8. When using a doublet, we must consider its height above ground.
For a 0 to 25-mile circuit using sky wave, we must erect the antenna
less than a quarter wavelength above the ground.
We might have to
vary the height from 15 to 30 feet. For short distance sky wave, we
want the radiation pattern straight up (Near Vertical Incidence Sky
Wave (NVIS)).
The highest frequency will determine our antenna
height.
For example: A quarter wave at 12 MHz is 19 feet.
Our
doublet antenna should not exceed 19 feet in height. As we lower the
doublet, the ground effects make the antenna electrically longer.
Therefore, you might have to lengthen the antenna.
Your SWR meter
will indicate whether you need to lengthen it or not.
9. You're probably thinking, "Now wait a minute! I don't have a
choice of frequencies.
I use what is listed in the CEOI." You're
right.
However, if those frequencies don't work, inform your
frequency manager at Division, Corps, Army, or Theater, and you will
be given additional frequencies. The bottom line is that you are not
stuck with any frequency that doesn't work. Remember also that there
is no such thing as a sole-user frequency.
You will share your
frequency with hundreds of users throughout the World. The following
frequency bands need to be avoided because of powerful ship-to-shore
or international broadcast stations located there:
BAND
STATIONS
2 - 3 MHz
Ship-to-Shore
4.75 - 4.95 MHz
Broadcast
5.95 - 6.2 MHz
Broadcast
9.50 - 9.77 MHz
Broadcast
11.70 - 11.97 MHz
Broadcast
15.10 - 15.45 MHz
Broadcast
17.70 - 17.90 MHz
Broadcast
21.45 - 21.75 MHz
Broadcast
25.60 - 26.10 MHz
Broadcast
These bands might seem to work during the day. At night, you might
experience severe interference from these stations.
Anytime you
experience interference submit a M1J1 report, then request another
frequency, until you get one that will work.
Don't keep using the
same frequency when you know it won't work. Keep reporting it until
you receive a better frequency. There are always spare frequencies.
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