Wireless telegraphy
Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables.[1][2] Before about 1910, the term wireless telegraphy was also used for other experimental technologies for transmitting telegraph signals without wires.[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. 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 the first means of radio communication. The first practical radio transmitters and receivers invented in 1894–1895 by Guglielmo Marconi used radiotelegraphy.[5] It continued to be the only type of radio transmission during the first few 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. Beginning about 1908, powerful transoceanic radiotelegraphy stations transmitted commercial telegram traffic between countries at rates up to 200 words per minute.
Radiotelegraphy was used for long-distance person-to-person commercial, diplomatic, and military text communication throughout the first half of the 20th century.[6] It became a strategically important capability during the two world wars[7] 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. Radiotelegraphy remains popular in amateur radio. It is also taught by the military for use in emergency communications. However, commercial radiotelegraphy is obsolete.[8]
Methods[edit]
The primitive spark-gap transmitters used until 1920 transmitted by a modulation method called damped wave. 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.[33] In a receiver's earphone, this sounded like a musical tone, rasp or buzz. Thus the Morse code "dots" and "dashes" sounded like beeps. Damped wave had a large frequency bandwidth, meaning that the radio signal was not a single frequency but occupied a wide band of frequencies. Damped wave transmitters had a limited range and interfered with the transmissions of other transmitters on adjacent frequencies.[34]
After 1905 new types of radiotelegraph transmitters were invented which transmitted code using a new modulation method: continuous wave (CW)[35] (designated by the International Telecommunication Union as emission type A1A).[36] As long as the telegraph key was pressed, the transmitter produced a continuous sinusoidal wave of constant amplitude.[35] Since all the radio wave's energy was concentrated at a single frequency, CW transmitters could transmit further with a given power, and also caused virtually no interference to transmissions on adjacent frequencies. The first transmitters able to produce continuous wave were the arc converter (Poulsen arc) transmitter, invented by Danish engineer Valdemar Poulsen in 1903,[37] and the Alexanderson alternator, invented 1906–1912 by Reginald Fessenden and Ernst Alexanderson.[38] These slowly replaced the spark transmitters in high power radiotelegraphy stations.
However, the radio receivers used for damped wave could not receive continuous wave. 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.[39] To receive a CW signal, some way had to be found to make the Morse code carrier wave pulses audible in a receiver.
This problem was solved by Reginald Fessenden in 1901. In his "heterodyne" receiver, the incoming radiotelegraph signal is mixed in the receiver's detector crystal or vacuum tube with a constant sine wave generated by an electronic oscillator in the receiver called a beat frequency oscillator (BFO). The frequency of the oscillator is offset from the radio transmitter's frequency . In the detector the two frequencies subtract, and a beat frequency (heterodyne) at the difference between the two frequencies is produced: .[40] If the BFO frequency is near enough to the radio station's frequency, the beat frequency is in the audio frequency range and can be heard in the receiver's earphones.[40] During the "dots" and "dashes" of the signal, the beat tone is produced, while between them there is no carrier so no tone is produced. Thus the Morse code is audible as musical "beeps" in the earphones.
The BFO was rare until the invention in 1913 of the first practical electronic oscillator, the vacuum tube feedback oscillator by Edwin Armstrong. After this time BFOs were a standard part of radiotelegraphy receivers. Each time the radio was tuned to a different station frequency, the BFO frequency had to be changed also, so the BFO oscillator had to be tunable. In later superheterodyne receivers from the 1930s on, the BFO signal was mixed with the constant intermediate frequency (IF) produced by the superheterodyne's detector. Therefore, the BFO could be a fixed frequency.[41]
Continuous-wave vacuum tube transmitters replaced the other types of transmitter with the availability of power tubes after World War I because they were cheap. CW became the standard method of transmitting radiotelegraphy by the 20s, damped wave spark transmitters were banned by 1930[10] and CW continues to be used today. Even today most communications receivers produced for use in shortwave communication stations have BFOs.[42]
Regulation[edit]
Continuous wave (CW) radiotelegraphy is regulated by the International Telecommunication Union (ITU) as emission type A1A.[36]
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.)[51]
Guglielmo Marconi, generally credited as first to develop practical radio-based wireless telegraphy communication, in 1901 with one of his first transmitters (right) and receivers (left)
German officers and troops manning a wireless field telegraph station during World War I
