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Shortwave radio received its name because the wavelengths in this band are shorter than 200 m (1500 kHz) which marked the original upper limit of the medium frequency band first used for radio communications. The broadcast medium wave band now extends above the 200 m/1500 kHz limit, and the amateur radio 1.8 MHz – 2.0 MHz band (known as the "top band") is the lowest-frequency band considered to be 'shortwave'.
Initially thought to be useless, shortwave radio is used for long distance communication by means of skywave or skip propagation, in which the radio waves are reflected back to Earth from the ionosphere, allowing communication around the curve of the Earth. Shortwave radio is used for broadcasting of voice and music, and long-distance communication to ships and aircraft, or to remote areas out of reach of wired communication or other radio services. Additionally, it is used for two-way international communication by amateur radio enthusiasts for hobby, educational and emergency purposes.
Amateur radio operators are credited with the discovery of long-distance communication on the shortwave bands. Early long-distance services used surface wave propagation at very low frequencies, which are attenuated along the path. Longer distances and higher frequencies using this method meant more signal attenuation. This, and the difficulties of generating and detecting higher frequencies, made discovery of shortwave propagation difficult for commercial services.
Radio amateurs conducted the first successful transatlantic tests in December 1921, operating in the 200 meter mediumwave band (1500 kHz)—the shortest wavelength then available to amateurs. In 1922 hundreds of North American amateurs were heard in Europe at 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 as the authorities mistakenly believed at first that such frequencies were useless for commercial or military use), amateurs began to experiment with those wavelengths using newly available vacuum tubes shortly after World War I.
Extreme interference at the upper edge of the 150-200 meter band—the official wavelengths allocated to amateurs 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 below 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 6000 miles (~9600 km) and more. On 21 September several amateurs in California completed two way contacts with an amateur in New Zealand. On 19 October amateurs in New Zealand and England completed a 90-minute two-way contact nearly halfway around the world. On October 10, 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.
In June and July 1923, Guglielmo Marconi's transmissions were completed during nights on 97 meters from Poldhu Wireless Station, Cornwall, to his yacht Elettra in the Cape Verde Islands. In September 1924, Marconi transmitted during daytime and nighttime on 32 meters from Poldhu to his yacht in Beirut. Marconi, in July 1924, 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 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.
Far more spectrum is available for long distance communication in the shortwave bands than in the longwave bands; and shortwave transmitters, receivers and antennas were orders of magnitude less expensive than the multi-hundred kilowatt transmitters and monstrous antennas needed for longwave.
Shortwave communications began to grow rapidly in the 1920s, similar to the internet in the late 20th century. 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 also ended the need for multi-million dollar investments in new transoceanic telegraph cables and massive longwave wireless stations, although some existing transoceanic telegraph cables and commercial longwave communications stations remained in use until the 1960s.
The cable companies began to lose large sums of money in 1927, and a serious financial crisis threatened the 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.
Shortwave radio frequency energy is capable of reaching any location on the Earth as it is influenced by ionospheric reflection back to the earth by the ionosphere, (a phenomenon known as "skywave propagation"). A typical phenomenon of shortwave propagation is the occurrence of a skip zone (see first figure on that page) 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, propagation often simultaneously occurs on different paths, scattered by the E or F region 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's energy and converting it to heat. Predictions of skywave propagation depend on:
Several different types of modulation are used to impress information on a short-wave transmission.
Amplitude modulation is the simplest type and the most commonly used for shortwave broadcasting. 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 recovers the desired modulation signal from the carrier.
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 the carrier frequency. If one set of these components is eliminated as well as the residual carrier, only the remaining set is transmitted. This saves power in the transmission, as roughly 2/3 of the energy sent by an AM signal is unnecessary to recover the information contained on it. It also saves "bandwidth", allowing about one-half the carrier frequency spacing to be used. The drawback is that the receiver is more complicated, since it must re-recreate the carrier to recover the signal. Small errors in the detector process can greatly affect the pitch of the received signal, so single side band is not usual for music or general broadcast. Single side band is used for long-range voice communications by ships and aircraft, Citizen's Band, and amateur radio operators. LSB (lower sideband) is generally used below 9 MHz and USB (upper sideband) above 9 MHz.
Vestigal sideband transmits the carrier and one complete side-band, but filters out the redundant side-band. It is a compromise between AM and SSB, allowing simple receivers to be used but requiring almost as much transmitter power as AM. One advantage is that only half the bandwidth of an AM signal is used. It can be heard in the transmission of certain radio time signal stations.[dubious ]
Narrow-band frequency modulation (NBFM) is mainly used in the higher HF frequencies (typically above 20 MHz). Because of the larger bandwidth required, NBFM is much more 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 is allowed to have a wider bandwidth. NBFM is limited to short-range SW transmissions due to the multiphasic distortions created by the ionosphere.
Digital Radio Mondiale (DRM) is a digital modulation for use on bands below 30 MHz.
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.
Some major uses of the shortwave radio band are:
The term DXing, in the context of listening to radio signals of any user of the shortwave band, is the activity of monitoring distant stations. In the context of amateur radio operators, the term "DXing" refers to the two-way communications with a distant station, using shortwave radio frequencies.
The Asia-Pacific Telecommunity estimates that there are approximately 600,000,000 shortwave broadcast-radio receivers in use in 2002. WWCR claims that there are 1.5 billion shortwave receivers worldwide.
See International broadcasting for details on the history and practice of broadcasting to foreign audiences.
See Shortwave relay station for the actual kinds of integrated technologies used to bring high power signals to listeners.
