Shortwave radio isradio transmission usingradio frequencies in theshortwave bands (SW). There is no official definition of the band range, but it always includes all of thehigh frequency band (HF), which extends from 3 to 30 MHz (approximately 100 to 10 metres in wavelength). It lies between themedium frequency band (MF) and the bottom of theVHF band.
Radio waves in the shortwave band can be reflected or refracted from a layer of electrically charged atoms in the atmosphere called theionosphere. Therefore, short waves directed at an angle into the sky can be reflected back to Earth at great distances, beyond the horizon. This is calledskywave 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 broadcasts of radio programs played an important role in international broadcasting for many decades, serving both to provide news and information and as apropaganda tool for an international audience. The heyday ofinternational shortwave broadcasting was during theCold War between 1960 and 1990.
With the wide implementation of other technologies for the long-distance distribution of radio programs, such assatellite radio,cable broadcasting andIP-based transmissions, shortwave broadcasting lost importance. Initiatives for thedigitization of broadcasting did not bear fruit either, and as of 2025[update], relatively few broadcasters continue to broadcast programs on shortwave. However,shortwave listening remains a niche hobby, with enthusiasts tuning into fringe stations.
Shortwave radio is important in war zones, such as in theRusso-Ukrainian war,[2][3][4][5][6] 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.
The name "shortwave" originated during the beginning of radio in the early 20th century, when theradio spectrum was divided intolong wave (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 themedium frequency bandfirst used for radio communications. The broadcast medium wave band now extends above the 200 m / 1,500 kHz limit.
Early long-distanceradio telegraphy used long waves, below 300 kilohertz (kHz) / above 1000 m. The drawbacks to this system included a very limited spectrum available for long-distance communication, and the very expensivetransmitters, 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.[7]
Guglielmo Marconi, pioneer of radio, commissioned his assistantCharles 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 atPoldhu Wireless Station,Cornwall, 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 yachtElettra in theCape Verde Islands.[8]
In September 1924, Marconi arranged for transmissions to be made day and night on 32 meters (about 9.4 MHz) from Poldhu to his yacht in the harbour atBeirut, to which he had sailed, and was "astonished" to find he could receive signals "throughout the day".[9] Franklin went on to refine the directional transmission by inventing thecurtain array aerial system.[10][11] In July 1924, Marconi entered into contracts with the BritishGeneral Post Office (GPO) to install high-speed shortwave telegraphy circuits from London to Australia, India, South Africa and Canada as the main element of theImperial Wireless Chain. 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.[8]
Shortwave communications began to grow rapidly in the 1920s. By 1928, more than half of long-distance communications had moved fromtransoceanic 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.[12] 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[13] 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 toCable and Wireless Ltd. in 1934.
A resurgence of long-distance cables began in 1956 with the laying ofTAT-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 radio operators also discovered that long-distance communication was possible on shortwave bands. Early long-distance services usedsurface wave propagation atvery low frequencies,[14] 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,[15] operating in the 200 metermediumwave 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 availablevacuum tubes shortly after World War I.
Extreme interference at the longer edge of the 150–200 meter band – the official wavelengths allocated toamateurs by the Second National Radio Conference[16] in 1923 – forced amateurs to shift to shorter and shorter wavelengths. However, regulations limited amateurs 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.[17]
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 inNew Zealand. 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[18] at80 meters (3.75 MHz),40 meters (7 MHz) and20 meters (14 MHz). These were allocated worldwide, while the10 meter band (28 MHz) was created by the Washington International Radiotelegraph Conference[19] on 25 November 1927. The15 meter band (21 MHz) was opened to amateurs in the United States on 1 May 1952.
Shortwave radio frequency energy is capable of reaching any location on the Earth as it is influenced byionospheric reflection back to Earth by theionosphere, (a phenomenon known as "skywave propagation"). A typical phenomenon of shortwave propagation is the occurrence of askip zone 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 theionosphere, 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 aradio frequency's energy and converting it to heat.[20] Predictions of skywave propagation depend on:
Several different types ofmodulation are used to incorporate information in a short-wave signal.
Amplitude modulation is the simplest type and the most commonly used forshortwave broadcasting. The instantaneous amplitude of the carrier is controlled by the amplitude of the signal (speech, or music, for example). At the receiver, a simpledetector recovers the desired modulation signal from the carrier.[22]
Single-sideband 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 thecarrier frequency. 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 roughly2⁄3 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 signalbandwidth, enabling less than one-half the AM signal bandwidth to be used.[22]
The drawback is the receiver is more complicated, because it must recreate 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, and amateur radio operators. In amateur radio operation lower sideband (LSB) is customarily used below 10 MHz and USB (upper sideband) above 10 MHz, non-amateur services use USB regardless of frequency.
Vestigial sideband 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 standardtime signal stationCHU.Vestigial sideband was used for analog television and byATSC, thedigital TV system used in North America.
Narrow-bandfrequency modulation (NBFM or NFM) is used typically above 20 MHz. Because of the larger bandwidth required, NBFM is commonly used forVHF 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.[23]
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.
