The polarisation of v.l.f. radiowaves reflected from the ionosphere at steep incidence has been determined from simultaneous measurements of the reflection and conversion coefficients. Changes due to the diurnal and seasonal variations in electron density are reported and the marked anomalies observed during sudden ionospheric disturbances produced by solar-flare activity are discussed.The propagation parameters have been calculated using the fullwave analysis for a number of idealised models of the electron density/height distribution. The calculated values are in close agreement with those obtained experimentally. It is suggested that these models could be used to determine the propagation parameters as an aid in the development of any new v.l.f. system.