Kohærer
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En kohærer er en primitiv radiobølge diodedetektor, som blev anvendt tidligt i radiofoniens historie. Men en kohærer er ikke en ensretterdiode. Det engelske navn coherer blev navngivet af professor Oliver Lodge i 1894.[1][2]
Der er to grundlæggende typer af kohærer: Den originale metalpulver type, kaldet Branly-kohærer; og en senere ikke-perfekte overgang-kohærer type.
Opbygning og virkemåde
En kohærer består af metalpulver rørindkapslet mellem to typisk skrånende elektroder (sorte) nogle få millimeter fra hinanden, forbundet til terminaler.
Når et radiosignal (eller jævnspænding) med tilstrækkelig styrke, påtrykkes den initielt højohms kohærer, vil den under påvirkning af radiosignaler få reduceret sin modstand betydeligt – og dette kan detekteres af et primitivt ohmmeter. Herefter skal kohæreren mekanisk rystes (dekohærer), så den er højohm igen – og så er den klar den klar til en ny radiodetektionscyklus.[2]
Historisk
En kohærerfunktion er først omtalt og anvendt af David Edward Hughes i 1878-1880 og blev muligvis opfundet af samme.[2] År 1891 anvender den franske videnskabsmand Édouard Branly en kohærerfunktion.[2]
Kohæreren blev grundlaget for radiomodtagelse omkring år 1900, og forblev bredt anvendt i omkring 10 år. Autokohæren blev opfundet af Jagadish Chandra Bose og anvendt af Guglielmo Marconi i 1901 til verdens første anerkendte interkontinentale radiotransmission.[2][3]
Kohæreren var en nøgleteknologi for datidens radio, og var den første teknologi anvendt til at detektere radiobølger og dermed muliggjorde praktisk radiobølgebaseret trådløs kommunikation sammen med gnistsendere og buesendere.
Kohæreren blev afløst af de mere følsomme elektrolytiske detektor og krystaldetektoren og blev forældet, selvom kohæreren i 1950'erne en overgang blev anvendt i mindst ét radiofjernstyret stykke legetøj.[4]
Se også
Kilder/referencer
- ^ thepotteries.org: Sir Oliver Lodge Citat: "...In wireless telegraphy Lodge perfected and named the "coherer" (1894)...", backup
- ^ a b c d e Jagadish Chandra Bose: The Real Inventor of Marconi’s Wireless Detector Citat: "...The present article puts forth the fact that the receiving device used by Marconi, a mercury autocoherer was invented by Sir Jagadish Chandra Bose. Bose was the first to report this invention in his paper [9] presented at the Royal Society...An improved version of the coherer called the autocoherer was used by Marconi in his 1901 experiment...Coherer: The name coherer was coined by Prof. Oliver Lodge [3]. Coherer is a two terminal device whose resistance drops sharply on application of a direct or alternating voltage more than a given threshold...David Edward Hughes uses some kind of a coherer. No use of mercury...3. V. J. Phillips, “The ‘Italian Navy Coherer’ Affair: A Turn of-the-Century Scandal,” Proc. IEE Series A., Vol 140, No. 3, pp. 175-185, May 1993. [Reproduced in IEEE, Vol 86, no. 1, pp. 248-258, January 1998]...", backup
- ^ nobelprize.org: Guglielmo Marconi: The Nobel Prize in Physics 1909 Citat: "...on an historic day in December 1901, determined to prove that wireless waves were not affected by the curvature of the Earth, he used his system for transmitting the first wireless signals across the Atlantic between Poldhu, Cornwall, and St. John’s, Newfoundland, a distance of 2100 miles...", backup
- ^ Lee, Thomas H. (2004). Planar Microwave Engineering: A Practical Guide to Theory, Measurement, and Circuits. London: Cambridge University Press. s. 11. ISBN 0521835267.
Eksterne henvisninger
 | Wikimedia Commons har medier relateret til: |
- Web archive backup: "The Coherer". World of Wireless, Virtual radiomuseum.
- "Coherer / Receiver Arkiveret 10. december 2004 hos Wayback Machine". Marconi Calling Company.
- Slaby, Adolphus, "The New Telegraphy, Recent experiments in telegraphy with sparks.". The Century Magazine. April, 1898. (Earlyradiohistory.us)
- Hirakawa Institute of Technology(Japan),"Coherer Arkiveret 3. marts 2016 hos Wayback Machine".
- "Tesla's US Patent: 613,809". ShareAPic.net.
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Medier brugt på denne side
Inductively coupled coherer radio receiver circuit from Guglielmo Marconi's April 26, 1900 patent.
Forfatter/Opretter: Everaldo Coelho (YellowIcon);, Licens: LGPL
Et ikon fra Crystal-temaet
A Branly coherer, 1902, an early primitive radio wave detector invented by Edouard Branly in 1890 and used in the first radio receivers until about 1906. This version, developed by Guglielmo Marconi, consists of an evacuated glass tube with metal filings between two metal electrodes. Radio waves from the antenna, applied across the electrodes, cause the filings to cohere, reducing the resistance of the device. This turns on a DC circuit attached to the electrodes, powered by a battery, that makes a "click" sound in earphones or a mark on a paper tape, to record the Morse code signal. The slanted faces of the electrodes allowed the gap between them, and thus the sensitivity of the detector, to be adjusted by rotating the tube.
A coherer, a crude radio detector used in the first radio receivers from about 1895 to 1906 during the wireless telegraphy era. This example was built by radio researcher Emile Guarini Foresio. It is a glass tube (right) containing metal powder between two electrodes. When a radio signal is applied between the electrodes, the metal powder coheres (clumps together) causing the tube to conduct electricity. The coherer is attached to a DC circuit (not shown) which makes a mark on a paper tape to record the Morse code radio message. Because the coherer remained in its conducting state after the radio wave stopped, the device includes a "decoherer" or "tapper" (left) which uses an electromagnet-activated arm to tap the coherer, agitating the powder, to return the device to its nonconducting state in preparation to receive another radio signal.
Fritter (Coherer)
A simple radio wave detector or coherer invented by French scientist Édouard Branly around 1890 and used in some of the first experiments at receiving radio communication. It consists of two copper bars which are roughened and oxidized and laid one over the other, creating a high resistance junction. A dipole radio antenna consisting of two wires were attached to the two bars. The bars were also connected to a DC circuit consisting of a battery and a galvanometer. When a radio signal from a spark gap radio transmitter was received by the antenna and applied to the bars, the resistance of the contact between them dropped to a low level. Electric current from the battery passed through the coherer and was detected by the galvanometer, registering the presence of the radio wave. Before receiving another signal, the device had to be restored to its receptive condition by tapping it, disturbing the junction between the bars to reset it to its previous high resistance state. Branly wrote that the device could be made more sensitive by grinding the top bar into a knife edge and resting it very lightly on the other.