A cavity magnetron is a high-powered vacuum tube that generates coherent microwaves. They are commonly found in the microwave oven, as well as various radar applications. In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. ... Coherence is a property of waves that measures the ability of the waves to interfere with each other. ... This page is about the radiation; for the appliance, see microwave oven. ... Microwave oven A microwave oven, (or microwave), is a kitchen appliance employing microwave radiation primarily to cook or heat food. ... This long range radar antenna (approximately 40m (130ft) in diameter) rotates on a track to observe activities near the horizon. ...

Construction and operation

Magnetron with section removed (magnet is not shown)

All cavity magnetrons consist 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 in a Daniells cell. ... To meet Wikipedias quality standards and appeal to a wider international audience, this article may require cleanup. ... A yagi antenna Most simply, an antenna (U.S.) or aerial (UK) is an electronic component designed to transmit or receive radio waves. ...

A cross-sectional diagram of a resonant cavity magnetron

The size of the cavities determine the resonant frequency, and so the frequency of the emitted microwaves. The frequency is thus not precisely controllable, which is not a problem in many applications such as heating or some forms of 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. Image File history File links Resonant_Cavity_Magnetron_Diagram. ... Image File history File links Resonant_Cavity_Magnetron_Diagram. ... This long range radar antenna (approximately 40m (130ft) in diameter) rotates on a track to observe activities near the horizon. ... A klystron is a specialized vacuum tube (evacuated electron tube) called a linear-beam tube. ...

The magnetron is a fairly efficient device. In a microwave oven, for instance, a 1100 watt input will generally create about 700 watts of microwave energy, an efficiency around 65%. This is far more efficient than the klystron, which typically operates around 30%. Modern solid state microwave sources typically operate around 25 to 30%, and are used primarily because they can generate a wide range of frequencies. Thus the magnetron remains in widespread use in roles where precise frequency control is not important.

Applications

Magnetron with magnet in its mounting box - the horizontal plates aid in cooling the device by airflow from a fan

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 began in the 1900s when engineers invented 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 Cutaway rendering of a color CRT The cathode ray tube or CRT, invented by Karl Ferdinand Braun, is the display device that was traditionally 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, it is recommended to add a glass of water as an energy sink, although care must be taken not to "superheat" the water. Microwave oven A microwave oven, (or microwave), 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

Simple two-pole magnetrons were developed in the 1920s by Albert Hull at General Electric, 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 two-pole magnetron, also known as a split-anode magnetron, gave relatively low efficiencies. The cavity version (properly referred to as a resonant-cavity magnetron) 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. ... Albert W. Hull (19 April 1880 – 22 January 1966) is most remembered for his early invention of the magnetron. ... The General Electric Company, or GE (NYSE: GE) is a multinational technology and services company. ... In electronics, a vacuum tube (American English) or (thermionic) valve (British English) is a device generally used to amplify a signal. ... Lee De Forest Lee De Forest, (August 26, 1873 - June 30, 1961), was an American inventor with over 300 patents to his name. ...

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 tube-based generators of the time. It was known that a multi-cavity resonant magnetron had been developed in 1935 by Hans Hollmann in Berlin. However the German military considered its frequency drift to be undesirable, and based their radar systems on the klystron instead. It was primarily for this reason that German night fighter radars were never a match for their British counterparts. This long range radar antenna (approximately 40m (130ft) in diameter) rotates on a track to observe activities near the horizon. ... Combatants Allied Powers Axis Powers Commanders {{{commander1}}} {{{commander2}}} Strength {{{strength1}}} {{{strength2}}} Casualties 17 million military deaths 7 million military deaths {{{notes}}} World War II, also known as the Second World War (sometimes WW2 or WWII or World War Two), was a mid-20th century conflict that engulfed much of the... This page is about the radiation; for the appliance, see microwave oven. ... The wavelength is the distance between repeating units of a wave pattern. ... Hans Eric Hollmann (4 November 1899 — ) was a German electronic specialist who made several breakthroughs in the development of radar. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... A night fighter is a fighter aircraft adapted for use at night, or in other times of bad visibility. ...

