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Shortwave radio is
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
transmission using shortwave (SW)
radio frequencies Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upper ...
. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 metres); above the medium frequency band (MF), to 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 The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
. 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 Propagation can refer to: * Chain propagation in a chemical reaction mechanism *Crack propagation, the growth of a crack during the fracture of materials * Propaganda, non-objective information used to further an agenda * Reproduction, and other fo ...
. 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 Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions travelin ...
) and are limited by the visual horizon, about 64 km (40 miles). Shortwave broadcasts of radio programs played an important role in the early days of radio history. In
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
it was used as a propaganda tool for an international audience. The heyday of international shortwave broadcasting was during the
Cold War The Cold War is a term commonly used to refer to a period of geopolitical tension between the United States and the Soviet Union and their respective allies, the Western Bloc and the Eastern Bloc. The term '' cold war'' is used because the ...
between 1960 and 1980. With the wide implementation of other technologies for the distribution of radio programs, such as
satellite radio Satellite radio is defined by the International Telecommunication Union (ITU)'s ITU Radio Regulations (RR) as a ''broadcasting-satellite service''. The satellite's signals are broadcast nationwide, across a much wider geographical area than ter ...
and cable broadcasting as well as IP-based transmissions, shortwave broadcasting lost importance. Initiatives for the digitization of broadcasting did not bear fruit either, and so , few broadcasters continue to broadcast programs on shortwave. However, shortwave remains important in war zones, such as in the
Russo-Ukrainian war The Russo-Ukrainian War; uk, російсько-українська війна, rosiisko-ukrainska viina. has been ongoing between Russia (alongside Russian separatist forces in Donbas, Russian separatists in Ukraine) and Ukraine since Feb ...
, and shortwave broadcasts can be transmitted over thousands of miles from a single transmitter, making it difficult for government authorities to censor them.


