5. Another consideration to make for sky wave communication when
using a whip is to bend the whip forming a 45 angle. We must also
consider adding additional whip mast sections so that the whip will
approach a quarter of a wavelength. We might have to guy the whip to
keep it from leaning too much if we make use of the ground wave.
However, tuning the whip will be easier when it is at least a quarter
wave in length. When the whip is shorter than a quarter wave, there
will be a power loss due to the matching done by the loading coils of
the transmitter. This is most evident when we operate the whip below
15 MHz. To calculate the length of the quarter wavelength whip, use
the formula 234/F (F is in MHz and 234 is 1/2 of 468).
Lets
calculate the power loss for an AN/GRC-142 with a 400 watt output and
operating at 2 MHz using a whip. What percent of a quarter wave is a
15-foot whip at 2 MHz?
F = 2 MHz
234/2 MHz = 117 feet
15 feet/117 feet = 12%
400 watts x 0.12 = 48 watts output (roughly)
48 watts is all that is actually going to the antenna.
The rest of
the power is used up by the transmitter's loading coils.
6. For our 25 mile radio circuit, we selected the highest FOT from
the 100 mile MUF/FOT chart on page 129.
We selected daytime
frequency, during a low sunspot (SSN10) period: FOT 5.7 MHz at 1200
hours and from the 15-foot whip LUF chart on page 134, we selected
the highest LUF of 6.7 MHz at 1200 hours. Our daytime frequency will
be 5.7 MHz. We shouldn't use the LUF of 6.7 MHz because it exceeds
the FOT.
As you can see all the LUF for the whip is less than 90
percent reliable, closer to 0 to 20 percent.
7. Nighttime presents other problems. The frequency band of 2 to 3
MHz is filled with powerful commercial stations. Even though our LUF
ISD charts indicate that this band is the one to use, there will be
too much interference from these stations. We are forced to go up in
frequency. Look at the ground wave chart for a 15-foot whip on page
159. We see that the ground wave range for 12 MHz is 25 miles. If
we select a frequency that makes the best use of a ground wave, we
might have one difficulty - the interference from an incoming sky-
wave signal might be stronger than our ground wave signal.
If that
happens, try other frequencies until you find one that is relatively
free of interference. Even if we were to use a 32-foot whip instead
of a 15-foot whip while operating on the same frequencies, we can
still expect a reliability of less than 90% (LUF chart on page 136).
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