Hull AW
The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic fieldwhile moving past a series of open metal cavities (cavity resonators). Bunches of electrons passing by the openings to the cavities excite radio wave oscillations in the cavity, much as a guitar's strings excite sound in its sound box. The frequency of the microwaves produced, the resonant frequency, is determined by the cavities' physical dimensions. Unlike other microwave tubes, such as the klystron and traveling-wave tube (TWT), the magnetron cannot function as an amplifier, increasing the power of an applied microwave signal, it serves solely as an oscillator, generating a microwave signal from direct current power supplied to the tube.
The first 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 little used. Similar devices were experimented with by many teams through the 1920s and 30s. On November 27, 1935, Hans Erich Hollmann applied for a patent for the first multiple cavities magnetron, which he received on July 12, 1938,[1] 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.[2] 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[3] so that they could be installed in anti-submarine aircraft[4] and escort ships.[3]
In the post-war era the magnetron became less widely used in the radar role. This was due to the fact that the magnetron's output changes from pulse to pulse, both in frequency and phase. This makes the signal unsuitable for pulse-to-pulse comparisons, which is widely used for detecting and removing "clutter" from the radar display.[5] The magnetron remains in use in some radars, but has become much more common as a low-cost microwave source for microwave ovens. In this form, approximately one billion magnetrons are in use today.[5][6]
Contents
- 1 Construction and operation
- 2 Common features
- 3 Applications
- 4 History
- 5 Health hazards
- 6 See also
- 7 References
- 8 External links
Albert Wallace Hull (19 April 1880 – 22 January 1966) is an American electrical engineer who made contributions to the development of vacuum tubes, and invented the magnetron.[1] He was a member of the National Academy of Sciences.
Contents
- 1 Biography
- 2 Dynatron
- 3 Magnetron
- 4 Gas-filled electron tubes
- 5 Publications
- 6 Awards
- 7 References
- 8 External links
Biography[edit]
He was born on 19 April 1880 in Southington, Connecticut.[1]
He majored in Greek and after taking one undergraduate course in physics, graduated from Yale University. He taught languages at The Albany Academy before returning to Yale, to take a doctorate in physics. He then undertook research on photoelectricity whilst teaching physics for five years at the Worcester Polytechnic Institute.[1]
In 1914 Hull joined the General Electric Research Laboratory (GERL) in Schenectady, New York and remained there until his retirement in 1949.[1]
During 1916, Hull began investigation into the use of magnetic control of thermionic valves (vacuum tubes) as an alternative to grid or electrostatic control and he had tested successfully magnetic control by applying a magnetic field parallel to the axis of the tube. Initially, Hull's work on these novel electron tubes was part of an effort at General Electric to develop amplifiers and oscillators that might be used to circumvent the vacuum- tube triode patents of Lee de Forest and Edwin Armstrong. Hull was promoted to assistant director of the GERL in 1928.[1]
Hull was promoted to assistant director of the GERL in 1928. He served as president of the American Physical Society in 1942. He retired fromGeneral Electric Research Laboratory (GERL) in 1949. He did consulting work and served on an advisory committee of the Army Ballistics Research Laboratories after retirement from General Electric.
He died on 22 January 1966 at the age of 85 in Schenectady, New York.[1][2]
Dynatron[edit]
He invented the dynatron vacuum tube which had three electrodes: a thermionic cathode, a perforated anode, and a supplementary anode or plate. In normal operation the supplementary anode was maintained at a lower positive voltage than the perforated anode. The secondary emission of electrons from the plate made the dynatron behave as a true negative resistance and so the tube could generate oscillations over a wide range of frequencies or be used as an amplifier. When a control grid was added between the cathode and the perforated anode, the device was called a "pliodynatron."
Magnetron[edit]
By 1920 his research led to his invention of the magnetron.[3] This took the form of a central cathode and a coaxial cylindrical anode split into two halves, with an axial magnetic field produced by an external coil. The Hull magnetron was tested as an amplifier in radio receivers and also as a low-frequency oscillator. It was reported in 1925 that a magnetron made at GERL could generate a power of 15 kW at a frequency of 20 kHz. At the time Hull anticipated that the magnetron would find greater use as a power converter than in communication applications. Hull's split-anode magnetron didn't prove to be capable of high frequency or high power output and was little used. However, during World War 2 John Randall and Harry Boot built on Hull's concept to develop the modern cavity magnetron, the first device which could produce high power at microwave frequencies, and the resulting centimeter-band radar proved a crucial advantage for the Allies in aerial warfare.
Gas-filled electron tubes[edit]
During the 1920s, Hull also was a major contributor to the development of gas-filled electron tubes at the GERL. He discovered how to protect thermionic cathodes from rapid disintegration under ion bombardment. This discovery enabled the successful development of hot-cathode thyratrons (gaseous triodes) and phanotrons (gaseous diodes).
Publications[edit]
In the 1918 issue of the Proceedings of the IRE he published a paper on the dynatron vacuum tube which he had invented. During his career in electronics he was the author or coauthor of 72 technical publications and was awarded 94 patents.[4]