History


Development

The name "shortwave" originated during the beginning of radio in the early 20th century, when the
radio spectrum The radio spectrum is the part of the electromagnetic spectrum with frequencies from 0  Hz to 3,000 GHz (3  THz). Electromagnetic waves in this frequency range, called radio waves, are widely used in modern technology, particula ...
was divided into
long wave In radio, longwave, long wave or long-wave, and commonly abbreviated LW, refers to parts of the radio spectrum with wavelengths longer than what was originally called the medium-wave broadcasting band. The term is historic, dating from the e ...
(LW), medium wave (MW), and short wave (SW) bands based on the length of the wave. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m (1,500 kHz) which marked the original upper limit of the
medium frequency Medium frequency (MF) is the ITU designation for radio frequencies (RF) in the range of 300  kilohertz (kHz) to 3  megahertz (MHz). Part of this band is the medium wave (MW) AM broadcast band. The MF band is also known as the ...
band first used for radio communications. The broadcast medium wave band now extends above the 200 m / 1,500 kHz limit. Early long-distance
radio telegraphy Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for t ...
used long waves, below 300 
kilohertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one h ...
(kHz). The drawbacks to this system included a very limited spectrum available for long-distance communication, and the very expensive
transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...
s, receivers and gigantic antennas. Long waves are also difficult to beam directionally, resulting in a major loss of power over long distances. Prior to the 1920s, the shortwave frequencies above 1.5 MHz were regarded as useless for long-distance communication and were designated in many countries for amateur use.
Guglielmo Marconi Guglielmo Giovanni Maria Marconi, 1st Marquis of Marconi (; 25 April 187420 July 1937) was an Italians, Italian inventor and electrical engineering, electrical engineer, known for his creation of a practical radio wave-based Wireless telegrap ...
, pioneer of radio, commissioned his assistant Charles Samuel Franklin to carry out a large-scale study into the transmission characteristics of short-wavelength waves and to determine their suitability for long-distance transmissions. Franklin rigged up a large antenna at Poldhu Wireless Station,
Cornwall Cornwall (; kw, Kernow ) is a historic county and ceremonial county in South West England. It is recognised as one of the Celtic nations, and is the homeland of the Cornish people. Cornwall is bordered to the north and west by the Atlantic ...
, running on 25 kW of power. In June and July 1923, wireless transmissions were completed during nights on 97 meters (about 3 MHz) from Poldhu to Marconi's yacht ''Elettra'' in the
Cape Verde Islands , national_anthem = () , official_languages = Portuguese , national_languages = Cape Verdean Creole , capital = Praia , coordinates = , largest_city = capital , demonym ...
. In September 1924, Marconi arranged for transmissions to be made day and night on 32 meters (9.4& MHz) from Poldhu to his yacht in the harbour at
Beirut Beirut, french: Beyrouth is the capital and largest city of Lebanon. , Greater Beirut has a population of 2.5 million, which makes it the third-largest city in the Levant region. The city is situated on a peninsula at the midpoint o ...
, to which he had sailed, and was "astonished" to find he could receive signals "throughout the day". Franklin went on to refine the directional transmission by inventing the
curtain array Curtain arrays are a class of large multielement directional radio transmitting wire antennas, used in the shortwave radio bands. They are a type of reflective array antenna, consisting of multiple wire dipole antennas, suspended in a vert ...
aerial system. In July 1924, Marconi entered into contracts with the British General Post Office (GPO) to install high-speed shortwave telegraphy circuits from London to Australia, India, South Africa and Canada as the main element of the
Imperial Wireless Chain The Imperial Wireless Chain was a strategic international communications network of powerful long range radiotelegraphy stations, created by the British government to link the countries of the British Empire. The stations exchanged commercial a ...
. The UK-to-Canada shortwave "Beam Wireless Service" went into commercial operation on 25 October 1926. Beam Wireless Services from the UK to Australia, South Africa and India went into service in 1927. Shortwave communications began to grow rapidly in the 1920s. By 1928, more than half of long-distance communications had moved from transoceanic cables and longwave wireless services to shortwave, and the overall volume of transoceanic shortwave communications had vastly increased. Shortwave stations had cost and efficiency advantages over massive longwave wireless installations. However, some commercial longwave communications stations remained in use until the 1960s. Long-distance radio circuits also reduced the need for new cables, although the cables maintained their advantages of high security and a much more reliable and better-quality signal than shortwave. The cable companies began to lose large sums of money in 1927. A serious financial crisis threatened viability of cable companies that were vital to strategic British interests. The British government convened the Imperial Wireless and Cable Conference in 1928 "to examine the situation that had arisen as a result of the competition of Beam Wireless with the Cable Services". It recommended and received government approval for all overseas cable and wireless resources of the Empire to be merged into one system controlled by a newly formed company in 1929, Imperial and International Communications Ltd. The name of the company was changed to Cable and Wireless Ltd. in 1934. A resurgence of long-distance cables began in 1956 with the laying of TAT-1 across the Atlantic Ocean, the first voice frequency cable on this route. This provided 36 high-quality telephone channels and was soon followed by even higher-capacity cables all around the world. Competition from these cables soon ended the economic viability of shortwave radio for commercial communication.


