Long-distance observations of radio waves of medium frequencies

The frequency-change technique of Appleton and Barnett has been applied to the analysis of the downcoming waves from a distant transmitter. The observations were carried out simultaneously at distances of 25 and 700 km. from the emitter, which operated on a frequency of 1415 kc./sec. It was found that several downcoming waves were present at the more distant receiving station. Each of these waves was identified by using the path-length of the singly reflected wave from the E layer as a reference. In this way it was found that the equivalent heights of both the E and the F layers are relatively stable over the 700-km. transmission path, and do not vary appreciably with the angle of incidence of the wave. The equivalent height of the F layer showed a pronounced minimum at about 3 a.m. each morning. The rate of propagation of the minimum height in the horizontal direction appears to be slower than the rate of sunset propagation in the same direction. The ionization-density in the E layer in the early morning was always greater than 2.4 × 10 3 electrons per cm 3 ., and during half the period of the observations was less than 8.3 × 10 3 electrons per cm 3 . The intermediate layer was observed regularly at sunrise. From the measurements of equivalent heights at different angles of incidence it is concluded that the gradient of ionization at the lower boundary of the E layer is sharp.