Frequency Spread in Ionospheric Radio Propagation

As part of a research program on the time- and frequency-dispersive characteristics of ionospheric radio propagation, techniques were developed for precise measurement of the power spectrum of a received CW signal. Frequency resolution of 0.025 Hz is obtained. This resolution allows the observation of the various propagating modes and, with the aid of oblique-incidence ionograms taken simultaneously, identification of the modes. Occasionally, even the ordinary and extraordinary components of the wave can be separated and identified. Power spectra were derived from measurements made on a midlatitude path and on a transauroral path over a period of one year near the solar cycle minimum. In addition to the plotting of the power spectra, the root-mean-square widths of the spectra (defined as the frequency spread) and the means (or average Doppler shift) were computed. Distributions of these data are presented, and the frequency spread is compared with the magnetic activity, the timedelay spread, and the number of modes.