Amateur use of shortwave propagation

Amateur radio operators An amateur radio operator is someone who uses equipment at an amateur radio station to engage in two-way personal communications with other amateur operators on radio frequencies assigned to the amateur radio service. Amateur radio operators hav ...
also discovered that long-distance communication was possible on shortwave bands. Early long-distance services used
surface wave In physics, a surface wave is a mechanical wave that propagates along the Interface (chemistry), interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occu ...
propagation at very low frequencies, which are attenuated along the path at wavelengths shorter than 1,000 meters. Longer distances and higher frequencies using this method meant more signal loss. This, and the difficulties of generating and detecting higher frequencies, made discovery of shortwave propagation difficult for commercial services. Radio amateurs may have conducted the first successful transatlantic tests in December 1921, operating in the 200 meter
mediumwave Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the dayti ...
band (near 1,500 kHz, inside the modern AM broadcast band), which at that time was the shortest wavelength / highest frequency available to amateur radio. In 1922 hundreds of North American amateurs were heard in Europe on 200 meters and at least 20 North American amateurs heard amateur signals from Europe. The first two-way communications between North American and Hawaiian amateurs began in 1922 at 200 meters. Although operation on wavelengths shorter than 200 meters was technically illegal (but tolerated at the time as the authorities mistakenly believed that such frequencies were useless for commercial or military use), amateurs began to experiment with those wavelengths using newly available
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
s shortly after World War I. Extreme interference at the longer edge of the 150–200 meter band – the official wavelengths allocated to
amateurs An amateur () is generally considered a person who pursues an avocation independent from their source of income. Amateurs and their pursuits are also described as popular, informal, self-taught, user-generated, DIY, and hobbyist. History ...
by the Second National Radio Conference in 1923 – forced amateurs to shift to shorter and shorter wavelengths; however, amateurs were limited by regulation to wavelengths longer than 150 meters (2 MHz). A few fortunate amateurs who obtained special permission for experimental communications at wavelengths shorter than 150 meters completed hundreds of long-distance two-way contacts on 100 meters (3 MHz) in 1923 including the first transatlantic two-way contacts. By 1924 many additional specially licensed amateurs were routinely making transoceanic contacts at distances of 6,000 miles (9,600 km) and more. On 21 September 1924 several amateurs in California completed two-way contacts with an amateur in
New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
. On 19 October amateurs in New Zealand and England completed a 90 minute two-way contact nearly halfway around the world. On 10 October the Third National Radio Conference made three shortwave bands available to U.S. amateurs at 80 meters (3.75 MHz), 40 meters (7 MHz) and 20 meters (14 MHz). These were allocated worldwide, while the 10 meter band (28 MHz) was created by the Washington International Radiotelegraph Conference on 25 November 1927. The 15 meter band (21 MHz) was opened to amateurs in the United States on 1 May 1952.


Propagation characteristics

Shortwave radio frequency energy is capable of reaching any location on the Earth as it is influenced by
ionospheric reflection In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. Since it is not limited by the curvature ...
back to the earth by the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
, (a phenomenon known as " skywave propagation"). A typical phenomenon of shortwave propagation is the occurrence of a
skip zone Skip or Skips may refer to: Acronyms * SKIP (Skeletal muscle and kidney enriched inositol phosphatase), a human gene * Simple Key-Management for Internet Protocol * SKIP of New York (Sick Kids need Involved People), a non-profit agency aidi ...
where reception fails. With a fixed working frequency, large changes in ionospheric conditions may create skip zones at night. As a result of the multi-layer structure of the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
, propagation often simultaneously occurs on different paths, scattered by the ‘E’ or ‘F’ layer and with different numbers of hops, a phenomenon that may be disturbed for certain techniques. Particularly for lower frequencies of the shortwave band, absorption of radio frequency energy in the lowest ionospheric layer, the ‘D’ layer, may impose a serious limit. This is due to collisions of electrons with neutral molecules, absorbing some of a
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
's energy and converting it to heat. Predictions of skywave propagation depend on: * The distance from the transmitter to the target receiver. * Time of day. During the day, frequencies higher than approximately 12 MHz can travel longer distances than lower ones. At night, this property is reversed. * With lower frequencies the dependence on the time of the day is mainly due to the lowest ionospheric layer, the ‘D’ Layer, forming only during the day when photons from the sun break up atoms into ions and free electrons. * Season. During the winter months of the Northern or Southern hemispheres, the AM/MW broadcast band tends to be more favorable because of longer hours of darkness. *
Solar flare A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phe ...
s produce a large increase in D region ionization – so great, sometimes for periods of several minutes, that skywave propagation is nonexistent.


Types of modulation

Several different types of
modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
are used to incorporate information in a short-wave signal.


Audio modes


AM

Amplitude modulation Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...
is the simplest type and the most commonly used for
shortwave broadcasting Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 m ...
. The instantaneous amplitude of the carrier is controlled by the amplitude of the signal (speech, or music, for example). At the receiver, a simple
detector A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
recovers the desired modulation signal from the carrier.


