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Encyclopedia > Magnetron

A cavity magnetron is a high-powered vacuum tube that generates coherent microwaves. In electronics, a vacuum tube (American English) or (thermionic) valve (British English) is a device generally used to amplify, or otherwise modify, a signal. ... This article needs to be cleaned up to conform to a higher standard of quality. ... This page is about the radiation; for the appliance, see microwave oven. ...

Contents


Construction and operation

Magnetron in cross-cut
Magnetron in cross-cut

A cavity magnetron consists of a hot filament (cathode) kept at or pulsed to a high negative potential by a high voltage direct current power supply. The cathode is built into the center of an evacuated, lobed, circular chamber. A perpendicular magnetic field is imposed by a permanent magnet. The magnetic field causes the electrons, attracted to the (relatively) positive outer part of the chamber, to spiral outward in a circular path rather than moving directly to this anode. Spaced about the rim of the chamber are cylindrical cavities. The cavities are open along their length and so connect the common cavity space. As electrons sweep past these openings they induce a resonant high frequency radio field in the cavity, which in turn causes the electrons to bunch into groups. A portion of this field is extracted with a short antenna that is connected to a waveguide (a metal tube usually of rectangular cross section). The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high gain antenna in the case of radar. Wikipedia does not have an article with this exact name. ... Wikipedia does not have an article with this exact name. ... Diagram of a copper cathode in a Daniells cell. ... Diagram of a zinc anode An anode (from the Greek άνοδος = going up) is the positive electrode in an electrolytic system or circuit. ... In physics, optics, and telecommunication, a waveguide is an inhomogeneous (structured) material medium that confines and guides a propagating electromagnetic wave. ... Aerial redirects here. ...


The size of the cavities determine the resonant frequency and so the frequency of the emitted RF energy microwaves. The frequency is thus not precisely controllable, which is not a problem in many applications such as heating (where it does not matter; usually microwave ovens are tuned at approximately 2450MHz = 2.450GHz, where the irradiated food heats up due to dielectric losses) and radar (where the receiver can be synchronised with the nonprecision output). (Where precise frequencies are required, other devices such as the klystron are used.) The voltage applied and the characteristics of the cathode determine the power of the device. Microwave Slang for small waves, like at a beach, often used by surfers. ... This long range radar antenna (approximately 40m (130ft) in diameter) rotates on a track to observe activities near the horizon. ... The klystrons in the superconducting section of the SNS Klystron Gallery of the linear particle accelerator (LINAC) A klystron is a specialized vacuum tube (evacuated electron tube) called a linear-beam tube. ...


Applications

Magnetron in its box
Magnetron in its box

Wikipedia does not have an article with this exact name. ... Wikipedia does not have an article with this exact name. ...

Radar

See also History of radar (Magnetron)

In radar devices the waveguide is connected to an antenna, which may be a slotted waveguide or a conical feedhorn pointing into a parabolic reflector. The magnetron is operated with very short high intensity pulses of applied voltage, resulting in a short pulse of microwave energy being emitted. A small portion of this energy is reflected back to the antenna and the waveguide where it is directed to a sensitive receiver. With further signal processing the signal is ultimately displayed as a radar map on a cathode ray tube (CRT) or a liquid crystal display. The history of radar begins in the 1900s as engineers invent reflection devices. ... This long range radar antenna (approximately 40m (130ft) in diameter) rotates on a track to observe activities near the horizon. ... A yagi antenna Most simply, an antenna is an electronic component designed to send or receive radio waves. ... A slotted waveguide is a waveguide that is used as an antenna in microwave radar applications. ... A parabolic reflector (also known as a parabolic dish or a parabolic mirror) is a reflective device formed in the shape of a paraboloid of revolution. ... Cathode ray tube employing electromagnetic focus and deflection The cathode ray tube or CRT, invented by Karl Ferdinand Braun, is the display device used in most computer displays, video monitors, televisions and oscilloscopes. ... Reflective twisted nematic liquid crystal display. ...