Continuous wave (CW) ison-and-off keying of a sine-wave carrier, used forMorse code communications andHellschreiberfacsimile-basedteleprinter transmissions. It is a data mode, although often listed separately.[24] It is typically received via lower or upper SSB modes.[22]
Radioteletype, fax, digital,slow-scan television, 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.
Some established users of the shortwave radio bands may include:
Sporadic or non-traditional users of the shortwave bands may include:
TheWorld Radiocommunication Conference (WRC), organized under the auspices of theInternational Telecommunication Union, allocates bands for various services in conferences every few years. The last WRC took place in 2023.[36]
As of WRC-97 in 1997, these bands were allocated forinternational broadcasting. AM shortwave broadcasting channels are allocated with a 5 kHz separation for traditional analog audio broadcasting:
| Metre Band | Frequency Range | Remarks |
|---|---|---|
| 120 m | 2.3–2.495 MHz | tropical band |
| 90 m | 3.2–3.4 MHz | tropical band |
| 75 m | 3.9–4 MHz | shared with the North Americanamateur radio 80m band |
| 60 m | 4.75–5.06 MHz | tropical band |
| 49 m | 5.9–6.2 MHz | |
| 41 m | 7.2–7.6 MHz | shared with theamateur radio 40m band |
| 31 m | 9.4–9.9 MHz | the most heavily used band |
| 25 m | 11.6–12.2 MHz | |
| 22 m | 13.57–13.87 MHz | |
| 19 m | 15.1–15.8 MHz | |
| 16 m | 17.48–17.9 MHz | |
| 15 m | 18.9–19.02 MHz | almost unused, could become a DRM band |
| 13 m | 21.45–21.85 MHz | |
| 11 m | 25.6–26.1 MHz | may be used for local DRM broadcasting |
Although countries generally follow the assigned bands, there may be small differences between countries or regions. For example, in the official bandplan of theNetherlands,[37] 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 newdigital audio broadcasting format for shortwaveDRM 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 andover-the-horizon radar. Shortwave transmitting centers often use specializedantenna designs (like theALLISS antenna technology) to concentrate radio energy at the target area.
Shortwave possesses a number of advantages over newer technologies:
Shortwave radio's benefits are sometimes regarded as being outweighed by its drawbacks, including:
The Asia-Pacific Telecommunity estimates that there are approximately 600 millionshortwave broadcast-radio receivers in use in 2002.[40]WWCR claims that there are 1.5 billion shortwave receivers worldwide.[41]
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 fromnumbers 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 asRadio Taiwan International,China Radio International,Voice of America,Radio France Internationale,BBC World Service,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.[42] 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.[43]
Shortwave listeners, or SWLs, can obtainQSL 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.
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 analoguetape loops ordigital 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.[44][45]
The first attempts by serious composers to incorporate radio effects into music may be those of the Russian physicist and musicianLéon Theremin,[46] who perfected a form of radio oscillator as a musical instrument in 1928 (regenerative circuits in radios of the time were prone to breaking intooscillation, adding various tonalharmonics to music and speech); and in the same year, the development of a French instrument called theOndes Martenot by its inventorMaurice Martenot, a French cellist and former wireless telegrapher.Karlheinz Stockhausen used shortwave radio and effects in works includingHymnen (1966–1967),Kurzwellen (1968) – adapted for the Beethoven Bicentennial inOpus 1970 with filtered and distorted snippets of Beethoven pieces –Spiral (1968),Pole,Expo (both 1969–1970), andMichaelion (1997).[44]
Cypriot composerYannis Kyriakides incorporated shortwavenumbers station transmissions in his 1999ConSPIracy cantata.[47]
Holger Czukay, a student of Stockhausen, was one of the first to use shortwave in arock music context.[45] In 1975, Germanelectronic music bandKraftwerk recorded a full lengthconcept album around simulated radiowave and shortwave sounds, entitledRadio-Activity.[48]The The's Radio Cineola monthly broadcasts drew heavily on shortwave radio sound.[49]
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 newdigital radio technology,Digital Radio Mondiale (DRM), is expected to improve the quality of shortwave audio from very poor to adequate.[50][51] The future of shortwave radio is threatened by the rise ofpower line communication (PLC), also known asBroadband 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.[52]
According to Andy Sennitt, former editor of theWorld Radio TV Handbook,
shortwave is a legacy technology, which is expensive and environmentally unfriendly. A few countries are hanging on to it, but most have faced up to the fact that the glory days of shortwave have gone. Religious broadcasters will still use it because they are not too concerned with listening figures.[50]
However, Thomas Witherspoon, editor of shortwave news siteSWLingPost.com wrote that
shortwave remains the most accessible international communications medium that still provides listeners with the protection of complete anonymity.[53]
In 2018, Nigel Fry, head of Distribution for theBBC World Service Group,
I still see a place for shortwave in the 21st century, especially for reaching areas of the world that are prone to natural disasters that destroy local broadcasting and Internet infrastructure.[50]
During the2022 Russian invasion of Ukraine, the BBC World Service launched two new shortwave frequencies for listeners inUkraine andRussia, broadcasting English-language news updates in an effort to avoidcensorship by the Russian state.[54] American commercial shortwave broadcastersWTWW andWRMI also redirected much of their programming to Ukraine.[55][56][57]
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