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Shortwave radio is radio transmission using shortwave radio frequencies, generally 1.6-30 MHz (187.4-10.0 m), just above the medium wave AM broadcast band.
Radio waves in this band can be reflected or refracted from a layer of electrically charged atoms in the atmosphere called the ionosphere. Therefore, short waves directed at an angle into the sky can be reflected back to Earth at great distances, beyond the horizon. This is called skywave or skip propagation. Thus shortwave radio can be used for very long distance communication, in contrast to radio waves of higher frequency which travel in straight lines (line-of-sight propagation) and are limited by the visual horizon, about 40 miles. Shortwave radio is used for broadcasting of voice and music to shortwave listeners over very large areas; sometimes entire continents or beyond. It is also used for military over-the-horizon radar, diplomatic communication, and two-way international communication by amateur radio enthusiasts for hobby, educational and emergency purposes.
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Frequency classifications
The widest popular definition of the shortwave frequency interval is the ITU Region 1 (EU+Africa+Russia...) definition, and is the span 1.6-30 MHz, just above the medium wave band, which ends approximately at 1.6 MHz.
There are also other definitions of the shortwave frequency interval:
- 1.71 to 30 MHz in ITU Region 2 (North and South America...)
- 1.8 (160 meter radio amateur band start) to 30 MHz
- 2.3 (120 meter band start) to 30 MHz
- 2.3 (120 meter band start) to 26.1 MHz (11 meter band end)
- In Germany and perhaps Austria the ITU Region 1 shortwave radio frequency interval can be subdivided in:
- de:Grenzwelle ("border waves"): 1.605-3.8 MHz
- In Germany these shortwave radio frequency intervals have also been seen used:
- the above other definitions
The term short wave is an historic one, dating from the early 20th century, when the radio spectrum was divided on the basis of wavelength into longwave (LW), medium wave (MW), and short wave (SW) radio bands. 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 band first used for radio communications. The broadcast medium wave band now extends above the 200 m/1,500 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'.
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History
Development
The name "shortwave" originated during the early days of radio in the early 20th century, when the radio spectrum was considered divided into long wave (LW), medium wave (MW) and short wave bands based on the wavelength of the radio waves. Early long distance radio telegraphy used long waves, below 300 kilohertz (kHz). The drawbacks to this system included a very limited spectrum available for long distance communication, and the very expensive transmitters, receivers and gigantic antennas that were required. It was also difficult to beam the radio wave directionally with long wave, 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, 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, running on 25 kW of power. In June and July 1923, wireless transmissions were completed during nights on 97 meters from Poldhu to Marconi's yacht Elettra in the Cape Verde Islands.
In September 1924, Marconi transmitted daytime and nighttime on 32 meters from Poldhu to his yacht in Beirut. Franklin went on to refine the directional transmission, by inventing the curtain array 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 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, 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 multimillion-dollar investments in massive longwave wireless stations, although some commercial longwave communications stations remained in use until the 1960s. Long distance radio circuits also reduced the load on the existing transoceanic telegraph cables and hence 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, 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.
Long-distance cables had a resurgence beginning 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. These sounded the death knell of shortwave radio for commercial communications.
Amateur use of shortwave propagation
Amateur radio operators also discovered that long-distance communication was possible on shortwave bands. Early long-distance services used surface wave 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 band (near 1,500 kHz in the modern AM broadcast band) - the shortest wavelength then available to amateurs. 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 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 longer 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 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. 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 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:
- 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 flares produce a large increase in D region ionization so high, sometimes for periods of several minutes, all skywave propagation is nonexistent.
Types of modulation
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. Vestigial sideband is used for over the air Television Broadcasts both analog and digital.
Continuous wave (CW) is on-and-off keying of a carrier, used for Morse code communications and Hellschreiber facsimile-based teleprinter transmissions.
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.
Uses
Some major uses of the shortwave radio band are:
- International broadcasting primarily by government-sponsored propaganda, international news (for example, the BBC World Service) or cultural stations to foreign audiences: the most common use of all.
- Domestic broadcasting: to widely dispersed populations with few longwave, mediumwave and FM stations serving them; or for specialty political, religious and alternative media networks; or of individual commercial and non-commercial paid broadcasts.
- Oceanic air traffic control uses the HF/shortwave band for long distance communication to aircraft over the oceans and poles, which are far beyond the range of traditional VHF frequencies. Modern systems also include satellite communications, such as ADS-C/CPDLC
- "Utility" stations transmitting messages not intended for the general public, such as aircraft flying between continents, encrypted diplomatic messages, weather reporting, or ships at sea.
- 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 Stations 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 the "Lincolnshire Poacher", named after the 18th century English folk song, which is transmitted just before the sequences of numbers.
- Amateur radio operators at the 80/75, 60, 40, 30, 20, 17, 15, 12, and 10-meter bands.
- Time signal 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.
- Over-the-horizon radar: From 1976 to 1989, the Soviet Union's Russian Woodpecker over-the-horizon radar system blotted out numerous shortwave broadcasts daily.
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.
Shortwave broadcasting
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.
Frequency allocations
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. 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, 90 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 (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.
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 the Internet, government authorities face technical difficulties monitoring which stations (sites) are being listened to (accessed). For example, during the attempted coup against Soviet President Mikhail Gorbachev, when his access to communications was limited (e.g. his phones were cut off, etc.), Gorbachev was able to stay informed by means of the BBC World Service on shortwave.
- Low-cost shortwave radios are widely available in all but the most repressive countries in the world. Simple shortwave regenerative receivers can be easily built with a few parts.
- In many countries (particularly in most developing nations and in the Eastern bloc during the Cold War era) ownership of shortwave receivers has been and continues to be widespread (in many of these countries some domestic stations also used shortwave).
- Many newer shortwave receivers are portable and can be battery-operated, making them useful in difficult circumstances. Newer technology includes hand-cranked radios which provide power without batteries.
- Shortwave radios can be used in situations where Internet or satellite communications service is temporarily or long-term 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 thousands of kilometers, including from one continent to another.
- Particularly in tropical regions, SW is somewhat less prone to interference from thunderstorms than 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 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 true enthusiasts, since most new standard radios do not receive the shortwave band. Therefore, Western audiences are limited.
- In the developed world, shortwave reception is very difficult in urban areas because of excessive noise from switched-mode power adapters, fluorescent or LED light sources, internet modems and routers, computers and many other sources of radio interference.
Shortwave listening
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, 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 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, 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 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.
Amateur radio
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
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.
Unusual signals
Unlicensed two way radio activity by individuals such as taxi drivers, bus drivers and fishermen in various countries can be heard on various shortwave frequencies. Such unlicensed transmissions by "pirate" or "bootleg" operators can often cause signal interference to licensed stations.
In addition, Pirate radio broadcasters who feature programming such as music, talk and other entertainment, can be heard sporadically and in various modes on the shortwave bands.
The short wave bands are also used for scientific experimentation by ionospheric heaters such as the High Frequency Active Auroral Research Program in Alaska, and the Sura ionospheric heating facility in Russia.
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 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, PressureWorks, 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.
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. 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, 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 - e.g. WSJT, WSPR.
Source of the article : Wikipedia
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