Sky-wave Propagation at the Low Frequencies
Author: Edmund A. Laport
If low-frequency-wave propagation was entirely by means of the ground wave, there would be negligible variations in received signal strengths. Actually there is some propagation by atmospheric paths above the earth with reflection or refraction of these sky waves back to earth. During the daylight hours, the lower radio frequencies seem to experience reflection from the D layer, an ionized stratum of atmosphere below the E layer. Like the latter, the D layer disappears during the hours of darkness, but this does not eliminate all signal-strength variations that are attributed to sky-wave propagation. Any such sky waves interfere with the dominant ground wave and produce the variations observed on long circuits, even at the lowest radio frequencies in use. However, the magnitude of these variations is relatively small with respect to those experienced at the higher frequencies.