off there is little change. At higher frequencies the locus of impedance variation is a circle on the Smith chart. This behaviour is shown to be due to a phase change at the input