page_title: Cavity magnetron
hierarchical_section_title: Cavity magnetron
caption_attribution_description: English: Magnetron section transverse to axis. Section through a magnetron from a microwave oven. The magnetron has a diameter of about 4 centimetres. There is a central cathode which is grey in colour. Everything else is copper and forms the anode. Around the anode, a total of 10 'spokes' are connected to the exterior of the magnetron. There are 5 'plain' spokes that are alternated with 5 'notched' spokes (the notches can be seen near the outer ring). Each pair of adjacent spokes (i.e. a plain spoke and a notched spoke) forms a resonant cavity. Around the cathode, two concentric rings (an inner ring and an outer ring) can be seen. The inner ring electrically connects the notched spokes; the outer ring electrically connects the plain spokes. The inner and outer rings are known as 'strapping' rings and ensures that the magnetron operates in the 'pi' mode. For the notched spoke that is roughly at the 'ten o'clock' position, the notch has an output coupling loop that extends upwards and out of the top of the photo. The output coupling loop is used to couple microwave energy out of the magnetron so that the microwave energy can heat up food in the microwave oven. Incidentally, note that the magnetron shown in this photo is almost exactly the same as the magnetron shown in another photo on the magnetron page:
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: A similar magnetron with a different section removed. Central cathode is visible; antenna conducting microwaves at the top; magnet is not shown.
exif: {"Image Make": "Canon", "Image Model": "Canon EOS 400D DIGITAL", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image DateTime": "2009:03:11 17:42:10", "Image YCbCrPositioning": "Co-sited", "Image ExifOffset": "196", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "6132", "Thumbnail JPEGInterchangeFormatLength": "8110", "EXIF ExposureTime": "3/5", "EXIF FNumber": "16", "EXIF ExposureProgram": "Aperture Priority", "EXIF ISOSpeedRatings": "100", "EXIF ExifVersion": "0221", "EXIF DateTimeOriginal": "2009:03:11 17:42:10", "EXIF DateTimeDigitized": "2009:03:11 17:42:10", "EXIF ComponentsConfiguration": "YCbCr", "EXIF ShutterSpeedValue": "24149/32768", "EXIF ApertureValue": "8", "EXIF ExposureBiasValue": "0", "EXIF MeteringMode": "Pattern", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "70", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "3888", "EXIF ExifImageLength": "2592", "Interoperability InteroperabilityIndex": "R98", "Interoperability InteroperabilityVersion": "[48, 49, 48, 48]", "EXIF InteroperabilityOffset": "5766", "EXIF FocalPlaneXResolution": "3888000/877", "EXIF FocalPlaneYResolution": "432000/97", "EXIF FocalPlaneResolutionUnit": "2", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF SceneCaptureType": "Standard"}