FREQUENCY SELECTION.
1. Prior to selecting frequencies for a radio circuit, thought must
be given as to what type of antenna will be used. Often, during
displacement or during an alert, at least two types of antennas will
be used -- a whip while moving; a doublet or suitable compromise
antenna while mobile at a halt. Antenna selection will determine the
choice of frequency, not the other way around.
Looking at the various Immediate Sky-Wave Distance (ISD) charts, page
125 to page 141 (we will use column 5 on the charts) for various
antennas, we see that the most reliable antenna is a doublet with
reliability dropping until we reach the poorest antenna -- the whip.
We also see on the ISD charts that for the same distance, the
frequency increases with each type of antenna, with the whip having
the highest frequency. What conclusions can we draw from these
comparisons? First, we must have two frequencies for sky wave use --
a day and a night frequency. Also, when forced to use a whip, a
higher frequency than that used by a doublet must be chosen in order
to offset the power loss caused by the short length of the whip. A
nighttime whip frequency should be selected and not a nighttime
doublet frequency. The reason is that in all cases, the poorest
antenna used will determine the frequency selection. Any frequency
selected for the whip will work with the doublet. A frequency
selected for a doublet will not necessarily work reliably with a whip
because of the shortness of whip and the inevitable power loss caused
by the transmitter's loading coils. If we increase the frequency in
order to compensate for the whip's power loss, we might exceed the
MUF for the radio circuit.
2. Antenna orientation is not a consideration for short distance sky
wave use. If possible, use a radial ground system, especially with a
whip, using 36 radials which should be as long as the antenna.
Sometimes a radial system makes communication worse when it is
connected to the equipment ground. To be sure, always try to
communicate first without the radial ground connected to the
equipment ground, then with it.
3. From a signal security standpoint, we are in trouble. In order
to increase the reliability of the whip, we use higher frequencies
which provide the enemy with a better opportunity to monitor and jam
our signal. Most of the time, if we use a whip, we must sacrifice
signal security in order to maintain the radio circuit. If we use a
doublet, we could use the LUF which makes monitoring and jamming more
difficult. During displacement, communications is very crucial but
also marginal in reliability if we use a whip. We must therefore
make a compromise and ease our signal security concerns in order to
increase the reliability of the radio circuit by using the whip's
LUF. With 100 nets requiring the same FOT/LUF, obviously not all
nets can use the same LUF or FOT. More compromises are necessary.
Frequency assignments will be close as possible to the FOT down to
the whip's LUF.
4. Usually, most of our units communicate less than 50 miles (closer
to 5 - 25 miles). One consideration to make is the use of ground
waves for short distance radio circuits. Let us compare some charts
for various antennas. Look at the ground wave charts, page 156 to
page 161, especially for a 15-foot whip (use column 7). Notice that
if we use any frequency from 12 -25 MHz, a ground wave will meet our
circuit path requirements of 25 miles.
Ground Wave Range (p 159) Ground Wave for 32-Foot Whip
At 0200 Hours (p 160)
For 15-Foot Whip
At 0200 Hours For
Column 7 (300 to 499
Watts) For RTTY 60
WPM (p 158)
02 Hours (7) (7)
15-Ft 32-Ft
Whip Whip
2 MHz 9.0 27
3 MHz 14 28
5 MHz 17 29
7 MHz 20 32
10 MHz 24 33
12 MHz 25 33
15 MHz 26 33
20 MHz 26 32
25 MHz 25 6.8
30 MHz 24 2.4
5. Another consideration to make for sky wave communication when
using a whip is to bend the whip forming a 45ï‚° 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.
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.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Your happy
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