The HF spectrum (3 MHz to 30 MHz) is capable of long-range communication with modest transmitter powers by the ionospheric reflection of low-angle sky waves. It is also used for short-range communication by ground wave or by the ionospheric reflection of high-angle sky waves, the latter technique being important in mountainous or forest terrain. Examples of HF users are international broadcasters, land-based fixed and mobile, maritime mobile and aeromobile operators. To enable organized use of the spectrum, the International Telecommunication Union defines a number of frequency allocations across the spectrum for each type of user.
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