b. A grounded antenna, in which the ground is part of the
antenna, can be compared to a person shining a flashlight into a
mirror.
While standing directly in front of the mirror, the light
from the flashlight shines directly into your eyes. The light we see
is coming from the flashlight's image. This is similar to the image
antenna in a grounded vertical antenna.
There is no quarter-wave
antenna in the ground.
However, the ground reflects the wave much
like the mirror reflects the light.
Grounded antennas are often
referred to as Marconi antennas. Ungrounded antennas are referred to
as Hertz antennas.
c. The disadvantage of a vertical antenna is the vertical support
needed.
It makes practical field antennas impossible to erect.
To
compensate for length requirements, loading coils are used to
stimulate the true length. The disadvantage of loading coils is that
they use up RF power while compensating electrically for the actual
physical required.
Six MHz is the lowest practical frequency for
which a vertical quarter-wave antenna can be erected with the present
issue mast kits (40 foot AB-155).
Figure 55. Distribution of voltage and current on
a grounded quarter-wave antenna.
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