10. The Immediate and Short Distance Sky-Wave books can be had just
for the asking. Write to:
Commanding General
USAESEIA
ATTN: ASC-E-TP
Fort Huachuca, Arizona 85613-5300
or call AUTOVON 879-7685. Every division or higher level unit should
have a book for their area. There are 35 volumes covering all areas
of the world.
For our use, we will use column 5 from the ISD book
and column 7 from the GW book.
11. Let's do a couple of sky-wave frequency selection problems.
a. Situation 1.
Let us say that we have a radio net with only two stations
which are 50 miles apart. The radio set used is the AN/GRC-142. We
are using radioteletype during a period of low sunspot activity. We
need to select two frequencies for 24-hour communications.
We will
use a doublet antenna erected less than a quarter wavelength above
the ground for our highest frequency so that our radiation pattern is
straight up.
Keep in mind that we might have to vary the antenna
height from 40 feet down to 10 feet (any lower than 10 feet, we have
safety problems).
Turn to the sky-wave extracts, the Index to the
Lowest Usable High frequency (LUF) on page 128.
Look down the left
side until you find Radioteletype, NSK, 60 WPM. Now, look across the
top of the columns for our power output, 400 watts. We will use the
column which has our power output (201-600) falling in between. Look
down this column until it intersects with our type of service. Five
is the column that we will use on our sky wave LUF charts. Turn to
the MUF/FOT chart on page 129. Look down the low sunspot column (SSN
10) for 100 miles. We will select the highest and lowest FOT. They
are 5.7 MHz and 2.5 MHz. Look at the doublet or dipole LUF chart on
page 130 for 100 miles, low sunspot, 33 feet high, column 5.
The
highest and lowest LUF is 2.0 MHz. We can now assume that a daytime
high frequency selection can be from 2.0 to 5.7 MHz.
These
frequencies will give us a 90% reliability.
(See page 125.) Let's
keep in mind the problem of ship-to-shore stations and eliminate from
consideration frequencies between 2 to 3 MHz. Now we have a daytime
high frequency between 3 to 5.7 MHz and a low nighttime frequency of
just above 3 MHz.
We must compromise because of possible
If we check the ISD extracts, we will see that a
doublet and a sloping long-wire antenna are the only antennas that
give 24-hour 90% reliability.
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