2-12
It depends on the polarization of the wave and the angle at which the wave strikes the reflecting surface.
Radio waves that keep their phase relationships after reflection normally produce a stronger signal at the
receiving site. Those that are received out of phase produce a weak or fading signal. The shifting in the
phase relationships of reflected radio waves is one of the major reasons for fading. Fading will be
discussed in more detail later in this chapter.
Figure 2-7.—Phase shift of reflected radio waves.
Refraction
Another phenomenon common to most radio waves is the bending of the waves as they move from
one medium into another in which the velocity of propagation is different. This bending of the waves is
called refraction. For example, suppose you are driving down a smoothly paved road at a constant speed
and suddenly one wheel goes off onto the soft shoulder. The car tends to veer off to one side. The change
of medium, from hard surface to soft shoulder, causes a change in speed or velocity. The tendency is for
the car to change direction. This same principle applies to radio waves as changes occur in the medium
through which they are passing. As an example, the radio wave shown in figure 2-8 is traveling through
the Earth's atmosphere at a constant speed. As the wave enters the dense layer of electrically charged ions,
the part of the wave that enters the new medium first travels faster than the parts of the wave that have not
yet entered the new medium. This abrupt increase in velocity of the upper part of the wave causes the
wave to bend back toward the Earth. This bending, or change of direction, is always toward the medium
that has the lower velocity of propagation.
