6.
Polarization.
a. The band of frequencies we use will determine the best
polarization.
At low and medium frequencies, vertical polarization
should be used.
This will take advantage of the surface wave which
travels vertically. A horizontal antenna has a horizontal wave that
will be short-circuited and will travel less than a vertical wave at
the same frequency. The disadvantage of using a vertical antenna at
these frequencies is that a sky hook will have to be used to hold the
antenna up. For example, a 2 MHz antenna that is a quarter wave long
is 117 feet.
It would not be possible to erect a practical field
antenna 117 feet high.
We, therefore, would be forced to use a
horizontal antenna.
We would be forced to make a compromise - like
it or not.
At frequencies above 3 MHz, the polarization is
immaterial. However, for a sky wave, a horizontal antenna should be
used.
For a ground wave, a vertical antenna should be used.
The
disadvantages of a vertical antenna are that it radiates in all
directions.
Also, if its a whip, a high loss occurs caused by the
loading coils trying to compensate for the whip being too short.
b. The choice of whether an antenna is vertical or horizontal, in
some cases, is out of our hands.
If we are mobile or mobile at a
halt, obviously, the only choice is a vertical antenna. Likewise, if
we are in a jungle area, our choice must be horizontal. A desert or
arctic location also presents a challenge of how to install a mast
section to support a horizontal antenna. In most cases, most of our
nets are of short distance (0 to 35 miles). This makes communication
difficult because you can't communicate by ground wave only, nor can
you communicate by sky wave only, especially if the antenna is a
whip.
For short distance sky wave a horizontal antenna should be
used erected a quarter wave or lower above ground.
Lower antenna
heights can be used with some degradation of the transmitted signal.
If a whip is used for sky wave then it should be bent at a 45 angle.
64
Previous Page
