Wireless telegraphy

Wireless telegraphy means transmission of telegraph signals by radio waves; a more specific term for this is radiotelegraphy. Before about 1910 when radio became dominant, the term wireless telegraphy was also used for various other experimental technologies for transmitting telegraph signals without wires, such as electromagnetic induction, and ground conduction telegraph systems. Guglielmo Marconi, the father of radio-based wireless telegraphy, in 1901, with one of his first wireless transmitters (right) and receivers (left)German troops erecting a wireless field telegraph station during World War IGerman officers and troops manning a wireless field telegraph station during World War I Wireless telegraphy means transmission of telegraph signals by radio waves; a more specific term for this is radiotelegraphy. Before about 1910 when radio became dominant, the term wireless telegraphy was also used for various other experimental technologies for transmitting telegraph signals without wires, such as electromagnetic induction, and ground conduction telegraph systems. Radiotelegraphy was the first means of radio communication; the first practical radio transmitters and receivers invented in 1894–5 by Guglielmo Marconi used radiotelegraphy. It continued to be the only type of radio transmission during the first three decades of radio, called the 'wireless telegraphy era' up until World War I, when the development of amplitude modulation (AM) radiotelephony allowed sound (audio) to be transmitted by radio. In radiotelegraphy, information is transmitted by pulses of radio waves of two different lengths called 'dots' and 'dashes', which spell out text messages, usually in Morse code. In a manual system, the sending operator taps on a switch called a telegraph key which turns the transmitter on and off, producing the pulses of radio waves. At the receiver the pulses are audible in the receiver's speaker as beeps, which are translated back to text by an operator who knows Morse code. Radiotelegraphy was used for long distance person-to-person commercial, diplomatic, and military text communication throughout the first half of the 20th century. It became a strategically important capability during the two world wars, since a nation without long distance radiotelegraph stations could be isolated from the rest of the world by an enemy cutting its submarine telegraph cables. Beginning about 1908, powerful transoceanic radiotelegraphy stations transmitted commercial telegram traffic between countries at rates up to 200 words per minute. Radiotelegraphy was transmitted by several different modulation methods during its history. The primitive spark gap transmitters used until 1920 transmitted damped waves, which had very large bandwidth and tended to interfere with other transmissions. This type of emission was banned by 1930. The vacuum tube (valve) transmitters which came into use after 1920 transmitted code by pulses of unmodulated sinusoidal carrier wave called continuous waves (CW), which is still used today. To make CW transmissions audible, the receiver requires a circuit called a beat frequency oscillator (BFO). A third type of modulation, frequency shift keying (FSK) was used mainly by radioteletypes. Morse code radiotelegraphy was gradually replaced by radioteletype networks (RTTY) in most high volume applications by World War II. Today it is nearly obsolete, the only remaining users are the radio amateur community and some limited training by the military for emergency use. Wireless telegraphy or radiotelegraphy, commonly called CW (continuous wave), ICW (interrupted continuous wave) transmission, or on-off keying, and designated by the International Telecommunication Union as emission type A1A, is a radio communication method in which the sending operator taps on a switch called a telegraph key, which turns the radio transmitter on and off, producing pulses of unmodulated carrier wave of different lengths called 'dots' and 'dashes', which encode characters of text, usually in Morse code. At the receiving location the code is audible in the radio receiver's earphone or speaker as a sequence of buzzes or beeps, which is translated back to text by an operator who knows Morse code. Although this type of communication has been mostly replaced since its introduction over 100 years ago by other means of communication it is still used by amateur radio operators as well as some military services. A CW coastal station, KSM, still exists in California, run primarily as a museum by volunteers, and occasional contacts with ships are made. Radio beacons, particularly in the aviation service, but also as 'placeholders' for commercial ship-to-shore systems, also transmit Morse but at very slow speeds. The US Federal Communications Commission issues a lifetime commercial Radiotelegraph Operator License. This requires passing a simple written test on regulations, a more complex written exam on technology, and demonstrating Morse reception at 20 words per minute plain language and 16 wpm code groups. (Credit is given for amateur extra class licenses earned under the old 20 wpm requirement.) Wireless telegraphy is still used widely today by amateur radio hobbyists where it is commonly referred to as radio telegraphy, continuous wave, or just CW. However, its knowledge is not required to obtain any class of amateur license. Continuous wave (CW) radiotelegraphy is regulated by the International Telecommunication Union (ITU) as emission type A1A. Efforts to find a way to transmit telegraph signals without wires grew out of the success of electric telegraph networks, the first instant telecommunication systems. Developed beginning in the 1830s, a telegraph line was a person-to-person text message system consisting of multiple telegraph offices linked by an overhead wire supported on telegraph poles. To send a message, an operator at one office would tap on a switch called a telegraph key, creating pulses of electric current which spelled out a message in Morse code. When the key was pressed, it would connect a battery to the telegraph line, sending current down the wire. At the receiving office the current pulses would operate a telegraph sounder, a device which would make a 'click' sound when it received each pulse of current. The operator at the receiving station who knew Morse code would translate the clicking sounds to text and write down the message. The ground was used as the return path for current in the telegraph circuit, to avoid having to use a second overhead wire. By the 1860s, telegraph was the standard way to send most urgent commercial, diplomatic and military messages, and industrial nations had built continentwide telegraph networks, with submarine telegraph cables allowing telegraph messages to bridge oceans. However installing and maintaining a telegraph line linking distant stations was very expensive, and wires could not reach some locations such as ships at sea. Inventors realized if a way could be found to send electrical impulses of Morse code between separate points without a connecting wire, it could revolutionize communications. The successful solution to this problem was the discovery of radio waves in 1887, and the development of practical radiotelegraphy transmitters and receivers by about 1899, described in the next section. However this was preceded by a 50 year history of ingenious but ultimately unsuccessful experiments by inventors to achieve wireless telegraphy by other means.