High frequency (HF) radio frequencies are 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:

HF is often used for continuous wave morse code transmissions.

See also: VHF, AM radio, Radio propagation