Sky wave Propagation offers long range communication with very little
power required.
The most difficult question regarding sky wave
propagation is what frequency to use.
The HF (3-30 MHz) band uses
ionospheric reflection most effectively.
a. Sky Wave Transmission Path.
Figure 23 indicates the many
varied paths a radio signal may take from the transmitter to the
receiver.
Notice that a receiving station located in the skip zone
would receive no signal. Through proper frequency selection, antenna
Notice also that from the point the radio signal leaves the
transmitter to the point it contacts the earth is called the skip
distance.
(1) Sky wave modes.
The distance the sky wave signal travels
before it returns to earth depends upon the ionospheric layer used.
When the signal strikes the earth, part of the signal is absorbed.
The rest is reflected back to the ionosphere. This is repeated until
the signal is too weak to be reflected either by the ionosphere or
the earth. This is called a multi-hop transmission.
(2) Frequency. The problem as to what frequency to use is not
an easy one to solve.
As mentioned earlier, the higher the
frequency, the higher the ionospheric density required to return the
frequency to earth.
Figure 28 shows radio signals of several
frequencies. Some are returned while others are not. The 5 and 20
MHz signals are returned, while the 100 MHz signal is not.
Notice
that the 20 MHz signal travels further. While this may hold true for
day time communication, it might not be true at night.
b. Maximum Usable Frequency (MUF).
(1) See Figure 23. For a given distance, there is a frequency
in which any further increase in frequency will result in no
communication. In other words, the station located in the skip zone
does not receive a signal.
The highest frequency that can be used
between two points is the maximum usable frequency. As the distance
increases the MUF increases.
(2) Care must be taken in selecting the frequency. Too high -
it passes through the ionosphere or overshoots the receiver. Too low
and it will be absorbed by either an ionospheric layer or the earth.
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