(from the article `radar`) ...lower than about 100 MHz usually are not desirable for radar applications. An example where lower frequencies can provide a unique and important ... High-frequency (HF) radio is in the 100- to 10-metre wavelength band, extending from 3 megahertz to 30 megahertz. Much of the HF band is allocated to ... [2... Found on http://www.britannica.com/eb/a-z/h/46
A High Frequency modulator transforms video and audio signals to the high frequency band used for television transmission. A HF decoder (e.g. in a TV set) transforms this HF signal into the frequencies used by the screen and the audio equipment. Found on http://www.transedit.se/glossary.htm