MAGNETRONS The Evolution & Operation of Chuck Hobson BA, BSc(hons)

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MAGNETRONS The Evolution
Operation ofChuck Hobson BA, BSc(hons)
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Introduction
Who invented the magnetron? When I started to
look into this, I soon realized that there was no
simple answer to this question.   Basically, the
magnetron is a simple electronic diode in a
strong magnetic field. Electrons move from the
cathode to the anode though a magnetic field,
which is at right angles to the direction of
electron motion. As such, the electrons
experience another force at right angles to both
their direction of motion and the magnetic field.
This results in the electrons taking a curved
path. The laws governing this motion are
identical to the laws governing the rotation of a
dc motor. The dc motor motor came about during
the early mid 19th century. The oscilloscope made
its entrance during the early 20th century. The
effect of a magnet on an oscilloscope beam gave
scientists a clue and something to investigate.
It wasnt long before scientists the world over
were experimenting with electron beams in strong
magnetic fields and observing oscillations. Up
until WW2 these scientists were in communication
with each other exchanging findings and
experimental results.
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Two such Scientists
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EVOLUTION OF THEMAGNETRON

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MAGNETRON TIME LINE

• 1921 A. W. Hull invented magnetron. Cylindrical
  anode
• 1927 Kinjiro Okabe at Osaka University
  introduced the split anode magnetron. Oscillated
  at 2.5gHz (12cm)
• 1933 1945 Japanese Navy experimented with
  Okabes magnetron and various anode
  configurations
• 1934 Posthumus at Philips developed 4 seg.
  Magnetron
• 1934 A. L Samuel Bell Tele filed patent 4 cavity
  magnetron
• 1935 Hans Hollmann Germany patented cavity
  magnetron
• 1936 Cleeton Wllliams reached 47gHz with split
  anode
• 1937 Aleksereff and Malearoff 4 cavity magnetron
• 1940 University of Birmingham GEC developed
  high power µ-wave magnetron suitable for radar
  application

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MAGNETRONS GERMANY
1920 Heinrich Barkhausen 0.3 - 6.4gHz at 5W
Electron cloud surrounds filament Pos. grid
attracts electrons Electrons accelerate through
grid Electrons near anode repelled back through
grid. Electrons oscillate around grid RF taken
off grid (glows white hot)
Barkhausen Oscillator (not a magnetron)
1935 Hans Hollmann patented cavity magnetron in
Berlin German military rejected it for radar
application because of excessive frequency drift.
However they used klystrons for their Wurzburg)
radar. 5 11kW peak pwr. 2µsec pulse width
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HULLS 1921 MAGNETRON (US)

• Cavity magnetron Coaxial configuration

Electron path
Frequency 200kHz increasing to 10MHz 1925 Elder
of GE (US) produced 8kW _at_ 30kHz 69 efficiency
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OKABES 1927 SPLIT ANODE MAGNETRON

• Plate and cathode enclosed in glass envelope

Electron path cathode to anode Strong magnetic
field parallel to cathode Oscillates at 2.5gHz
(12cm)
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MAGNETRON WAR TIME JAPAN
Shimada Laboratory at the Technical Institute of
the Japanese Navy had been carrying out
experiments on high power microwaves since
1933 Below are some magnetron anode
configurations involved. Frequency was 2.5cm
(12gHz)
Various configurations named after Japanese
flowers C Kosumosu (Rising sun) U Umebachi
(Apricot flower)
Above information from paper by Professor Koichi
Shimoda
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MAGNETRON WAR TIME JAPAN
Shimada Laboratory, Technical Institute of
Japanese Navy, Shizuoka Prefecture in 1944
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MAGNETRON RUSSIA
4 cavity magnetron Russia 1937 Aleksereff and
Malearoff 300W CW _at_ 10cm 20 efficiency No record
of Russian military using it in radars
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MAGNETRON WAR TIME UK

• 1938 Admiralty awarded GEC a development
  contract.
• 1940 April, GEC bread-boarded a 25cm operating
  radar
• Transmitter produced 25kW pulses using Hi-Pwr.
  Triodes

• University of Birmingham J. T. Randall and H. A.
  H. Boot
• Literature on Magnetrons world-wide but
  unobtainable
• 1940 Feb. Developed 9.4cm (3.91gHz) 400W CW
  Magnetron
• GEC produced two magnetrons using R B as a
  model
• 1940 June Pulse powers of 10 to 40kW at 10cm
  achieved
• 1940 Aug. Tizard and team brought magnetron to
  the U. S.
• Sept. Mag. at MIT Labs. Bell Labs Raytheon Co.
  x-rayed Mag. reproduced it.By Nov. it was in
  mass production

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MAGNETRON WAR TIME UK
Experimental magnetron University of Birmingham
Randall and Boots first experimental
magnetron. Produced 400W CW at 3.91gHz (a true
break through) The anode had six cavities and
was water cooled Used 0.75mm tungsten rod as a
filament for the cathode Tube was continuously
pumped and placed between the poles of an
electromagnet.
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J. T. Randall H. T. Boot
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Dr. Eric Stanley Megaw
Born in Belfast Educated at Queens
University Avid radio enthusiast Transmitted the
first amateur Radio signals out of Ireland in
1924. First QSOs with West Coast US and
Australia

