4. Behavior of radio waves in different media.
a. Radio waves are subject to the influence of the environment in which they are propagated.
When a radio wave leaves the boundary of one medium and enters another, the wave changes direction.
Three things can occur when a wave passes from one medium to another:
(1) Some of the energy can be reflected back into the initial medium.
(2) Some of the energy may be transmitted into the second medium where it may continue at
a different velocity.
(3) Some of the energy may be absorbed by the medium.
b. Reflected waves are neither transmitted nor absorbed, but are reflected from the surface of
the medium they encounter. The basic analogy that illustrates this concept is the mirror. Figure 1-10
illustrates the reflection of a light beam from a flashlight. If a wave is directed against a mirror, the
wave that strikes the surface is called the incident wave, and the one that bounces back is called the
reflected wave. This also occurs when a wave is transmitted skyward, reflect off the ionosphere, and
returns to a receiving station. The angle of reflection equals the angle of incidence.
c. When a wave passes from one medium into a medium that has a different propagation
velocity, a change in the direction of the wave will occur. This changing of direction is called
refraction. Possibly the most common recollection of this is when you dip a spoon into a glass of water;
the spoon handle appears bent. Figure 1-10 also shows how a wave is refracted (bent) as it travels from
one medium to another, such as a radio wave bending as it passes through the atmosphere.
d. When a radio wave encounters an obstacle in its path, it bends around the obstacle. This
bending is called diffraction, and results in a change of direction of part of the energy from the normal
line-of-sight path. Figure 1-11 shows a signal that is diffracted around a mountain range. Between the
mountains and the house, the closer you are to the house, the stronger the signal. Conversely, the closer
you are to the mountains, the weaker the signal. When reconnoitering a hilly area prior to an operation,
radio checks should be made to determine areas of poor radio reception.
e. Absorption occurs when radio waves are transmitted from one medium to another, with a
resultant loss of energy. For example, if a radio signal is propagated through trees during the summer
months, the foliage will absorb some of the energy of the signal. The same signal transmitted during the
winter months may pass because the trees have shed their leaves and do not absorb the signal. A
receiving antenna should be erected so that it is in the best position possible to absorb incoming
electromagnetic energy.
SS0130
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