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Information Systems Technician Training Series, Module 4 - Communications Hardware | Next
Quarter-Wave Antenna |
dummy load, instead of the antenna, is conected to the
output of the transmitter, and the normal transmitter
tuning procedure is followed.
Most Navy transmitters have a built-in dummy
load. This permits you to switch between the dummy
load or the actual antenna, using a switch. For
transmitters that do not have such a switch, the
transmission line at the transmitter is disconnected and
connected to the dummy load (figure 2-18). When
transmitter tuning is complete, the dummy load is
disconnected and the antenna transmission line is again
connected to the transmitter.
ELECTROMAGNETIC WAVELENGTH
Electromagnetic waves travel through free space at
186,000 miles per second. But, because of resistance,
the travel rate of these waves along a wire is slightly
slower. An antenna must be an appropriate length so
that a wave will travel from one end to the other and
return to complete one cycle of the RF voltage. A
wavelength is the distance traveled by a radio wave in
one cycle. This means that wavelength will vary with
frequency.
If we increase the frequency, the time required to
complete one cycle of alternating current (at) is
naturally less; therefore, the wavelength is less. If we
decrease the frequency, the time required to complete
one cycle of ac is longer; therefore, the wavelength is
longer. Another word used to represent wavelength is
LAMBDA (designated by the symbol i).
The term “wavelength” also refers to the length of
an antenna. Antennas are often referred to as half wave,
quarter wave, or full wave. These terms describe the
Figure 2-18.—DA-91/U dummy load.
relative length of an antenna, whether that length is
electrical or physical.
Earlier, we said that when tuning an antenna, we are
electrically lengthening or shortening the antenna to
achieve resonance at that frequency. We are actually
changing the wavelength of the antenna. The electrical
length of an antenna may not be the same as its physical
length.
You know that RF energy travels through space at
the speed of light. However, because of resistance, RF
energy on an antenna travels at slightly less than the
speed of light. Because of this difference in velocity, the
physical length no longer corresponds to the electrical
length of an antenna. Therefore, an antenna may be a
half-wave antenna electrically, but it is physically
somewhat shorter. For information on how to compute
wavelengths for different frequencies, consult NEETS,
Module 12, Modulation Principles.
BASIC ANTENNAS
Many types and variations of antenna design are
used in the fleet to achieve a particular directive
radiation pattern or a certain vertical radiation angle.
However, all antennas are derived from two basic types:
the half wave and the quarter wave.
HALF-WAVE ANTENNA
An antenna that is one-half wavelength long is the
shortest antenna that can be used to radiate radio signals
into free space. The most widely used antenna is the
half-wave antenna, commonly called a dipole, or hertz,
antenna. This antenna consists of two lengths of wire
rod, or tubing, each one-fourth wavelength long at a
certain frequency.
Many complex antennas are constructed from this
basic atenna design. This type of antenna will not
function efficiently unless its length is one-half
wavelength of the frequency radiated or received.
Figure 2-19 shows a theoretical half-wave antenna
with a center feed point. Both sections of the antenna
Figure 2-19.—Half-wave antenna with center feed point.
2-18
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