Propagation Of Radio Waves
Radio waves travel by three different routes from their point of propagation to their point of detection. These three routes are through the troposphere, through the ground, and by reflection off the ionosphere. The first of these routes is the most direct. A radio wave generated and transmitted from point A may travel in a relatively straight line through the lower atmosphere to a second point, B, where its presence can be detected by a receiver. This "line of sight" propagation is similar to the transmission of a beam of light from one point to another on Earth's surface. And, as with light, this form of radio wave propagation is limited by the curvature of Earth's surface.
This description is, however, overly simplified. Radio waves are deflected in a number of ways as they move through the troposphere. For example, they may be reflected, refracted, or diffracted by air molecules through which they pass. As a consequence, radio waves can actually pass beyond Earth's optical horizon and, to an extent, follow Earth's curvature.
Line-of-sight transmission has taken on a new dimension with the invention of communications satellites. Today a radio wave can be aimed at an orbiting satellite traveling in the upper part of the atmosphere. That satellite can then retransmit the signal back to Earth's surface, where it can be picked up by a number of receiving stations. Communications satellites can be of two types. One, a passive satellite, simply provides a surface off which the radio wave can be reflected. The other type, an active satellite, picks up the signal received from Earth's surface, amplifies it, and then retransmits it to ground-based receiving stations.
Since radio waves are propagated in all directions from a transmitting antenna, some may reflect off the ground to the receiving antenna, where they can be detected. Such waves can also be transmitted along Earth's surface in a form known as surface waves. Radio waves whose transmission takes place in connection with Earth's surface may be modified because of changing ground conditions, such as irregularities in the surface or the amount of moisture in the ground.
Finally, radio waves can be transmitted by reflection from the ionosphere. When waves of frequencies up to about 25 megahertz (sometimes higher) are projected into the sky, they bounce off a region of the ionosphere known as the E layer. The E layer is a region of high electron density located about 50 mi (80 km) above Earth's surface. Some reflection occurs off the F layer of the ionosphere also, located about 120 mi (200 km) above Earth's surface. Radio waves reflected by the ionosphere are also known as sky waves.