HF radio transmission
70 Manpack Transceiver 2110 series Getting Started Guide
Sky wave
The sky wave is the most important form of HF propagation.
The HF radio wave is transmitted toward the sky and is
reflected by the ionosphere to a distant receiver on earth.
The reflective properties of the ionosphere change throughout
the day, from season to season, and yearly.
Figure 7: The reflective properties of the ionosphere
Frequency, distance and time of day
The extent to which an HF radio wave is reflected depends on
the frequency that is used. If the frequency is too low, the
signal is absorbed by the ionosphere. If the frequency is too
high, the signal passes straight through the ionosphere. Within
the HF band, low frequencies are generally considered to be in
the range of 2 to 10 MHz. High frequencies are above
10 MHz.
A frequency chosen for daytime transmission may not
necessarily be suitable for night-time use. During the day, the
layers of the ionosphere are thick. The layers absorb lower
frequencies and reflect higher frequencies. At night, the
ionosphere becomes very thin. The low frequencies that were
absorbed during the day are reflected, and the high frequencies
that were reflected during the day pass straight through.
ionosphere
transmitter
receiver
emitted HF
radio wave
reflected
HF radio
wave
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