horizontally or vertically. The half-wave antenna can be used with FM radio
equipment for high frequency operation. The half-wave antenna is normally
installed horizontally and used with AM radio equipment.
b. Quarter-wave antenna.
A quarter-wave antenna is about one-quarter
the wavelength of the signal transmitted.
When a quarter-wave antenna is
erected vertically and is connected electrically to ground, it becomes the
equivalent of a half-wave antenna (fig 2). The ground takes the place of
the missing quarter wave, and the ground reflections supply the radiated
energy which would normally be supplied by the lower half of an ungrounded
half-wave antenna. This ground-reflected quarter wave is designated as an
image antenna--a reflection of the grounded quarter wave.
Figure 2. Quarter-wave antenna connected to ground.
c. Ground. When grounding an antenna; it is important that the ground
be highly conductive. This reduces the loss of radiated energy and provides
a reflecting surface for most of the radiated energy from the antenna. For
simple field operations, a single ground rod is usually all that is
necessary. Metal buildings or underground pipe systems are commonly used as
ground connections .
d. Counterpoise.
When an actual ground connection cannot be used
because of soil conditions or because a large buried ground system is not
practical, a counterpoise may be used to replace ground. The counterpoise
(Figure 3) can be constructed of wire and mounted below the antenna a short
distance off the ground. The area of the counterpoise should be equal to,
and preferably larger than, the antenna.
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