Lesson 2/Learning Event 4
Learning Event 4:
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.
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