In 1940, at the University of Birmingham in the UK, John Randall and Dr. Harry Boot produced a working prototype similar to Hollman's cavity magnetron, but added liquid cooling and a stronger cavity. Randall and Boot soon managed to increase its power output 100-fold. Instead of giving up on the magnetron due to its frequency innaccuracy, they instead sampled the output signal and synced their receiver to whatever frequency was actually being generated. The University of Birmingham is the oldest of three universities in the English city of Birmingham. ... Sir John Randall (March 23, 1905 – June 16, 1984) was a British physicist, credited with radical improvement of cavity magnetron, an essential component of the centimetre radar, which was one of the keys to the Allied victory in the Second World War. ... Dr Henry Albert Howard Harry Boot (29 July 1917 — 8 February 1983) was a physicist who with Sir John Randall and James Sayers developed the cavity magnetron, which was one of the keys to the Allied victory in the Second World War. ...

An early 6kW version built by G.E.C. and given to the U.S. government in September 1940 was called "the most valuable cargo ever brought to our shores" (see Tizard Mission). At the time the most powerful equivalent microwave-producer available in the US (a klystron) had a power of only ten watts. 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 kilowatt (symbol: kW) is a unit for measuring power, equal to one thousand watts. ... ... In the late Sepember 1940 during the Battle of Britain in the Second World War, a delegation arrived from the UK in the United States on a mission instigated by Henry Tizard, known as the Tizard Mission. ... Combatants Allied Powers Axis Powers Commanders {{{commander1}}} {{{commander2}}} Strength {{{strength1}}} {{{strength2}}} Casualties 17 million military deaths 7 million military deaths {{{notes}}} World War II, also known as the Second World War (sometimes WW2 or WWII or World War Two), was a mid-20th century conflict that engulfed much of the...

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 likewise far more accurate than the older technology. They made the big gunned Allied battleships more deadly and along with the newly developed proximity fuze 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. An early H2S picture of the Pembroke and Milford Haven area 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. ... Look up Proximity fuze in Wiktionary, the free dictionary A proximity fuze (also called a VT fuze) is a fuze that is designed to detonate an explosive automatically when close enough to the target to destroy it. ... The Fieseler Fi 103/FZG-76 (Vergeltungswaffe-1, V-1), known as the Flying bomb, Buzz bomb or Doodlebug, was the first modern guided missile used in wartime and the forerunner of todays cruise missile. ... 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 far more domestic — the microwave oven. The use in radar itself has dwindled to some extent, as more accurate signals have generally been needed and developers have moved to klystron and travelling wave tube systems for these needs. Microwave oven A microwave oven, (or microwave), is a kitchen appliance employing microwave radiation primarily to cook or heat food. ...

Health hazards

Caution:radiowaves hazard

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. Image File history File links Radiowaves_hazard_symbol. ... Image File history File links Radiowaves_hazard_symbol. ... Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... For the band with this name, see Cataract (band). ...

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
• Tuxedo Park, Jennet Conant. New York, Simon & Schuster, c2002. ISBN 0-684-87287-0

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.
• Traveling wave tube - Another microwave amplifier device, capable of greater bandwidths than a klystron.
• Crossed-field amplifier - A device combining characteristics of magnetrons and TWTs, resulting in a high-power narrowband amplifier.
• 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
• Sputtering - An important industrial application of magnetrons

60-inch cyclotron, circa 1939, showing beam of accelerated ions (perhaps protons or deuterons) escaping the accelerator and ionizing the surrounding air causing a blue glow. ... 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. ... It has been suggested that this article or section be merged with Traveling wave tube amplifier. ... A crossed-field amplifier (CFA) is a specialized vacuum tube, frequently used as a microwave amplifier in very-high-power transmitters. ... This article needs to be cleaned up to conform to a higher standard of quality. ... A Hydrogen RF discharge, the first element inside an Hydrogen Maser (see description below), courtesy NASA/JPL-Caltech. ... The range of sizes in which lasers exist is immense, extending from microscopic diode lasers (top) to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion. ... Sputtering is a physical process whereby atoms in a solid target material are ejected into the gas phase due to bombardment of the material by energetic ions. ...

External link

Information

• Magnetrons
• 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