• Born in Belfast

Worked for GEC for 16 years. Headed group which
took the Boot and Randell magnetron design and
developed the E-1189 Magnetron. This included
improvements making it suitable for airborne
radar use. It was actually Megaw who added the
straps which made the magnetron a stable µ-wave
oscillator Megaw was awarded the MBE in 1951 for
his µ-wave work Became Director of Physical
Research with the Admiralty
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E-1189 MAGNETRON
E-1189 GEC no. 12
Megaw
Photo of actual magnetron Tizard took to N.
America E-1189 The 1st GEC magnetron had 6
cavities Subsequently modified to have 8
cavities (No. 12) Freq. 3297MHz peak Pwr. 12kW
Peak anode current 7A Magnetic field 1050 gauss
(0.105 Tesla) Dr. Boot used a Colt 45
revolving chamber as a drill fixture at U. of
Birmingham for his first magnetron.
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E-1198 MAGNETRON
E-1198 8 cavity 12.5kW 3gHz (10cm) 1500
Oersteds
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MAGNETRONS
CV 38 E-1198 8 cavity magnetron Fil. 6V Nom. Freq
3297MHz Pk. Pwr. 7kW Magnet 1050 gauss
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MAGNETRONS
X-band magnetrons CV-208 glass enclosed probe
which is inserted in wave-guide 2J49, 725A, 730A
shows x-band wave-guide outputs
725A output 9375MHz at 60kW Western Electric
manufactured and delivered 89000 units to the
British Empire during WW2
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MAGNETRON OPERATION
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MAGNETRON APPLICATION

• Magnetrons are used primarily in
• Radar Transmitters (pulsed)
• Peak power from 10kW to 3MW
• Frequency from 600MHz to 47gHz

• Microwave Ovens (CW
• Frequency 2.45gHz
• Output power 650 1200W Efficiency
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• Specialized Industrial applications

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MAGNETRON CONSTRUCTION
Typical S band 50 kW magnetron used in military
radars Driven by a 30kV 1.0µsec pulse. Efficiency
30 (WW2) now 65 Input peak power 167kW
Peak current 5.6A With 1000 repetition rate,
average input ave. power 167W
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MAGNETRON CONSTRUCTION
Cutaway view of the magnetron
Open area between cathode anode called
Interaction space E H fields interact on
electrons to get µ-waves in cavities
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MAGNETRON CONSTRUCTION
Another cutaway view of the magnetron
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MAGNETRON CONSTRUCTION
Magnetron eight cavity anode
µ-wave energy is induced in all cavities by
moving electrons Cavities in series. Energy
coupled to output loop as shown
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MAGNETRON CONSTRUCTION
Equivalent circuit of one cavity
One of 8 cavities
Eight equivalent circuits shown in series
Typical of German and Japanese
magnetrons Unstable
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MAGNETRON CONSTRUCTION
Alternate cavities strapped together with solid
copper rings Dr. Megaws addition to the Boot
Randall magnetron configuration
Schematic of eight strapped cavities Note that
all cavities are connected in parallel This
insures that oscillations in all cavities are in
phase
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HOW DOES A MAGNETRON WORK?
Various anode forms
Magnetic field provided by strong permanent magnet

• Producing µ-waves can be subdivided into four
  phases
• Production and acceleration of an electron beam
• Velocity-modulation of the electron beam
• Forming of a Space-Charge Wheel
• Dispense energy to the ac field

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MAGNETRON OPERATION PHASE 1
Cathode centre at high negative volts Anode at
zero volts No magnetic field Electrons move in
straight line
Magnet added North pole on top South pole at
bottom Electrons curve to the right Electrons
curve more when the magnetic field is
increased
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MAGNETRON OPERATION PHASE 1
Green path Weak magnet. All cathode electrons
reach anode Red path Magnetic field increased to
critical value. Anode current decreases to a
small value. White path Magnetic field increased
further. Anode current drops to zero Magnetic
field adjusted to where electrons just fail to
reach the anode, the magnetron can oscillate
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MAGNETRON OPERATION PHASE 2
Interaction space between cathode and cavities 2
electric fields, ac dc in interaction
space Polarity is one instant of ac (µ-wave)
field The dc field extends radially from cavities
to cathode
Electrons near cavities move tangentially to
cavities Electrons approaching the positive
sides are speeded up Electrons departing the
positive side and approaching the negative side
are slowed down.
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MAGNETRON OPERATION PHASE 3
12 cavity magnetron Rotating 6 spoke space
charge Space charge gives µ-wave energy to the
cavity keeping it oscillating
8 cavity magnetron 4 spoke wheel
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MAGNETRON OPERATION PHASE 4
Assume dc field rf fields on cavities
(magnetron oscillating Electron approaching
cavity gives up energy to cavity Electron slows
down accordingly Then electron speeds up gaining
energy from dc field Electron eventually reaches
cavity (anode current)
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MAGNETRON RADAR CIRCUIT

• PFN charges up to 12kV (dc resonance phenomena)
• Trigger switches thyratron on
• PFN discharges through transformer and thyratron
• During discharge PFN develops rectangular pulse
• Transformer steps negative 6kV pulse up to 30kV
• Magnetron oscillates for duration of pulse ( 0.5
  to 4µsec

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Thank you for viewing my Magnetron
presentation.I hope you found it informative and
enjoyable.Chuck Hobson BA, BSc(hons)
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