Figure 2-8.Radio wave refraction.
Radio waves passing through the atmosphere are affected by certain factors, such as temperature,
pressure, humidity, and density. These factors can cause the radio waves to be refracted. This effect will
be discussed in greater detail later in this chapter.
A radio wave that meets an obstacle has a natural tendency to bend around the obstacle as illustrated
in figure 2-9. The bending, called diffraction, results in a change of direction of part of the wave energy
from the normal line-of-sight path. This change makes it possible to receive energy around the edges of
an obstacle as shown in view A or at some distances below the highest point of an obstruction, as shown
in view B. Although diffracted rf energy usually is weak, it can still be detected by a suitable receiver.
The principal effect of diffraction extends the radio range beyond the visible horizon. In certain cases, by
using high power and very low frequencies, radio waves can be made to encircle the Earth by diffraction.
Figure 2-9.Diffraction around an object.