SSB

Single-sideband In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude m ...
transmission is a form of amplitude modulation but in effect filters the result of modulation. An amplitude-modulated signal has frequency components both above and below the
carrier frequency In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a ...
. If one set of these components is eliminated as well as the residual carrier, only the remaining set is transmitted. This reduces power in the transmission, as roughly of the energy sent by an AM signal is in the carrier, which is not needed to recover the information contained in the signal. It also reduces signal
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
, enabling less than one-half the AM signal bandwidth to be used. The drawback is the receiver is more complicated, since it must re-create the carrier to recover the signal. Small errors in the detection process greatly affect the pitch of the received signal. As a result, single sideband is not used for music or general broadcast. Single sideband is used for long-range voice communications by ships and aircraft,
citizen's band Citizens band radio (also known as CB radio), used in many countries, is a land mobile radio system, a system allowing short-distance person-to-many persons bidirectional voice communication among individuals, using two way radios operating on ...
, and amateur radio operators. Lower sideband (LSB) is customarily used below 9 MHz and USB (upper sideband) above 9 MHz.


VSB

Vestigial sideband In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude m ...
transmits the carrier and one complete sideband, but filters out most of the other sideband. It is a compromise between AM and SSB, enabling simple receivers to be used, but requires almost as much transmitter power as AM. Its main advantage is that only half the bandwidth of an AM signal is used. It is used by the Canadian standard
time signal A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day. Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, a ...
station CHU.
Vestigial sideband In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude m ...
was used for analog television and by ATSC, the
digital TV Digital television (DTV) is the transmission of television signals using digital encoding, in contrast to the earlier analog television technology which used analog signals. At the time of its development it was considered an innovative adva ...
system used in North America.


NFM

Narrow-band
frequency modulation Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and Run-length limited#FM: .280. ...
(NBFM or NFM) is used typically above 20 MHz. Because of the larger bandwidth required, NBFM is commonly used for VHF communication. Regulations limit the bandwidth of a signal transmitted in the HF bands, and the advantages of frequency modulation are greatest if the FM signal has a wide bandwidth. NBFM is limited to short-range transmissions due to the multiphasic distortions created by the ionosphere.


DRM

Digital Radio Mondiale (DRM) is a digital modulation for use on bands below 30 MHz. It is a digital signal, like the data modes, below, but is for transmitting audio, like the analog modes above.


Data modes


CW

Continuous wave A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration. It may refer to e.g. a laser or particle ...
(CW) is on-and-off keying of a sine-wave carrier, used for
Morse code Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...
communications and
Hellschreiber The Hellschreiber, Feldhellschreiber or Typenbildfeldfernschreiber (also Hell-Schreiber named after its inventor Rudolf Hell) is a facsimile-based teleprinter invented by Rudolf Hell. Compared to contemporary teleprinters that were based on ty ...
facsimile A facsimile (from Latin ''fac simile'', "to make alike") is a copy or reproduction of an old book, manuscript, map, Old master print, art print, or other item of historical value that is as true to the original source as possible. It differs from ...
-based
teleprinter A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Initia ...
transmissions. It is a data mode, although often listed separately. It is typically received via lower or upper SSB modes.


RTTY, FAX, SSTV

Radioteletype Radioteletype (RTTY) is a telecommunications system consisting originally of two or more electromechanical teleprinters in different locations connected by radio rather than a wired link. Radioteletype evolved from earlier landline teleprinter ...
, fax, digital,
slow-scan television Slow-scan television (SSTV) is a picture transmission method, used mainly by amateur radio operators, to transmit and receive static pictures via radio in monochrome or color. A literal term for SSTV is narrowband television. Analog broadcast tel ...
, and other systems use forms of frequency-shift keying or audio subcarriers on a shortwave carrier. These generally require special equipment to decode, such as software on a computer equipped with a sound card. 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.


