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
interference. 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.
he sloping long-wire antenna and a sloping quarter-wave wire antenna
have similar performance characteristics. Since we are using short
distance sky wave, again, antenna orientation is not a factor. Let's
take a look at the ground wave charts to see how far our ground wave
will reach. Turn to the index to ground wave ranges on page 158.
Look down the left side to find our type of service, RTTY, single
channel, FSK, 60 W/M. Next, look across the top for our power in
watts, 300 to 499. Where these two columns intersect, is the column
number for our charts. We will use 7. Now, turn to our 15-foot whip
antenna chart on page 159, the 32-foot whip on page 160, and sloping-
wire on page 161. As you can see, our ground wave range for 3 to 5.7
MHz is approximately 14 to 37 miles. During a 24-hour period this
means that ground wave propagation cannot be used for this circuit.
b. Situation 2.
(1) Let us assume that we have a radio circuit in which our
stations are 100, 750, and 1500 miles away from the net control
station (NCS). We are using an AN/GRC-142 in radioteletype mode
during a low sunspot period. Let's select two frequencies for 24-
hour operation. One of the problems in operating in a net like this
is that some of the stations might be close while others are far
away. Will the same two frequencies work for all stations all of the
time? Probably not. You might have to set up two or three nets with
two or three radios at the NCS location to accommodate the stations
of varying distance. Of course, it is easy to say. However, if you
don't have the resources, what will you do? Select frequencies that
provide communication for the majority of stations and then rely on
relaying to get the traffic through. The stations with which you
will have the most difficulty are the closest stations, while the
ones further away will be easier to communicate with.
(2) We will use a doublet at least 40 feet high above the
ground. Long distance communications is best when our antenna is
over a half wavelength (at the lowest frequency, if possible) above
the ground. The antenna should be broadside to the majority of the
stations. Let's find the highest and lowest FOT and then the highest
and lowest LUF for 100, 750, and 1500 miles.
It will probably be impossible to select a frequency or frequencies
that will provide communications to all the stations all of the time.
We will select frequencies which will allow communications to the
middle-distance station (750 miles). There will be times, probably
at night, when all stations hear, and other times when only one
station hears. In that case, the station that hears must act as a
relay for the others. For daytime, use any frequency between the
highest FOT 12.0 MHz and the highest LUF 7.2 MHz, probably in the
middle or 9.6 MHz. For nighttime, any frequency between the lowest
FOT 4.5 MHz and lowest LUF 2.0 MHz will work (move to 3 MHz to escape
ship-to-shore stations). To make communications better, a third
frequency might be considered. You can see from the LUF charts that
direct communication with station C (1500 miles) is less than 90%
reliable. Upon checking other antennas, you will find that they are
even less reliable.
(3) Suppose that we were forced to use a 32-foot whip for this
net. What would be the consequences? Ground wave would be out of
the question. As you look at the LUF charts, you can see that the
overall reliability is very low, on the order of 50%. Station A (100
miles) is the one we will have the most trouble with. Station B (750
miles) will be doing a lot of relaying. The reason that the
reliability of station A is poor is because a whip is a vertical
antenna and it favors sky wave with a low take-off angle. Station A
is too close for these low take-off angles and too far for ground
wave. Station B is at the optimum range for a vertical antenna.
Even so, the reliability for a 32-foot whip is 60 to 80 percent. It
also radiates in all directions - another disadvantage.VOLUME 8, CENTRAL EUROPE
There are 35 separate volumes covering all areas of the world.
Predictions are for minimum and maximum sunspot periods. Some of the
more common terms are:
MUF (Maximum Usable Frequency): The highest frequency which is
expected to be completely reflected from the ionosphere on at least
50 percent of the days of the month.
FOT (Optimum Traffic Frequency): The highest frequency that will be
reflected from the ionosphere on at least 90 percent of the days of
the month.
LUF (Lowest Usable Frequency): The lowest frequency that will be
reflected from the ionosphere on at least 90 percent of the days of
the month. When there is no frequency that will provide at least 90
percent reliability, the LUF will be listed followed by a letter to
represent the reliability.
B = 80 to 89 percent reliability
C = 60 to 79 percent reliability
D = 40 to 59 percent reliability
E = 20 to 39 percent reliability
F = Less than 20 percent reliability
The volumes cover six two-month periods: January-February, March-
April, May-June, July-August, September-October, and November-
December.
There is a chart that covers each two-month period listing the FOT
and MUF over a 24 hour period. Additional charts list the LUF over a
24 hour period. The charts cover the following distances: 100, 250,
500, 750, 1,000 and 1500 tiles. Predictions are given for sunspot
minimum and sunspot maximum. The LUF charts are prepared for the
following antenna types:
Half-wave horizontal dipole thirty-three feet high.
Half-wave horizontal dipole sixty-six feet high.
Fifteen foot vertical whip.
Thirty-two foot whip.
Sloping long wire.
Inverted vee.
Other antenna types are listed, but this subcourse covers only those
antennas that are considered practical from a field point of view.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Your happy
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