The polarisation of low frequency radio waves in the terrestrial wave-guide

Abstract

The propagation of very low frequency radio waves over great distances is often treated by regarding the space between the earth and the ionosphere as a wave-guide. Most of the previous work has assumed the ionosphere to be isotropic and with this restriction the possible modes are either of type TM or type TE in the usual wave-guide nomenclature. A radio transmitter consisting of a vertical wire aerial can excite only TM modes, so these have received most attention. The electric field is everywhere in the plane of propagation and the modes may be said to have a linear polarisation with the magnetic field horizontal. Similarly in TE modes there is a linear polarisation with the electric field horizontal. When the anisotropy of the ionosphere is included, however, simple TM and TE modes are not excitable. Each of the two crossing waves comprising a mode is elliptically polarised and they combine to give a resultant field in which the electric vector at any point describes an ellipse whose features depend on the height. The purpose of this paper is to study this polarisation ellipse in some special cases. It is found that the fields depart only slightly in form from those in true TM and TE modes, so that it is convenient to associate them with propagation in “ quasi-TM” and “ quasi-fE” modes. Curves are given showing examples of how the wave polarisation depends upon height in these two modes. The ionosphere is represented by an exponential variation in electron density and by a collision frequency which is independent of height. The earth is taken to be flat and perfectly conducting. Propagation is confined to a magnetic east—west plane representative of northern temperate latitudes and the mode characteristics are found to depend on the direction of propagation. The dominant field in quasi- TE modes is affected considerably by the propagation direction whereas the stronger field component in quasi- TM modes is affected only slightly. This non-reciprocity is accounted for by the direction dependence of the conversion coefficients of the ionosphere at large angles of incidence.