Users

Some established users of the shortwave radio bands may include: *
International broadcasting International broadcasting, in a limited extent, began during World War I, when German and British stations broadcast press communiqués using Morse code. With the severing of Germany's undersea cables, the wireless telegraph station in Nauen was ...
primarily by government-sponsored
propaganda Propaganda is communication that is primarily used to influence or persuade an audience to further an agenda, which may not be objective and may be selectively presenting facts to encourage a particular synthesis or perception, or using loaded ...
, or international news (for example, the
BBC World Service The BBC World Service is an international broadcasting, international broadcaster owned and operated by the BBC, with funding from the Government of the United Kingdom, British Government through the Foreign Secretary, Foreign Secretary's o ...
) or cultural stations to foreign audiences: The most common use of all. * Domestic broadcasting: to widely dispersed populations with few
longwave In radio, longwave, long wave or long-wave, and commonly abbreviated LW, refers to parts of the radio spectrum with wavelengths longer than what was originally called the medium-wave broadcasting band. The term is historic, dating from the e ...
,
mediumwave Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the dayti ...
and FM stations serving them; or for speciality political, religious and
alternative media Alternative media are media sources that differ from established or dominant types of media (such as mainstream media or mass media) in terms of their content, production, or distribution.Downing, John (2001). ''Radical Media''. Thousand Oaks, ...
networks; or of individual commercial and non-commercial paid broadcasts. * Oceanic
air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airs ...
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. For example, two way shortwave communication is still used in remote regions by the
Royal Flying Doctor Service of Australia The Royal Flying Doctor Service (RFDS), commonly known as the Flying Doctor, is an air medical service in Australia. It is a non-profit organisation that provides emergency and primary health care services for those living in rural, remote an ...
. * "Utility" stations 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 Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communic ...
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 A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day. Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, a ...
and radio clock stations: In North America, WWV radio and WWVH radio 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. *
Numbers station A numbers station is a shortwave radio station characterized by broadcasts of formatted numbers, which are believed to be addressed to intelligence officers operating in foreign countries. Most identified stations use speech synthesis to vocal ...
s. 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 A two-way radio is a radio that can both transmit and receive radio waves (a transceiver), unlike a broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and receiver in one unit, used for bidirec ...
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 Pirate radio or a pirate radio station is a radio station that broadcasts without a valid license. In some cases, radio stations are considered legal where the signal is transmitted, but illegal where the signals are received—especially w ...
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. *
Over-the-horizon radar Over-the-horizon radar (OTH), sometimes called beyond the horizon radar (BTH), is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is ...
: From 1976 to 1989, the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen national ...
's
Russian Woodpecker ''Duga'' (, ) was an over-the-horizon radar (OTH) system used in the Soviet Union as part of its early-warning radar network for missile defense. It operated from July 1976 to December 1989. Two operational ''duga'' radars were deployed, wit ...
over-the-horizon radar system blotted out numerous shortwave broadcasts daily. *
Ionospheric heater An ionospheric heater, or an ionospheric HF pump facility, is a powerful radio wave transmitter with an array of antennas which is used for research of plasma turbulence, the ionosphere and upper atmosphere.Powerful electromagnetic waves for activ ...
s used for scientific experimentation such as the High Frequency Active Auroral Research Program in Alaska, and the
Sura ionospheric heating facility The Sura Ionospheric Heating Facility, located near the small town of Vasilsursk about 100 km eastward from Nizhniy Novgorod in Russia, is a laboratory for ionosphere research . Sura is capable of radiating about 80 megawatts at 4.3 MHz, in ...
in Russia.


Shortwave broadcasting

* ''See
International broadcasting International broadcasting, in a limited extent, began during World War I, when German and British stations broadcast press communiqués using Morse code. With the severing of Germany's undersea cables, the wireless telegraph station in Nauen was ...
for details on the history and practice of broadcasting to foreign audiences.'' * ''See
List of shortwave radio broadcasters This is a List of shortwave radio broadcasters updated on Aug 22, 2021: By country By frequency See also * International broadcasting International broadcasting, in a limited extent, began during World War I, when German and British stations ...
for a list of international and domestic shortwave radio broadcasters.'' * ''See
Shortwave relay station Shortwave relay stations are transmitter sites used by international broadcasters to extend their coverage to areas that cannot be reached easily from their home state. For example, the BBC operates an extensive net of relay stations. These days ...
for the actual kinds of integrated technologies used to bring high power signals to listeners.''


