1. Radio wave propagation. A radio wave is a form of radiant energy (electromagnetic radiation) that
propagates at the speed of light (186,000 miles or 300,000,000 meters per second). The following
description of wave motion is based on FM 11-64, Transmission Lines, Wave Propagation, and
a. A wave can be defined as a disturbance (sound, light, radio waves) which moves through a
medium (air, water, vacuum). To help you understand what this means, think of the following picture.
You are standing in the middle of a wheat field. As the wind blows across the field toward you, you 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 is not. Instead, the stalks are actually moving back and forth.
We can then say that the medium is the wheat, and the disturbance is the wind moving the stalks of
wheat.
b. Wave motion can be defined as a recurring disturbance advancing through space with or
without the use of a physical medium. Therefore, it is a means of moving or transferring energy from
one point to another point. For example, when sound waves strike a microphone, sound energy is
converted into electrical energy. When light waves strike an antenna, they are likewise converted into
electrical energy. Thus, sound, light, and radio waves are all forms of energy that are moved by wave
motion.
c. The analogy of wave motion in water helps to explain the basic concept of how a radio wave
propagates. Dropping a stone into a pool of water result in a disturbance of the water (the medium).
From the point of impact, the disturbance is transmitted on the surface of the water as an expanding
series of circular waves. Figure 1-1 depicts thin wave motion.
(1) View A shows the falling stone an instant before it strikes the water.
(2) View B illustrates the action that occurs when the stone strikes the surface, pushing the
water upward and outward.
(3) In View C, the stone has sunk deeper into water, which has closed over it, while the
leading wave has moved outward.
(4) View D shows the leading wave continuing to move outward, followed by a series of
waves gradually diminishing in amplitude. Meanwhile, the disturbance at the original point of impact
has gradually subsided.
(5) Note that the leading wave has amplitude and wavelength corresponding to one compete
cycle. The water is not actually being moved by the outward motion of the wave, but rather by the up
and down motion of the water. The up and down motion is traverse, or at right angles to the outward
motion of the waves. This is called traverse wave motion.
SS0130
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