are ~/4 (one-fourth wavelength) at the operating
frequency. Together, of course, the sections make the
effective length of the antenna L/2 (one-half
wavelength) at the operating frequency.
One feature of the dipole antenna is that it does not
need to be connected to the ground like other antennas.
Antennas shorter than a half wavelength must use the
ground to achieve half-wave characteristics. The half-
wave antenna is already long enough to radiate the
signal properly.
Because of sophisticated antenna systems and
tuning processes, half-wave antennas can be
electrically achieved aboard ship. Therefore
wavelength is becoming less and less the criteria for
determining the types of antennas to be used on ships.
Dipole antennas can be mounted horizontally or
vertically, depending upon the desired polarization, and
can be fed at the center or at the ends. Because it is
ungrounded, the dipole antenna can be installed above
energy-absorbing structures.
QUARTER-WAVE ANTENNA
A quarter-wave antenna is a grounded antenna that
is one-fourth wavelength of the transmitted or received
frequency. You will hear the quarter-wave antenna
referred to as a “Marconi antenna.” The quarter-wave
antenna is also omnidirectional.
As we mentioned earlier, a half-wave antenna is the
shortest practical length that can be effectively used to
radiate radio signals into free space. The natural
question, then is, “How do we use a quarter-wavelength
antenna if a half-wavelength is the shortest length that
can be used?” The answer is simple.
Figure 2-20.—Direct and image signal of a quarter-wave
antenna.
Figure 2-21.—Current distribution in a real antenna and its
image.
Two components make up the total radiation from
an antenna. One component is that part of the radiated
signal which leaves the antenna directly. The other is a
ground reflection that appears to come from an
underground image of the real antenna (figure 2-20).
This image is sometimes called the mirror image and is
considered to be as far below the ground as the real
antenna is above it.
Figure 2-21 shows basic current distribution in a
real and image antenna. There are certain directions in
which the direct wave from the real antenna and the
reflected wave from the image are exactly equal in
amplitude but opposite in phase. Conversely, there are
other directions in which the direct and reflected waves
are equal in amplitude and in phase. Therefore,
depending on the direction and location of the point at
which the field strength is measured, the actual field
strength may be (1) twice the field strength from the real
antenna alone, (2) zero field strength, or (3) some
intermediate value between maximum and minimum.
It is this “real” and “image” radiated field that forms the
basis for using quarter-wavelength antennas.
This reflected-energy principle is very useful in the
lower frequency ranges, although ground reflections
occur in the high-frequency range as well.
The antenna does not always need to be placed at
the Earth’s surface to produce an image. Another
method of achieving reflected images is through the use
of ground planes. This means that a large reflecting
metallic surface is used as a substitute for the ground or
Earth. This method is frequently used in the VHF/UHF
2-19