Frequency allocations

The
World Radiocommunication Conference The World Radiocommunication Conference (WRC) is a conference organized by the ITU to review and, as necessary, revise the Radio Regulations, the international treaty governing the use of the radio-frequency spectrum as well as geostationary and ...
(WRC), organized under the auspices of the
International Telecommunication Union The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
, allocates bands for various services in conferences every few years. The last WRC took place in 2019. At WRC-97 in 1997, the following bands were allocated for
international broadcasting International broadcasting, in a limited extent, began during World War I, when German and British stations broadcast press communiqués using Morse code. With the severing of Germany's undersea cables, the wireless telegraph station in Nauen was ...
. AM shortwave broadcasting channels are allocated with a 5 kHz separation for traditional analog audio broadcasting. Although countries generally follow the table above, 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. Additionally, international broadcasters sometimes operate outside the normal 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 Digital Radio Mondiale, 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 (radio), antenna designs (like the ALLISS antenna technology) to concentrate radio energy at the target area.


Advantages

Shortwave does possess a number of advantages over newer technologies, including the following: * Difficulty of censoring programming by authorities in restrictive countries. Unlike their relative ease in monitoring and internet censorship, censoring the Internet, over-the air television, cable television and satellite television, government authorities face technical difficulties monitoring which stations (sites) are being listened to (accessed). For example, during the 1991 Soviet coup d'état attempt, 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
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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 The Cold War is a term commonly used to refer to a period of geopolitical tension between the United States and the Soviet Union and their respective allies, the Western Bloc and the Eastern Bloc. The term '' cold war'' is used because the ...
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 Terrestrial television, over-the-air television, cable television, satellite television, Landline, landline phones, mobile phones, satellite phones, Communications satellite, 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, medium wave radio, 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, Olivia are even more robust, allowing successful reception of signals well below the noise floor of a conventional receiver.


Disadvantages

Shortwave radio's benefits are sometimes regarded as being outweighed by its drawbacks, including: * 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 supply, 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.


Shortwave listening

The Asia-Pacific Telecommunity estimates that there are approximately 600 million Shortwave receiver, shortwave broadcast-radio receivers in use in 2002. WWCR claims that there are 1.5 billion shortwave receivers worldwide. Many hobbyists listen to shortwave broadcasters. In some cases, the goal is to hear as many stations from as many countries as possible ''(DXing)''; others listen to specialized shortwave utility, or "ute", transmissions such as maritime, naval, aviation, or military signals. Others focus on intelligence signals from numbers stations, stations which transmit strange broadcast usually for intelligence operations, or the two way communications by amateur radio operators. Some short wave listeners behave analogously to "lurkers" on the Internet, in that they listen only, and never attempt to send out their own signals. Other listeners participate in clubs, or actively send and receive QSL cards, or become involved with amateur radio and start transmitting on their own. Many listeners tune the shortwave bands for the programmes of stations broadcasting to a general audience (such as Radio Taiwan International, China Radio International, Voice of America, Radio France Internationale,
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, Voice of Korea, Radio Free Sarawak etc.). Today, through the evolution of the Internet, the hobbyist can listen to shortwave signals via remotely controlled or web controlled shortwave receivers around the world, even without owning a shortwave radio. Many international broadcasters offer live streaming audio on their websites and a number have closed their shortwave service entirely, or severely curtailed it, in favour of internet transmission. Shortwave listeners, or SWLs, can obtain QSL cards from broadcasters, utility stations or amateur radio operators as trophies of the hobby. Some stations even give out special certificates, pennants, stickers and other tokens and promotional materials to shortwave listeners.


