Skanti TRP 9250, Technical Manual, Page: 71 / 289

Skywave propagation
At frequencies between 5 MHz and 30 MHz long distance propagation is achieved by skywave
propagation. Certain layers of the Earth’s upper atmosphere, in the Ionosphere, called the
E-layer and the F1- and the F2-layers are able to act as a sort of mirror to the radio waves in the
above mentioned frequency range. A signal of the appropriate frequency will therefore be
reflected by the mirror and will return to the Earth beyond the horizon of the transmitter antenna.
The reflection properties of the Ionosphere are strongly dependent of the state of the Ionosphere.
During the day the Sun’s radiation will increase the reflection, while the reflection during the night
will be lower. At the same time the radiation will however, increase the attenuation of the radio
waves.
The highest frequency that can be used to communication on a certain distance is called the
Maximum Usable Frequency (MUF). This frequency is, however, subject to great variation in
propagation. The best frequency to use is about 15 pct. lower than the MUF. This frequency is
called the FOT, Frequence Optimal du Travail.
Besides the diurnal variations due to the Sun’s radiation, the propagation is also dependent of
the solar activity in general. During periods of high solar activity the MUF will be higher, but at the
same time disturbances in the Ionosphere, due to the high activity, will be more frequent.
Tables of MUFs covering various radio paths are published monthly by many telecom
administrations.
OPERATION PROPAGATION OF HF AND MF RADIO WAVES
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