Shortwave bands

Shortwave bands are frequency allocations for use within the shortwave radio spectrum (the upper medium frequency [MF] band and all of the high frequency [HF] band). Radio waves in these frequency ranges can be used for very long distance (transcontinental) communication because they can reflect off layers of charged particles in the ionosphere and return to Earth beyond the horizon, a mechanism called skywave or “skip” propagation. They are allocated by the ITU for radio services such as maritime communications, international shortwave broadcasting and worldwide amateur radio. The bands are conventionally named by their wavelength (λ) in metres, for example the ‘20 meter band’. Radio propagation and possible communication distances vary depending on the time of day, the season and the level of solar activity. These bands are used by powerful long range AM radio stations, many operated by governments, which broadcast to multiple countries. Most international broadcasters use amplitude modulation with 5 kHz steps between channels; a few use single sideband or reduced carrier single sideband modulation. The World Radiocommunication Conference (WRC), organized under the auspices of the International Telecommunication Union, allocates bands for various services in periodic conferences. The most recent WRC took place in 2012. At WRC-97 in 1997, the following bands were allocated for international broadcasting: Particularly in the United States and at frequencies under 10 MHz, shortwave broadcasters may operate in between those bands in bands allocated for fixed service, with the 60-meter band extending as high as 5.13 MHz, the 49-meter band down to 5.8 MHz, the 41-meter band as high as 7.78 MHz and the 31-meter band extending as low as 9.265 MHz.