TC 9-64 _________________________________________________________________________
WHAT IS PROPAGATION?
1-2. Early man was quick to recognize the need to communicate beyond the
range of the human voice. To satisfy this need, he developed alternate
methods of communication, such as hand gestures, beating on a hollow log,
and smoke signals. Although these methods were effective, they were still
greatly limited in range. Eventually, the range limitations were overcome by
the development of courier and postal systems; but there was then a problem
of speed. For centuries the time required to deliver a message depended on
the speed of a horse.
1-3. During the latter part of the nineteenth century, both distance and
time limitations were largely overcome. The invention of the telegraph made
possible instantaneous communication over long wires. Then a short time
later, man discovered how to transmit messages in the form of radio waves.
1-4. As you will learn in this chapter, radio waves are propagated.
Propagation means "movement through a medium." Light rays most easily
illustrate this concept. When a light is turned on in a darkened room, light
rays travel from the light bulb throughout the room. When a flashlight is
turned on, light rays also radiate from its bulb, but are focused into a narrow
beam. You can use these examples to picture how radio waves propagate.
Like the light in the room, radio waves may spread out in all directions. They
can also be focused (concentrated) like the flashlight, depending upon the
need. Radio waves are a form of radiant energy, similar to light and heat.
Although they can neither be seen nor felt, their presence can be detected
through the use of sensitive measuring devices. The speed at which both
forms of waves travel is the same; they both travel at the speed of light.
1-5. You may wonder why you can see light but not radio waves, which
consist of the same form of energy as light. The reason is that you can see
only what your eyes can detect. Your eyes can detect radiant energy only
within a fixed range of frequencies. Because the frequencies of radio waves
are below the frequencies your eyes can detect, you cannot see radio waves.
1-6. All things on the land, or in the water are showered continually with
waves of energy. Some of these waves stimulate our senses and can be seen,
felt, or heard. For instance, we can see light, hear sound, and feel heat.
However, there are some waves that do not stimulate our senses. For
example, radio waves, such as those received by our portable radio or
television sets, cannot be seen, heard, or felt. A device must be used to
convert radio waves into light (TV pictures) and sound (audio) for us to sense
1-7. A wave can be defined as a disturbance (e.g., sound, light, radio waves)
that moves through a medium (e.g., air, water, vacuum). To help you
understand what is meant by "a disturbance that moves through a medium,"
picture the following illustration. You are standing in the middle of a wheat
field. As the wind blows across the field toward you, you can see the wheat
stalks bending and rising as the force of the wind moves into and across
them. The wheat appears to be moving toward you, but it isn't. Instead, the
stalks are actually moving back and forth. We can then say that the