c. Lowest Usable Frequency (LUF). For a given distance, there is
also a frequency which will be returned and which any further
decrease in frequency will result in no communication. The decrease
in frequency results in having all lower frequencies absorbed by the
ionosphere or the earth. This is called the LUF.
d. Optimum Working Frequency (FOT).
The frequency we select
should be a compromise between the MUF and the LUF.
With the
fluctuations of the ionosphere, communication might not be possible
using the MUF or LUF. We therefore choose a frequency that is lower
than the MUF and higher than the LUF. This frequency is referred to
as the FOT.
e. Signal Strength.
There are several factors that affect the
received signal strength.
The orientation of the transmitting
antenna, if possible, should be broadside to the direction of the
receiving station (s).
Likewise, the receiving antenna should be
broadside to the transmitting station(s). As the radio signal passes
through the layers, partial absorption takes place.
Part of the
signal is also lost when the signal is reflected from the earth.
Fading is the rapid fluctuations of ionization of the layers, causing
the signal to reflect off different layers.
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