Shortwave radio

Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (approximately 100 to 10 metres in wavelength). It lies between the medium frequency band (MF) and the bottom of the VHF band.

Radio waves in the shortwave band can be reflected or refracted from a layer of electrically charged atoms in the atmosphere called the ionosphere. Therefore, short waves directed at an angle into the sky can be reflected back to Earth at great distances, beyond the horizon. This is called skywave or "skip" propagation. Thus shortwave radio can be used for communication over very long distances, in contrast to radio waves of higher frequency, which travel in straight lines (line-of-sight propagation) and are generally limited by the visual horizon, about 64 km (40 miles).

Shortwave radio is used for broadcasting of voice and music to shortwave listeners over very large areas; sometimes entire continents or beyond. Other uses include military over-the-horizon radar, diplomatic communication, amateur radio enthusiasts' two-way international communication for hobby, education, and emergency service, and long-distance aviation and marine communications.

Shortwave broadcasts of radio programs played an important role in international broadcasting for many decades, serving both to provide news and information and as a propaganda tool for an international audience. Other technologies have come to replace shortwave for long-distance distribution of radio programs, such as satellite radio, cable broadcasting, and Internet radio. However, shortwave listening remains a niche hobby, with enthusiasts tuning in to fringe stations.

Shortwave radio is important in war zones, such as in the Russo-Ukrainian war, and shortwave broadcasts can be transmitted over thousands of miles from a single transmitter, making it difficult for government authorities to censor them. Shortwave radio is also often used by aircraft.