Shortwave broadcasts and music

Some musicians have been attracted to the unique aural characteristics of shortwave radio which – due to the nature of amplitude modulation, varying propagation conditions, and the presence of interference – generally has lower fidelity than local broadcasts (particularly via FM stations). Shortwave transmissions often have bursts of distortion, and "hollow" sounding loss of clarity at certain aural frequencies, altering the harmonics of natural sound and creating at times a strange "spacey" quality due to echoes and phase distortion. Evocations of shortwave reception distortions have been incorporated into rock and classical compositions, by means of delays or feedback loops, equalizers, or even playing shortwave radios as live instruments. Snippets of broadcasts have been mixed into electronic sound collages and live musical instruments, by means of analogue tape loops or digital sampler, digital samples. Sometimes the sounds of instruments and existing musical recordings are altered by remixing or equalizing, with various distortions added, to replicate the garbled effects of shortwave radio reception. The first attempts by serious composers to incorporate radio effects into music may be those of the Russian physicist and musician Léon Theremin, who perfected a form of radio oscillator as a musical instrument in 1928 (regenerative circuits in radios of the time were prone to breaking into oscillation, adding various tonal harmonics to music and speech); and in the same year, the development of a French instrument called the Ondes Martenot by its inventor Maurice Martenot, a French cellist and former wireless telegrapher. Karlheinz Stockhausen used shortwave radio and effects in works including ''Hymnen'' (1966–1967), ''Kurzwellen'' (1968) – adapted for the Beethoven Bicentennial in ''Opus 1970'' with filtered and distorted snippets of Beethoven pieces – ''Spiral (Stockhausen), Spiral'' (1968), ''Pole (Stockhausen), Pole'', ''Expo (Stockhausen), Expo'' (both 1969–1970), and ''Licht#Wednesday, Michaelion'' (1997). Cyprus, Cypriot composer Yannis Kyriakides incorporated shortwave numbers station transmissions in his 1999 ''ConSPIracy cantata''. Holger Czukay, a student of Stockhausen, was one of the first to use shortwave in a rock music context. In 1975, German electronic music band Kraftwerk recorded a full length concept album around simulated radiowave and shortwave sounds, entitled ''Radioactivity (album), Radio-Activity''. The The's Radio Cineola monthly broadcasts drew heavily on shortwave radio sound.


Shortwave's future

The development of direct broadcasts from satellites has reduced the demand for shortwave receiver hardware, but there are still a great number of shortwave broadcasters. A new digital radio technology, Digital Radio Mondiale (DRM), is expected to improve the quality of shortwave audio from very poor to standards comparable to the FM broadcast band. The future of shortwave radio is threatened by the rise of power line communication (PLC), also known as Broadband over Power Lines (BPL), which uses a data stream transmitted over unshielded power lines. As the BPL frequencies used overlap with shortwave bands, severe distortions can make listening to analog shortwave radio signals near power lines difficult or impossible. According to Andy Sennitt, former editor of the World Radio TV Handbook, However, Thomas Witherspoon, editor of shortwave news site ''SWLingPost.com'' wrote that In 2018, Nigel Fry, head of Distribution for the
BBC World Service The BBC World Service is an international broadcasting, international broadcaster owned and operated by the BBC, with funding from the Government of the United Kingdom, British Government through the Foreign Secretary, Foreign Secretary's o ...
Group, During the 2022 Russian invasion of Ukraine, the BBC World Service launched two new shortwave frequencies for listeners in Ukraine and Russia, broadcasting English-language news updates in an effort to avoid Media freedom in Russia#Coverage on Ukraine, censorship by the Russian state.


See also

* ALLISS–a very large rotatable antenna system used in international broadcasting * List of American shortwave broadcasters * List of European short wave transmitters *
List of shortwave radio broadcasters This is a List of shortwave radio broadcasters updated on Aug 22, 2021: By country By frequency See also * International broadcasting International broadcasting, in a limited extent, began during World War I, when German and British stations ...


References


External links

* * * View live and historical data and images of space weather and radio propagation. * article describing pros and cons of short wave radio since the
Cold War The Cold War is a term commonly used to refer to a period of geopolitical tension between the United States and the Soviet Union and their respective allies, the Western Bloc and the Eastern Bloc. The term '' cold war'' is used because the ...
. * describes experiments carried out for the French and British governments. * {{Authority control International broadcasting Radio Guglielmo Marconi Radio spectrum Short wave radio