Shortwave radio

 ( International Broadcasting ) 

Note that on modern computer-driven systems, digital modes are typically sent by coupling a computer's sound output to the SSB input of a radio.

• International broadcasting primarily by government-sponsored propaganda, or international news (for example, the BBC World Service), religious or cultural stations to foreign audiences: The most common use of all.
• Domestic broadcasting: to widely dispersed populations with few longwave, mediumwave and FM stations serving them; or for speciality political, religious and alternative media networks; or of individual commercial and non-commercial paid broadcasts.
• Oceanic air traffic control uses the HF/shortwave band for long-distance communication to aircraft over the oceans and poles, which are far beyond the range of traditional VHF frequencies. Modern systems also include satellite communications, such as ADS-C/CPDLC.
• Two-way radio communications by marine and maritime HF stations, aeronautical users, and ground based stations.
  (2019). 9783030306328, Springer Nature. . ISBN 9783030306328
  For example, two way shortwave communication is still used in remote regions by the Royal Flying Doctor Service of Australia.
  (2013). 9780786474110, McFarland. . ISBN 9780786474110
• Utility station transmitting messages not intended for the general public, such as merchant shipping, marine weather, and ship-to-shore stations; for aviation weather and air-to-ground communications; for military communications; for long-distance governmental purposes, and for other non-broadcast communications.
• Amateur radio operators at the 80/75, 60, 40, 30, 20, 17, 15, 12, and 10–meter bands. Licenses are granted by authorized government agencies.
• Time signal and radio clock stations: In North America, WWV radio and WWVH transmit at these frequencies: 2.5 MHz, 5 MHz, 10 MHz, and 15 MHz; and WWV also transmits on 20 MHz. The CHU radio station in Canada transmits on the following frequencies: 3.33 MHz, 7.85 MHz, and 14.67 MHz. Other similar radio clock stations transmit on various shortwave and longwave frequencies around the world. The shortwave transmissions are primarily intended for human reception, while the longwave stations are generally used for automatic synchronization of watches and clocks.

Sporadic or non-traditional users of the shortwave bands may include:

• Clandestine stations. These are stations that broadcast on behalf of various political movements such as rebel or insurrectionist forces. They may advocate civil war, insurrection, rebellion against the government-in-charge of the country to which they are directed. Clandestine broadcasts may emanate from transmitters located in rebel-controlled territory or from outside the country entirely, using another country's transmission facilities.
• . These stations regularly appear and disappear all over the shortwave radio band, but are unlicensed and untraceable. It is believed that numbers stations are operated by government agencies and are used to communicate with clandestine operatives working within foreign countries. However, no definitive proof of such use has emerged. Because the vast majority of these broadcasts contain nothing but the recitation of blocks of numbers, in various languages, with occasional bursts of music, they have become known colloquially as "number stations". Perhaps the most noted number station is called the "Lincolnshire Poacher", named after the 18th century English folk song, which is transmitted just before the sequences of numbers.
• Unlicensed two way radio activity by individuals such as taxi drivers, bus drivers and fishermen in various countries can be heard on various shortwave frequencies. Such unlicensed transmissions by "pirate" or "bootleg" two way radio operators can often cause signal interference to licensed stations. Unlicensed business radio (taxis, trucking companies, among numerous others) land mobile systems may be found in the 20–30 MHz region while unlicensed marine mobile and other similar users may be found over the entire shortwave range.
• Pirate radio broadcasters who feature programming such as music, talk and other entertainment, can be heard sporadically and in various modes on the shortwave bands. Pirate broadcasters take advantage of the better propagation characteristics to achieve more range compared to the AM or FM broadcast bands.
  Andrew R. Yoder (2025). 9780071375634, McGraw Hill Professional. . ISBN 9780071375634
• Over-the-horizon radar: From 1976 to 1989, the Soviet Union's Duga radar over-the-horizon radar system blotted out numerous shortwave broadcasts daily.
• Ionospheric heaters used for scientific experimentation such as the High Frequency Active Auroral Research Program in Alaska, and the Sura ionospheric heating facility in Russia.
• High-frequency trading firms have asked the FCC to open shortwave frequencies to datacasting that would allow faster data transmission than fiber optic cables. Concerns about the reliability, capacity, security and contagion risk presented by this use of shortwave technology have been raised.

