Maximum Usable Frequencies for Radio Sky-Wave Transmission, 1933 to 1937

Graphs are presented to indicate the maximum usable frequencies for sky-wave transmission from June, 1933, to December, 1937. The graphs are given for March, June, and December to show equinoctial, summer, and winter conditions for each year. The factors which must be considered in deriving these graphs from vertical-incidence ionosphere measurements are outlined briefly. The principal factors are Snell´s law, variation of virtual height of the ionosphere with frequency, the curvature of the ionosphere, and the effect of the earth´s magnetic field. The method of applying these graphs to simple and complex transmission paths is indicated. The effect of sporadic E reflections, absorption, and scattered reflections are discussed briefly. In conclusion it is pointed out that the graphs and the ionosphere data, from which they were derived, may be used to estimate future diurnal, seasonal, and longtime variations of maximum usable frequencies.