Heating

In microwave ovens the waveguide leads to a radio frequency transparent port into the cooking chamber. It is important that there be food in the oven when it is operated so that these waves are absorbed, rather than reflecting back into the waveguide where the intensity of standing waves can cause arcing. The arcing, if allowed to occur for long periods will destroy the magnetron. If a very small object is being microwaved for whatever reason, it is probably best to add a glass of water as a sink for microwaves, although care must be taken to not "superheat" the water. Microwave oven A microwave oven is a kitchen appliance employing microwave radiation primarily to cook or heat food. ... A standing wave, also known as a stationary wave, is a wave that remains in a constant position. ...


History

Influence of a magnetron on a DVD
Influence of a magnetron on a DVD

Simple two-pole magnetrons were developed in the 1920s but gave relatively low power outputs. The cavity version proved to be far more useful. Image File history File links Download high resolution version (708x702, 493 KB) Description: Influence of Magnetron to a Compact Disk Source: http://www. ... Image File history File links Download high resolution version (708x702, 493 KB) Description: Influence of Magnetron to a Compact Disk Source: http://www. ... Sometimes referred to as the Jazz Age or primarily in North America as the Roaring Twenties. // Events and trends Technology John T. Thompson invents Thompson submachine gun, also known as Tommy gun John Logie Baird invents the first working mechanical television system (1925) Charles Lindbergh becomes the first person to...


There was an urgent need during radar development in World War II for a high-power microwave generator that worked in shorter wavelengths - around 10 cm rather than 150 cm - available from generators of the time. In 1940, at Birmingham University in the UK, Professor John Randall and Doctor Harry Boot produced a working prototype of the cavity magnetron, (properly referred to as a resonant-cavity magnetron) and soon managed to increase its power output 100-fold. This long range radar antenna (approximately 40m (130ft) in diameter) rotates on a track to observe activities near the horizon. ... World War II was a truly global conflict with many facets: immense human suffering, fierce indoctrinations, and the use of new, extremely devastating weapons like the atom bomb World War II, also known as the Second World War, was a mid-20th-century conflict that engulfed much of the globe... This page is about the radiation; for the appliance, see microwave oven. ... The wavelength is the distance between repeating units of a wave pattern. ... 1940 was a leap year starting on Monday (link will take you to calendar). ... The University of Birmingham is the oldest of three universities in the English city of Birmingham. ...


An early 6kW version built by G.E.C. and given to the U.S. government in August 1940 was called "the most valuable cargo ever brought to our shores". At the time the most powerful equivalent microwave-producer available in the US (a klystron) had a power of ten watts. Curiously, in order not to draw attention to the value of the package, it was not dispatched under guard but sent by regular parcel post. The cavity magnetron was widely used during World War II in microwave radar equipment, and is often credited with giving Allied radar a considerable performance advantage over German and Japanese radars, thus directly influencing the outcome of the war. Short wave centimetric radar, which was made possible by the cavity magnetron, allowed for the detection of much smaller objects and the use of much smaller antennas. The combination of the small sized cavity magnetron, small antennas and high resolution allowed small high quality radars to be installed in aircraft. They could be used by maritime patrol aircraft to detect objects as small as a submarine periscope, which allowed aircraft to attack and destroy submerged submarines which had previously been undetectable from the air. Centimetric contour mapping radars like H2S improved the accuracy of Allied bombers used in the strategic bombing campaign. Centimetric gun laying radars were much more accurate than the older technology. They made the big gunned Allied battleships more deadly and along with the newly developed proximity fuse made anti-aircraft guns much more dangerous to attacking aircraft. The two coupled together and used by anti-aircraft batteries, placed along on the German V-1 flying bomb flight paths to London, are credited with destroying many of the flying bombs before they reached their target. Power kilowatt (symbol: kW) is a unit for measuring power, equal to one thousand watts. ... ... 1940 was a leap year starting on Monday (link will take you to calendar). ... The H2S radar was used in bombers of RAF Bomber Command. ... Strategic Bombing during World War II was unlike anything the world had previously witnessed. ... A proximity fuse (sometimes spelled fuze) is a fuse that is designed to detonate an explosive automatically when close enough to the target to destroy it. ... The Vergeltungswaffe 1 Fi 103 / FZG-76 (V-1), known as the Flying bomb, Buzz bomb or Doodlebug, was the first modern guided missile used in wartime and the first cruise missile. ... The clock tower of the Palace of Westminster, which contains Big Ben London is the capital city of the United Kingdom and of England. ...


