Transcript RADAR - TalkTalk

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MAGNETRONS
The Evolution & Operation of
Chuck Hobson BA, BSc(hons)
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Introduction
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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.
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Two such Scientists
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EVOLUTION
OF THE
MAGNETRON
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MAGNETRON TIME LINE
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 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
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1920 Heinrich Barkhausen 0.3 - 6.4gHz at 5W
Barkhausen Oscillator
(not a magnetron)
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)
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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HULL’S 1921 MAGNETRON (US)
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• Cavity magnetron Coaxial configuration
Electron path
Frequency: 200kHz increasing to 10MHz
1925 Elder of GE (US) produced 8kW @ 30kHz 69% efficiency
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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
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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
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Shimada Laboratory, Technical Institute of Japanese Navy,
Shizuoka Prefecture in 1944
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MAGNETRON RUSSIA
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4 cavity magnetron Russia 1937
Aleksereff and Malearoff
300W CW @ 10cm 20% efficiency
No record of Russian military using
it in radars
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MAGNETRON WAR TIME UK
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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
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
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MAGNETRON WAR TIME UK
Experimental magnetron
University of Birmingham
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.
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J. T. Randall & H. T. Boot
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Dr. Eric Stanley Megaw
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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
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E-1189 MAGNETRON
E-1189 GEC no. 12
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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:
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Radar Transmitters (pulsed)
Peak power from ~10kW to 3MW +
Frequency from ~600MHz to 47gHz +
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Microwave Ovens (CW}
Frequency 2.45gHz
Output power 650 – 1200W Efficiency ~ 65%
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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
One of 8 cavities
Equivalent circuit of one cavity
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. 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?
Various anode forms
Magnetic field provided by strong permanent magnet
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
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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
8 cavity magnetron
Rotating 6 spoke space charge
4 spoke wheel
Space charge gives µ-wave
energy to the cavity keeping it
oscillating
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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
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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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