High frequency

High Frequency or HF refers to radio frequencies between 3 and 30 MHz. This range is often called shortwave.

Since the ionosphere often reflects HF radio waves quite well, this range is extensively used for medium and long range terrestial radio communication. However, suitability of this portion of the spectrum for such communication varies greatly with a complex combination of factors:

Sunlight/darkness at site of transmission and reception
Transmitter/receiver proximity to terminator
Season
Sunspot cycle
Solar activity
Polar aurora
Maximum usable frequency
Lowest usable frequency
Frequency of operation within the HF range

HF is often used for continuous wave morse code transmissions.

See also: VHF, AM radio, Radio propagation