Since then, many millions of cavity magnetrons have been manufactured; some for radar, but the vast majority for another application that was completely unanticipated at the time - the microwave oven. Microwave oven A microwave oven is a kitchen appliance employing microwave radiation primarily to cook or heat food. ...


Health hazards

Among more speculative hazards, at least one in particular is well known and documented. As the lens of the eye has no cooling blood flow it is particularly prone to overheating when exposed to microwave radiation. This heating can in turn lead to a higher incidence of cataracts in later life. A microwave oven with a warped door or poor microwave sealing can be hazardous. The lens or crystalline lens is a component of the eye. ... An eye is an organ that detects light. ... A cataract is a large waterfall or place where the flow of a river changes dramatically. ...


References

  • T. J. Morgan - RADAR - The Mechanical Age Library - Mullen - circa 1952
  • A. P. Rowe: One Story of Radar - Camb Univ Press - 1948
  • Dudley Saward, Bernard Lovell: A Biography - Robert Hale - 1984

See also

  • Cyclotron - An atomic accelerator that also directs particles in a spiral with a transverse magnetic field.
  • Klystron - A device for amplifying or generating microwaves with greater precision and control than is available from the magnetron.
  • Travelling wave tube - Another microwave amplifier device, capable of greater bandwidths than a klystron
  • Free electron laser - A device for amplifying or generating microwaves, infrared light, UV, and X-Rays.
  • Maser - A device for generating microwaves that produces a very low noise and stable signal, a predecessor of the laser.
  • Laser - A device for generating coherent light, an evolution of the maser

60-inch cyclotron, circa 1939, showing beam of accelerated ions (perhaps protons or deuterons) causing a blue glow, almost certainly the Cherenkov effect. ... Reflex klystron Type 2K25 or 723 A/B. The threaded adjustment rod on the right side allows the position of the reflector to be adjusted (by compressing the reflex cavity), and thus the natural resonant frequency of the device. ... A TWTA or travelling-wave tube amplifier is an electronic device used to produce high-power radiofrequency signals. ... This article needs to be cleaned up to conform to a higher standard of quality. ... A maser is any amplifier that produces coherent electromagnetic waves due to stimulated emission. ... Laser (US Air Force) A LASER (Light Amplification by Stimulated Emission of Radiation) is an optical device which uses a quantum mechanical effect called stimulated emission (discovered by Einstein while researching the photoelectric effect) in order to generate a coherent beam of light from a lasing medium of controlled purity...

External link

Information

  • Magnetron collection in the Virtual Valve Museum

Patents

  • U.S. Patent 2315313 - Cavity resonator - H. Bushholz
  • U.S. Patent 2357313 - High frequency resonator and circuit therefor -- P. S. Carter
  • U.S. Patent 2357314 - Cavity resonator circuit - P. S. Carter
  • U.S. Patent 2444152 - Cavity resonator circuit - P. S. Carter
  • U.S. Patent 2611094 - Inductance-capacitance resonance circuit - H. B. Rex
  • U.S. Patent 2408236 - Magnetron casing - P. L. Spencer

  Results from FactBites:
 
cavity magnetron: Definition and Much More from Answers.com (1664 words)
The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high gain antenna in the case of radar.
Simple two-pole magnetrons were developed in the 1920s by Albert Hull at General Electric's Research Laboratories (Schenectady, New York), as an outgrowth of his work on the magnetic control of vacuum tubes in an attempt to work around the patents held by Lee DeForest on electrostatic control.
The cavity magnetron was widely used during World War II in microwave radar equipment, and is often credited with giving Allied radar a considerable performance advantage over German and Japanese radars, thus directly influencing the outcome of the war.
Power supply means for magnetron - Patent 4005370 (2821 words)
Magnetron power supply means as claimed in claim 2, wherein said signal generating means comprises a bridge circuit having a transistor therein for producing a signal corresponding to power input applied to said magnetron.
Magnetron power supply means as claimed in claim 2, wherein said signal generating means comprises photocoupler connected to said secondary side of said step-up transformer connected to said secondary side of said step-up transformer for producing a signal corresponding to the power input to said magnetron.
3 and a magnetron 33 corresponding to the magnetron 19 of FIG.
  More results at FactBites »

 
 

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