Wireless telegraphy
[3][4] 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.
Beginning about 1908, powerful transoceanic radiotelegraphy stations transmitted commercial telegram traffic between countries at rates up to 200 words per minute.
The primitive spark-gap transmitters used until 1920 transmitted damped waves, which had very wide bandwidth and tended to interfere with other transmissions.
Morse code radiotelegraphy was gradually replaced by radioteletype in most high volume applications by World War II.
In manual radiotelegraphy the sending operator manipulates 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 in Morse code.
[8] However it is still used by amateur radio operators, and military services require signalmen to be trained in Morse code for emergency communication.
[15][16] A CW coastal station, KSM, still exists in California, run primarily as a museum by volunteers,[17] and occasional contacts with ships are made.
Radiotelegraphy is popular amongst radio amateurs world-wide, who commonly refer to it as continuous wave, or just CW.
The operator at the receiving station who knew Morse code would translate the clicking sounds to text and write down the message.
[25] 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.
[29] Preece and the General Post Office (GPO) in Britain at first supported and gave financial backing to Marconi's experiments conducted on Salisbury Plain from 1896.
[31] With this development, wireless telegraphy came to mean radiotelegraphy, Morse code transmitted by radio waves.
Instead, the operator would send the text message on a telegraph key, which turned the transmitter on and off, producing short ("dot") and long ("dash") pulses of radio waves, groups of which comprised the letters and other symbols of the Morse code.
As long as the telegraph key was pressed, the transmitter would produce a string of transient pulses of radio waves which repeated at an audio rate, usually between 50 and several thousand hertz.
[36] As long as the telegraph key was pressed, the transmitter produced a continuous sinusoidal wave of constant amplitude.
Because the CW signal produced while the key was pressed was just an unmodulated carrier wave, it made no sound in a receiver's earphones.
[43] When the United States entered World War I, private radiotelegraphy stations were prohibited, which put an end to several pioneers' work in this field.
[46] The ultimate implementation of wireless telegraphy was telex, using radio signals, which was developed in the 1930s and was for many years the only reliable form of communication between many distant countries.
[48] Today, due to more modern text transmission methods, Morse code radiotelegraphy for commercial use has become obsolete.
[49][50] Continuous wave (CW) radiotelegraphy is regulated by the International Telecommunication Union (ITU) as emission type A1A.
