Cavity magnetron

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 radio waves to oscillate within, similar to the way a guitar resonates sound from its sound box via the oscillation of its strings. 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 in 1920, but it wasn't capable of high frequencies and was of little use. Similar devices were experimented with by many teams through the 1920s and 1930s. On November 27, 1935, Hans Erich Hollmann applied for a patent for the first multiple-cavity magnetron, which he received on July 12, 1938, but the more stable klystron was preferred for most German radars during World War II. The cavity magnetron tube was later improved by John Randall and Harry Boot in 1940 at the University of Birmingham, England. The high power of pulses from their 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.

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