Propagation of Sky Waves

Author: J.B. Hoag

Fig. 9 C. Bending of sky waves

Figure 9 C shows the paths of three sky waves through the ionosphere.

Low-frequency waves will be returned from the ionosphere even if sent straight up. As the frequency is increased, a critical frequency is reached for which the wave does not return to the earth. This is a useful measure of transmitting conditions. If waves are sent upward at an angle appreciably less than 90° from the ground, they may be refracted or bent back to the earth even though of higher frequency than the critical value. The maximum usable frequency, for waves transmitted at small angles above the ground, is about three times the critical frequency.

Fig. 9 D. Maximum usable frequencies for reliable radio transmission, for Sept. 1941, at Washington, D.C.

Figure 9 D shows the values as measured by The National Bureau of Standards during September 1941. It can be seen that for transmission to a given distance, lower frequencies must be used at night than during daylight hours.

After returning to the earth, the sky wave can be reflected by the ground, to travel a second time to the ionosphere and back at a more remote point. This is called a multi-hop. In fact, radio waves have been received which have traveled the entire distance around the earth.

If two rays from the transmitter have traveled along slightly different paths to the receiver, they may be out of phase (crest for trough) and cancel each other, or they may be in phase with each other and give a strong signal. Since the paths through the ionosphere are subject to changes, the received signal may be strong at times and weak at others. This is called fading.