In addition to natural and human-made changes, electrical characteristics of
the atmosphere affect the propagation of electrical signals. Figure 1.1 shows various layers of the ionosphere and the troposphere that are formed due to
the ionization of atmospheric air. As illustrated in Figure 1.2(a) and (b), an RF
signal can reach the receiver by propagating along the ground or after reflection
from the ionosphere. These signals may be classified as ground and sky waves,
respectively. The behavior of a sky wave depends on the season, day or night,
and solar radiation. The ionosphere does not reflect microwaves, and the signals
propagate line of sight, as shown in Figure 1.2(c). Hence, curvature of the Earth
limits the range of a microwave communication link to less than 50 km. One
way to increase the range is to place a human-made reflector up in the sky. This
type of arrangement is called a satellite communication system. Another way to
increase the range of a microwave link is to place repeaters at periodic intervals.
This is known as a terrestrial communication system.