Muffled wave
| Damped and undamped wave |
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 Series of dampened waves generated by a pop-spark transmitter. The pauses between the signals, which no longer occur with undamped waves, are characteristic. |
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The term evanescent wave ( english damped wave ), also damped wave is a historical designation of communications technology for a special form of electromagnetic waves , as used in the first working method of the wireless radio transmission arise. This method was used exclusively for the wireless transmission of short or long radio signals, the Morse code . A transmission of tones was not possible with the devices that generated dampened waves.
function
With this earliest method of wireless transmission, the transmission, the radiation of the transmission energy takes place with the discharge of the transmission capacitor through two reactions:
- a spark gap that arises between the electrodes and
- a bang
Through the radio link and the loud bang that transmission method got the name spark-gap transmitter ( english spark-gap transmitter ). Its principle is based on an experimental setup with a spark inductor , the Hertz oscillator , with which the physicist Heinrich Hertz detected the electromagnetic waves predicted by James Clerk Maxwell and thus wirelessly transmitted them for the first time over a distance of 10 m.
The capacitor, which has two electrodes, is one pole connected to one end of an induction coil via a wire, which initially creates the “first half of a circuit”. From the second pole, which is still free, a wire does not lead to the coil, but to an electrode in the form of a metal ball. The wire from the other end of the coil, which is still free, also leads to an identical spherical electrode that is a few centimeters opposite the other. This creates the "second half" of the open circuit, albeit with the short beeline between the electrodes. From above, from the perspective of the top view looking, has the circuit so symbolically the shape of the letter C .
If the capacitor is now charged by briefly switching a voltage source on and off again, the capacitor is discharged via a spark that skips between the two balls. Since the discharge current flows through the coil on one side, an electromagnetic wave is created that is radiated all around.
disadvantage
In order to be able to send not just a single but several characters, the capacitor has to be charged and discharged again and again. This creates the following sequence: switch on the power source - switch off the power source - capacitor charges, device sends - capacitor is discharged - switch on power source - switch off power source - capacitor charges, device sends - capacitor is discharged. The power source is switched on - in simple terms - using the Morse code key . If it is pressed briefly, the Morse code dot appears, and if it is pressed longer, the dash is created.
These processes of repeating charging and discharging, which always happened one after the other, were carried out as often as how many characters are to be transmitted. On the one hand, this created a waveform with a gradually increasing and decreasing amplitude and a sudden pause.
The second striking thing about this waveform was that it was emitted in a very broadband manner because the transmitter cannot yet regulate the transmission frequency. The consequence of these disturbances is that during the transmission process, no other transmitter in the vicinity, for example another radio operator, could work at the same time. As also, a parallel reply from the receiving station was not yet possible.
Pop spark transmitters - presented independently for the first time by David Edward Hughes (1893) and Guglielmo Marconi (1895), as well as the further developments to the shock or extinguishing spark transmitter , which also could not solve the central problem, were therefore considered English in the English-speaking world dirty called 'dirty' .
Due to this dampened signal from the spark transmitter, they could practically only be used for message transmission as Morse code for telegraphy. Despite Marconi's range records, such as the Atlantic crossing in 1901, her era in wireless telegraphy ended around 1920. Muted waves were designated as “Class B” emissions by the 1938 “ International Telecommunications Union ” and were in fact banned. A contemporary example of the problems broadband waves pose for other communications, for example radio astronomy , are ultra- broadband waves (UWB). Although they are emitted with less power than the historical waves, the dispersion of UWB is still over 500 megahertz .
Prohibition in broadcasting stations
The International Telecommunication Union ( ITU) prohibits the use of damped vibrations in the radio regulations under section 3.15 because of their bandwidth in transmission systems.
successor
At the turn of the century, experiments followed in order to be able to produce the counterpart in the historical sense, the undamped waves . The first step was to replace an electrode with technical carbon, as Nikola Tesla did, for example . The then fully developed transmitter principle, including sound transmission, which enabled usable, undamped waves for the first time, was Valdemar Poulsen's arc transmitter from 1904 and later the machine transmitter presented by Reginald Fessenden in 1905 .
Individual evidence
- ↑ Thomas Piper (Ed.): “Prof. DE Hughes' Researches in Wireless Telegraphy ”. In: The Electrician. May 5, 1899. London 1899, pp. 40-41.
- ↑ Fred Gardiol, Yves Fournier: In: Salvan, the cradle of telecommunications. Marconi and his first attempts at wireless communication in the Swiss Alps. (PDF; 486 kB) , Bulletin SEV / VSE 21/2007, pp. 24–28
- ↑ Kazimierz Siwiak and Debra McKeown. In: "Ultra-wideband radio technology". Publisher: John Wiley and Sons, 2004. ISBN 0-470-85931-8 . P. 15.