Abstract :
Multiple signals have been observed in a frequency range of 20,000 to 16,667 kc, or 15 to 18 m, only when the great circle between transmitter and receiver passes through the twilight zone, in which conditions are especially favorable for these frequencies. The conditions for various positions of great circles are discussed in detail, and the expected times of occurrence of double signals are given and compared with those measured. As the greatest angle which the plane of the circle must form with the twilight plane at the most favorable time of day and year, in order to receive multiple signals, the value ϕ ≦ 10 deg. is assigned. The propagation conditions in the twilight zone appear, at least for Berlin as receiving point, to be more favorable in the winter in the indirect sense, and in the summer in the direct. For reception from the west, this has been clearly confirmed for North and South America, while the direction of summer double signals from the east is not yet entirely determined. Next year, after the increase and improvement of transmitters, the observation of the lines favorable for double signals from Osaka, Mukden, and Manila will be possible. Indirect double signals also appear in the range of 25,000 to 13,636 kc (12 to 22 m), outside of the twilight zone, when the indirect night path is in summer.
