Radio Transmission at VHF by Scattering and Other Processes in the Lower Ionosphere

The results of a program extending over the last four and one-half years to investigate the nature and characteristics of high-loss regular VHF propagation by means of the lower ionosphere are presented. For the most part continuous-wave transmissions are employed, with the result that the effects of the different elements in the propagation are superposed. Three different ionizing agents operating in the lower ionosphere can be distinguished by means of their differing effects on the behavior of the signal, after the exclusion of behavior associated with sporadic-E propagation. They are solar radiation, corpuscular radiation presumably of solar origin, and meteors. The part played by each of these is discussed in the account and interpretation of the principal results obtained. Both short- and long-term characteristics of the observed composite signals are described, as well as the results of experiments with spaced and other antenna arrangements. The intensity and nature of the signals are discussed as a function of time of day, season, geographical position of the transmission path, and level of solar activity. The dependence of the strength of the signals on path length, on frequency, and on scattering angle has been studied. In the latter connection, pulse experiments were performed with equipment permitting fairly direct height determinations, which are found to be in satisfactory agreement with earlier less direct height determinations. Attempts to compare the observations with various existing theories designed to account for the propagation as a scattering process have been made. The results are not entirely satisfactory.