Shortwave radio
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.
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.
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.
[10][11] 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.
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.
Early long-distance services used surface wave propagation at very low frequencies,[15] which are attenuated along the path at wavelengths shorter than 1,000 meters.
This, and the difficulties of generating and detecting higher frequencies, made discovery of shortwave propagation difficult for commercial services.
[18] By 1924 many additional specially licensed amateurs were routinely making transoceanic contacts at distances of 6,000 miles (9,600 km) and more.
[21] Predictions of skywave propagation depend on: Several different types of modulation are used to incorporate information in a short-wave signal.
Single sideband is used for long-range voice communications by ships and aircraft, citizen's band, and amateur radio operators.
Continuous wave (CW) is on-and-off keying of a sine-wave carrier, used for Morse code communications and Hellschreiber facsimile-based teleprinter transmissions.
[23] Radioteletype, fax, digital, slow-scan television, and other systems use forms of frequency-shift keying or audio subcarriers on a shortwave carrier.
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.
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.
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.
Other listeners participate in clubs, or actively send and receive QSL cards, or become involved with amateur radio and start transmitting on their own.
[39] 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.
[40] Shortwave listeners, or SWLs, can obtain QSL cards from broadcasters, utility stations or amateur radio operators as trophies of the hobby.
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).
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.
[41][42] The first attempts by serious composers to incorporate radio effects into music may be those of the Russian physicist and musician Léon Theremin,[43] 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.
[41] Cypriot composer Yannis Kyriakides incorporated shortwave numbers station transmissions in his 1999 ConSPIracy cantata.
[42] In 1975, German electronic music band Kraftwerk recorded a full length concept album around simulated radiowave and shortwave sounds, entitled Radio-Activity.
[49] According to Andy Sennitt, former editor of the World Radio TV Handbook, shortwave is a legacy technology, which is expensive and environmentally unfriendly.
