page_title: Cavity magnetron
hierarchical_section_title: Cavity magnetron
caption_attribution_description: Deutsch: Impulsmagnetron MI-189W(Flugzeug-Bordradar, ca. 9GHz, Sowjetunion) English: A Russian MI-189W magnetron tube from an obsolete aircraft radar. It generates a pulsed output at 9 GHz in the X band. The end of the tube with filament contacts projects inside the red pipe at right. The rest of the tube itself is concealed between the two powerful horseshoe-shaped alnico magnets that provide the magnetic field it requires to operate. The pole pieces of the magnets embrace each end of the tube, creating a magnetic field along the tubes axis. The microwaves produced radiate out of the rectangular waveguide aperture at top, which in use is attached to a waveguide which conducts the microwaves to the antenna. The text on the front side reads: "Do not bring ferromagnetic materials closer than 100 millimeters".
context_page_description: The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field while moving past a series of open metal cavities. Electrons pass by the openings to these cavities and cause microwaves to oscillate within, similar to the way a whistle produces a tone when excited by an air stream blown past its opening. The frequency of the microwaves produced, the resonant frequency, is determined by the cavities' physical dimensions. Unlike other vacuum tubes such as a klystron or a traveling-wave tube, the magnetron cannot function as an amplifier in order to increase the intensity of an applied microwave signal; the magnetron serves solely as an oscillator, generating a microwave signal from direct current electricity supplied to the vacuum tube.
  An early form of magnetron was invented by H. Gerdien in 1910. Another form of magnetron tube, the split-anode magnetron, was invented by Albert Hull of General Electric Research Laboratory in 1920, but it achieved a frequency of only 30 kHz. Similar devices were experimented with by many teams through the 1920s and 1930s.
context_section_description: The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field while moving past a series of open metal cavities (cavity resonators). Electrons pass by the openings to these cavities and cause microwaves to oscillate within, similar to the way a whistle produces a tone when excited by an air stream blown past its opening. The frequency of the microwaves produced, the resonant frequency, is determined by the cavities' physical dimensions.  Unlike other vacuum tubes such as a klystron or a traveling-wave tube (TWT), the magnetron cannot function as an amplifier in order to increase the intensity of an applied microwave signal; the magnetron serves solely as an oscillator, generating a microwave signal from direct current electricity supplied to the vacuum tube.
  An early form of magnetron was invented by H. Gerdien in 1910. Another form of magnetron tube, the split-anode magnetron, was invented by Albert Hull of General Electric Research Laboratory in 1920, but it achieved a frequency of only 30 kHz.  Similar devices were experimented with by many teams through the 1920s and 1930s. Hans Erich Hollmann filed a patent on a design similar to the modern tube in 1935, but the more frequency stable klystron was preferred for most German radars during World War II. An important advance was the multi-cavity magnetron, first proposed in 1934 by A. L. Samuel of Bell Telephone Laboratories. However, the first truly successful example was developed by Aleksereff and Malearoff in USSR in 1936, which achieved 300 watts at 3 GHz (10 cm wavelength).
  The cavity magnetron was radically improved by John Randall and Harry Boot at the University of Birmingham, England in 1940. They invented a valve that could produce multi-kilowatt pulses at 10 cm wavelength, an unprecedented achievement. The high power of pulses from the device made centimeter-band radar practical for the Allies of World War II, with shorter wavelength radars allowing detection of smaller objects from smaller antennas. The compact cavity magnetron tube drastically reduced the size of radar sets so that they could be more easily installed in night-fighter aircraft, anti-submarine aircraft and escort ships.
  At the same time, Yoji Ito was experimenting with magnetrons in Japan, and proposed a system of collision avoidance using frequency modulation. Only low power output was achieved. Visiting Germany, where he had earlier received his doctorate, Ito learned that the Germans were using pulse modulation at VHF with great success. Back in Japan, he produced a prototype pulse magnetron with 2 kW output in October 1941, which was then widely deployed.
  In the post-war era the magnetron was less widely used for radar applications, because the output changes from pulse to pulse, both in frequency and phase. This renders the method unsuitable for pulse-to-pulse comparisons for detecting and removing "clutter" from the radar display. The magnetron remains in use in some radar systems, but has become much more common as a low-cost source for microwave ovens. In this form, over one billion magnetrons are in use today.
caption: Obsolete 9 GHz magnetron tube and magnets from a Soviet aircraft radar.  The tube is embraced between the poles of two horseshoe-shaped alnico magnets (top, bottom), which create a magnetic field along the axis of the tube. The microwaves are emitted from the waveguide aperture (top) which in use is attached to a waveguide conducting the microwaves to the radar antenna. Modern tubes use rare earth magnets, electromagnets or ferrite magnets which are much less bulky.
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