• See International broadcasting for details on the history and practice of broadcasting to foreign audiences.
• See List of shortwave radio broadcasters for a list of international and domestic shortwave radio broadcasters.
• See Shortwave relay station for the actual kinds of integrated technologies used to bring high power signals to listeners.

As of WRC-97 in 1997, these bands were allocated for international broadcasting. AM shortwave broadcasting channels are allocated with a 5 kHz separation for traditional analog audio broadcasting:

Although countries generally follow the assigned bands, there may be small differences between countries or regions. For example, in the official bandplan of the Netherlands, the 49 m band starts at 5.95 MHz, the 41 m band ends at 7.45 MHz, the 11 m band starts at 25.67 MHz, and the 120 m, 90 m, and 60 m bands are absent altogether. International broadcasters sometimes operate outside the normal the WRC-allocated bands or use off-channel frequencies. This is done for practical reasons, or to attract attention in crowded bands (60 m, 49 m, 40 m, 41 m, 31 m, 25 m).

The new digital audio broadcasting format for shortwave DRM operates 10 kHz or 20 kHz channels. There are some ongoing discussions with respect to specific band allocation for DRM, as it mainly transmitted in 10 kHz format.

The power used by shortwave transmitters ranges from less than one watt for some experimental and amateur radio transmissions to 500 kilowatts and higher for intercontinental broadcasters and over-the-horizon radar. Shortwave transmitting centers often use specialized antenna designs (like the ALLISS antenna technology) to concentrate radio energy at the target area.

• Difficulty of censoring programming by authorities in restrictive countries. Unlike their relative ease in monitoring and censoring the Internet, over-the air television, cable television, satellite television, satellite radio, mobile phones, landline phones, and satellite phones, government authorities face technical difficulties monitoring which stations (sites) are being listened to (accessed). For example, during the attempted coup against Soviet President Mikhail Gorbachev, when his access to communications was limited (e.g. his phones, television and radio were cut off), Gorbachev was able to stay informed by means of the BBC World Service on shortwave.
• Low-cost shortwave radios are widely available in all but the most repressive countries in the world. Simple shortwave regenerative receivers can be easily built with a few parts.
• In many countries (particularly in most developing nations and in the Eastern bloc during the Cold War era) ownership of shortwave receivers has been and continues to be widespread (in many of these countries some domestic stations also used shortwave).
• Many newer shortwave receivers are portable and can be battery-operated, making them useful in difficult circumstances. Newer technology includes hand-cranked radios which provide power without batteries.
• Shortwave radios can be used in situations where over-the-air television, cable television, satellite television, Landline, mobile phones, satellite phones, satellite communications, or the Internet is temporarily, long-term or permanently unavailable (or unaffordable).
• Shortwave radio travels much farther than broadcast FM (88–108 MHz). Shortwave broadcasts can be easily transmitted over a distance of several thousand miles, including from one continent to another.
• Particularly in tropical regions, SW is somewhat less prone to interference from thunderstorms than medium wave, and is able to cover a large geographic area with relatively low power (and hence cost). Therefore, in many of these countries it is widely used for domestic broadcasting.
• Very little infrastructure is required for long-distance two-way communications using shortwave radio. All one needs is a pair of transceivers, each with an antenna, and a source of energy (such as a battery, a portable generator, or the electrical grid). This makes shortwave radio one of the most robust means of communications, which can be disrupted only by interference or bad ionospheric conditions. Modern digital transmission modes such as MFSK and Olivia MFSK are even more robust, allowing successful reception of signals well below the noise floor of a conventional receiver.

• In most Western countries, shortwave radio ownership is usually limited to enthusiasts, since most new standard radios do not receive the shortwave band. Therefore, Western audiences are limited.
• In the developed world, shortwave reception is very difficult in urban areas because of excessive noise from switched-mode power adapters, fluorescent or LED light sources, internet modems and routers, computers and many other sources of radio interference.
• Audio quality may be limited due to interference and the modes that are used.