The World Radiocommunication Conference (WRC), organized under the auspices of the International Telecommunication Union, allocates bands for various services in conferences every few years. The last WRC took place in 2007.
At WRC-97 in 1997, the following bands were allocated for international broadcasting. (listed in the table):
|Metre Band||Frequency Range||Remarks|
|120 m||2300–2495 kHz||tropic band|
|90 m||3200 – 3400 kHz||tropic band|
|75 m||3900 – 4000 kHz||shared with the North American amateur radio 80m band|
|60 m||4750 – 5060 kHz||tropic band|
|49 m||5900 – 6200 kHz|
|41 m||7200 – 7600 kHz||shared with the amateur radio 40m band|
|31 m||9400 – 9900 kHz||currently the most heavily used band|
|25 m||11,600 - 12,200 kHz|
|22 m||13,570 - 13,870 kHz|
|19 m||15,100 - 15,800 kHz|
|16 m||17,480 - 17,900 kHz|
|15 m||18,900 - 19,020 kHz||almost unused, could become a DRM band|
|13 m||21,450 - 21,850 kHz|
|11 m||25,600 - 26,100 kHz||may be used for local DRM broadcasting|
AM shortwave broadcasting channels are allocated with a 5 kHz separation for traditional analog audio broadcasting.
International broadcasters for practical reasons sometimes operate outside the normal WRC-allocated bands or use off-channel frequencies to attract attention in crowded bands (60m, 49m, 40m, 41m, 31m, 25m).
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.
Shortwave does possess a number of advantages over newer technologies, including the following:
Shortwave radio's benefits are sometimes regarded as being outweighed by its drawbacks, including:
Many hobbyists listen to shortwave broadcasters without operating their own transmitters. 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, 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 make any 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, Voice of Russia, China Radio International, Radio Canada International, Voice of America, Radio France Internationale, BBC World Service, Radio Australia, Radio Netherlands, Voice of Korea, Sarawak Report etc.). Today, through the evolution of the Internet, the hobbyist can listen to shortwave signals via remotely controlled shortwave receivers around the world, even without owning a shortwave radio. Many international broadcasters (such as Radio Canada International , the BBC and Radio Australia) offer live streaming audio on their websites. 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.
The practice of operating a shortwave radio transmitter for non-commercial two-way communications is known as amateur radio. Licenses are granted by authorized government agencies.
Amateur radio operators have made many technical advancements in the field of radio, and make themselves available to transmit emergency communications when normal communications channels fail. Some amateurs practice operating off the power grid so as to be prepared for power loss. Many amateur radio operators started out as Shortwave Listeners (SWLs) and actively encourage SWLs to become amateur radio operators.
Utility stations are stations that do not intentionally broadcast to the general public (although their signals can be received by anybody with appropriate equipment). There are shortwave bands allocated to the use of 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. Many radio hobbyists specialize in listening to "ute" broadcasts, which often originate from geographic locations without known shortwave broadcasters.
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The short wave bands are also used by unlicensed individuals who may want mostly short-range "party line" like communications. Two examples are the use of HF for communication between fishing boats in many areas of the world, and the unlicensed use of the 11-meter band, which is effectively permitted in some areas of the world. Unlicensed operators, called "pirates", can cause signal interference to licensed stations. Many third-world countries have shops selling HF transmitter radios to any customer without regard to license or operator knowledge. As of 2012, there were virtually no national or international efforts to control such pirate operations.
The short wave bands are also used for various experiments, some continuing for years. In 2011, signals traceable to China regularly sent powerful HF transmissions scanning wide ranges of HF frequencies, perhaps to determine the maximum usable frequency (MUF) or other variables.
Numbers stations are broadcasts on shortwave radio that are coded into groups of numbers. Their content is generally encrypted and their purpose remains a mystery.
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 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. A notable chamber piece by Mexican composer Silvestre Revueltas—Ocho x radio, 1933—features a complex texture of pseudo-mariachi musics, overlapping and cross-fading as if heard from distant stations: quite similar to shortwave radio signal propagation disturbance. John Cage used actual radios (of unspecified wavelength) live on several occasions, starting in 1942 with Credo in Us, while Karlheinz Stockhausen used shortwave radio and effects in works including Hymnen (1966–67), Kurzwellen (1968)—adapted for the Beethoven Bicentennial in Opus 1970 with filtered and distorted snippets of Beethoven pieces—Spiral (1968), Pole, Expo (both 1969–70), and Michaelion (1997).
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 Radio-Activity. Among others, The The whose Radio Cineola monthly broadcasts draw heavily on shortwave radio sound, The B-52s, Shearwater, Tom Robinson, Peter Gabriel, Pukka Orchestra, AMM, John Duncan, Orchestral Manoeuvres in the Dark (on their Dazzle Ships album), Pat Metheny, Aphex Twin, Boards of Canada, Rush, Able Tasmans, Team Sleep, Underworld, Meat Beat Manifesto, Tim Hecker, Jonny Greenwood of Radiohead, Roger Waters (on Radio KAOS album), Wilco, code 000 and Samuel Trim have also used or been inspired by shortwave broadcasts.
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. However, because shortwave is a cheap and effective way to receive communications in countries with poor infrastructure, it will be around for years to come.
Shortwave use by hobbyists and licensed amateur ham radio operators continues, and after declining interest for a few years due to competing interests in computers and other communication devices,[when?] a new resurgence of interest has occurred as evidenced by the increase of new amateur operator licenses issued worldwide. Some hobbyists have combined amateur radio HF with computers for experimental and established data modes that can communicate very close to under the noise floor of receivers.