a. Radio waves. The traveling electromagnetic field referred to as a
radio wave in paragraph 4 behaves in the same general manner as the waves of
disturbed water.
As it leaves the antenna, it creates a continuous wave
disturbance which travels outward from the antenna at a constant velocity (a
frequency of 300,000,000 meters per second) and at a constant amplitude in
free space. Of course, the term "free space" implies that the radio waves
are traveling in the atmosphere or a vacuum. The presence of hills, trees,
lakes, and other terrain features (natural and man-made) modifies the
effective radiation of an electromagnetic (radio) wave and distorts the
radiation pattern.
b. Quarter-wave antenna.
The full- or solid-radiation pattern is a
three-dimensional figure that looks somewhat like a doughnut with the
transmitting antenna in the center (Fig 6).
The quarter-wave, vertically
mounted antenna is omnidirectional (radiates in all directions).
The
vertically mounted antenna has minimum radiation off the top.
c. Half-wave antenna. The half-wave antenna, because of its physical
length, is normally mounted horizontally. The radiation pattern (Fig 6) is
bidirectional (in two directions) and is at right angles (broadside) to the
antenna length.
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