g0mdk 1 magnetrons the evolution & operation of chuck hobson ba, bsc(hons)
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MAGNETRONS The Evolution & Operation of
Chuck 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 wasn’t 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.
G0MDK3Two such Scientists
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EVOLUTION OF THE
MAGNETRON
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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 Okabe’s 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
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
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)
G0MDK7HULL’S 1921 MAGNETRON (US)
• Cavity magnetron Coaxial configuration
Frequency: 200kHz increasing to 10MHz
1925 Elder of GE (US) produced 8kW @ 30kHz 69% efficiency
Electron path
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OKABE’S 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
Various configurations named after Japanese flowers
C Kosumosu (Rising sun)
U Umebachi (Apricot flower)
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)
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 @ 10cm 20% efficiency
No record of Russian military using it in radars
G0MDK12MAGNETRON WAR TIME UK
4. University of Birmingham: J. T. Randall and H. A. H. Boot
5. Literature on Magnetrons world-wide but unobtainable
6. 1940 Feb. Developed 9.4cm (3.91gHz) 400W CW Magnetron
7. GEC produced two magnetrons using R & B as a model
8. 1940 June Pulse powers of 10 to 40kW at 10cm achieved
9. 1940 Aug. Tizard and team brought magnetron to the U. S.
10.Sept. Mag. at MIT Labs. Bell Labs & Raytheon Co. x-rayed Mag. & reproduced it.By Nov. it was in mass production
1. 1938 Admiralty awarded GEC a development contract.
2. 1940 April, GEC bread-boarded a 25cm operating radar
3. Transmitter produced 25kW pulses using Hi-Pwr. Triodes
G0MDK13MAGNETRON WAR TIME UK
Randall and Boot’s 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.
Experimental magnetron
University of Birmingham
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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 QSO’s with West Coast US and Australia
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
Born in Belfast
G0MDK16E-1189 MAGNETRON
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.
MegawE-1189 GEC no. 12
G0MDK17E-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 ~ 65%
• 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 CONSTRUCTIONCutaway 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 CONSTRUCTIONAnother cutaway view of the magnetron
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MAGNETRON CONSTRUCTIONMagnetron 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
Eight equivalent circuits shown in series Typical of German and Japanese magnetrons [Unstable]
One of 8 cavities
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MAGNETRON CONSTRUCTION
Alternate cavities strapped together with solid copper rings
Dr. Megaw’s 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?
Producing µ-waves can be subdivided into four phases:
1. Production and acceleration of an electron beam
2. Velocity-modulation of the electron beam
3. Forming of a “Space-Charge Wheel”
4. Dispense energy to the ac field
Various anode forms
Magnetic field provided by strong permanent magnet
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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
1. PFN charges up to 12kV (dc resonance phenomena)
2. Trigger switches thyratron on
3. PFN discharges through transformer and thyratron
4. During discharge PFN develops rectangular pulse
5. Transformer steps negative 6kV pulse up to 30kV
6. 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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