The effect of multipath distortion on the choice of operating frequencies for high-frequency communication circuits

Harmful multipath distortion on high-frequency facsimile services and telegraphic services operating at high speeds occurs when the received signal is composed of two or more components arriving by different modes over the same great-circle path with comparable intensities, but having travel times which differ by an amount equal to an appreciable fraction of the duration of a signal element. The dependence of multipath distortion on the relationship of the operating frequency to the MUF is discussed and a new term, the multipath reduction factor (MRF), is introduced which permits calculation in terms of the MUF of the lowest frequency which can be used to provide a specified measure of protection against multipath distortion. The MRF has a marked path-length dependence and is calculated as a function of path length for representative values of the other parameters involved by making use of an ionospheric model. It is then shown how the MRF can be used in connection with world-wide MUF prediction material to determine the minimum number of frequencies which must be assigned to a high-frequency communication service of continuous availability operating at high speed. Some comparisons with observations are discussed, and finally conclusions are drawn concerning manner of operation and choice of operating frequencies to reduce or to eliminate harmful multipath distortion.