Shortwave radio
Shortwave radio is
Radio waves in the shortwave 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 communication over very long distances, 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 64 km (40 miles).
Shortwave broadcasts of radio programs played an important role in the early days of radio history. In World War II it was used as a propaganda tool for an international audience. The heyday of international shortwave broadcasting was during the Cold War between 1960 and 1980.
With the wide implementation of other technologies for the distribution of radio programs, such as satellite radio 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 as of 2024[update], few broadcasters continue to broadcast programs on shortwave.
However, shortwave remains important in war zones, such as in the
History
Development
The name "shortwave" originated during the beginning of radio in the early 20th century, when the
Early long-distance
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 (about 3 MHz) from Poldhu to Marconi's yacht Elettra in the Cape Verde Islands.[3]
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 at Beirut, to which he had sailed, and was "astonished" to find he could receive signals "throughout the day".[4] Franklin went on to refine the directional transmission by inventing the curtain array aerial system.[5][6] 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.[3]
Shortwave communications began to grow rapidly in the 1920s.[7] 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.[8] 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[9] 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 also discovered that long-distance communication was possible on shortwave bands. Early long-distance services used surface wave propagation at very low frequencies,[10] 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,
Extreme interference at the longer edge of the 150–200 meter band – the official wavelengths allocated to
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[14] 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[15] 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
As a result of the multi-layer structure of the
- 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.
- D regionionization – so great, sometimes for periods of several minutes, that skywave propagation is nonexistent.
Types of modulation
Several different types of modulation are used to incorporate information in a short-wave signal.
Audio modes
AM
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.[18]
SSB
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,
VSB
NFM
Narrow-band
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
RTTY, FAX, SSTV
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.
Users
Some established users of the shortwave radio bands may include:
- International broadcasting primarily by government-sponsored propaganda, or international news (for example, the BBC World Service), religious or cultural stations to foreign audiences: The most common use of all.
- Domestic broadcasting: to widely dispersed populations with few mediumwave and FM stations serving them; or for speciality political, religious and alternative medianetworks; or of individual commercial and non-commercial paid broadcasts.
- Oceanic CPDLC.
- Two-way radio communications by marine and maritime HF stations, aeronautical users, and ground based stations.Royal Flying Doctor Service of Australia.[22]
- "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.
- 12, and 10–meterbands. Licenses are granted by authorized government agencies.
- 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.
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.[23]
- 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 called the "Lincolnshire Poacher", named after the 18th century English folk song, which is transmitted just before the sequences of numbers.
- Unlicensed two way radio activity by individuals such as taxi drivers, bus drivers and fishermen in various countries can be heard on various shortwave frequencies. Such unlicensed transmissions by "pirate" or "bootleg" two way radio operators[24] 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.[25]
- Pirate radio broadcasters who feature programming such as music, talk and other entertainment, can be heard sporadically and in various modes on the shortwave bands. Pirate broadcasters take advantage of the better propagation characteristics to achieve more range compared to the AM or FM broadcast bands.[26]
- 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.
- High Frequency Active Auroral Research Program in Alaska, and the Sura ionospheric heating facility in Russia.[27]
Shortwave broadcasting
- See International broadcasting for details on the history and practice of broadcasting to foreign audiences.
- See List of shortwave radio broadcasters for a list of international and domestic shortwave radio broadcasters.
- See Shortwave relay station for the actual kinds of integrated technologies used to bring high power signals to listeners.
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 2019.[28]
As of WRC-97 in 1997, these bands were allocated for international broadcasting. AM shortwave broadcasting channels are allocated with a 5 kHz separation for traditional analog audio broadcasting:
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 American amateur 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 the amateur 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 the Netherlands,[29] the 49 m band starts at 5.95 MHz, the 41 m band ends at 7.45 MHz, the 11 m band starts at 25.67 MHz, and the 120 m, 90 m, and 60 m bands are absent altogether. International broadcasters sometimes operate outside the normal the WRC-allocated bands or use off-channel frequencies. This is done for practical reasons, or to attract attention in crowded bands (60 m, 49 m, 40 m, 41 m, 31 m, 25 m).
The new
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 possesses a number of advantages over newer technologies:
- Difficulty of censoring programming by authorities in restrictive countries. Unlike their relative ease in monitoring and attempted coup against Soviet President Mikhail Gorbachev, when his access to communications was limited (e.g. his phones, television and radio were cut off), Gorbachev was able to stay informed by means of the BBC World Service on shortwave.[30]
- Low-cost shortwave radios are widely available in all but the most repressive countries in the world. Simple shortwave regenerative receiverscan be easily built with a few parts.
- In many countries (particularly in most developing nations and in the (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 radioswhich provide power without batteries.
- Shortwave radios can be used in situations where satellite phones, satellite communications, or the Internetis 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 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 floorof 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 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
Many hobbyists listen to shortwave broadcasters. In some cases, the goal is to hear as many stations from as many countries as possible (
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 or web controlled shortwave receivers around the world, even without owning a shortwave radio.[34] 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.[35]
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
The first attempts by serious composers to incorporate radio effects into music may be those of the Russian physicist and musician
Cypriot composer Yannis Kyriakides incorporated shortwave numbers station transmissions in his 1999 ConSPIracy cantata.[39]
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
According to Andy Sennitt, former editor of the World 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.[42]
However, Thomas Witherspoon, editor of shortwave news site SWLingPost.com wrote that
shortwave remains the most accessible international communications medium that still provides listeners with the protection of complete anonymity.[45]
In 2018, Nigel Fry, head of Distribution for the BBC 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.[42]
During the
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
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External links
- "A beginner's guide to shortwave radio listening". SWLing.com.
- Hauser, Glenn. "World of Radio".
- "Space Weather and Radio Propagation Center". propagation.hfradio.org. View live and historical data and images of space weather and radio propagation.
- "Short-wave radio, snap and crackle goes pop. Life in the old wireless yet". The Economist. article describing pros and cons of short wave radio since the Cold War.
- "Short-wave radio telephone is success in tests". Popular Mechanics. Hearst Magazines. July 1931. mid page 114. describes experiments carried out for the French and British governments.
- "Que Escuchar en la Onda Corta en Español". queescucharenlaoc.blogspot.com (in Spanish).