Propagation of low-frequency radio waves

Radio waves at frequencies less than around 100 kHz can propagate to great distances with little attenuation in the cavity formed by the earth and the ionosphere. At these frequencies, many active propagation modes can occur between the transmitter and receiver. Changes in the ionospheric conductivity or reflection height influence the relative phases and amplitudes of the propagating modes and hence their vector sum. The propagation is less stable at LF than at VLF and the received field strength is more difficult to predict. In this investigation, the Wavehop program has been employed in conjunction with previously unpublished summer-time ionospheric models to estimate the received field strength over a number of experimental paths. The predicted values have been compared with the measurements to validate the ionospheric models and the method of calculation.