MadSci Network: Physics
Query:

Re: How does a magnetron tube work?

Date: Mon Feb 16 15:19:06 1998
Posted By: Adrian Popa, Staff Optical/Microwave Physics
Area of science: Physics
ID: 885538910.Ph
Message:

Greetings:

Before I address your question letís review a few facts that relate to the 
physics of vacuum tubes.

1. When specially coated cathode electrodes are heated in a vacuum they 
release large numbers of negatively charged electrons. This is called 
thermionic emission.

2. The anode electrode, some times called the plate, is usually biased with 
a highly positive voltage relative to the cathode. The positive anode 
attracts electrons which in turn travel in a straight line from the cathode 
until they slam into the anode, producing waste heat in the anode. The 
higher the anode to cathode voltage the faster the electrons move between 
the electrodes. The velocity of the electrons in an ordinary vacuum tube 
are a small fraction of the speed of light. In high power radio tubes the 
anodes often glow red to white hot from the electron bombardment.

3. A two electrode vacuum tube (anode and cathode) is called a diode. 
Magnetrons are a special form of diode.

4. The direction that the electrons travel between the cathode and anode 
can be modified into a curve by having them move through a crosswise 
(normal to the electron beam) electric field or through a cross wise 
magnetic field. Both these techniques have been used in cathode ray tubes 
to observe and measure electrical voltages (e.g. oscilloscopes) or to 
produce a raster scan (e.g. TV picture tube). 

5. When an electron is slowed down by an electric or magnetic field it 
gives up energy making the field stronger. If an electron is increased in 
speed by an electric or magnetic field it weakens the field.

6. Simple radio amplifier tubes, called triodes, have a third control 
electrode, usually called a grid, placed between the anode and cathode. A 
small voltage placed between the grid and the cathode can control a large 
current between the cathode and anode producing electrical amplification of 
the grid signal in the anode - cathode circuit.

7. An amplifier tube circuit that generates radio frequency signals is 
called an oscillator and a resonant circuit consisting of an inductor 
(coil) in parallel with a capacitor determines the frequency and wavelength 
of the oscillator. The less the number of turns in the coil and the smaller 
the capacitor plates are, the higher will be the radio frequency the 
oscillator generates.

In the ultra high frequency (UHF) range between 300 Mega Hertz (MHz) and 
3000 MHz the resonant circuit coil becomes less than one turn and the 
capacitor is formed by the spacing between the two closely spaced wires 
(called a hair pin resonator). This lack of control causes conventional 
resonant circuit techniques to fail in the higher microwave frequency range 
between 1000 MHz to 30,000 MHz. By paralleling hairpins the inductance of a 
single hairpin can be reduced and by making a large number of hairpins in 
parallel in a circle you can form an enclosed metal cylinder, a resonant 
cavity!

The controlled resonant circuit problem at microwave frequencies was solved 
in the 1930s by using hollow metal cylinders as a resonator. The round 
cylinder walls are similar to a one turn coil and the cylinder end plates 
are like a very small capacitor. These cylindrical resonators are called 
microwave cavities and by moving one of the cavity end plates in or out of 
the cylinder the frequency of the oscillator can be tuned. The cylinder 
must be about a wavelength in diameter and about one half wave length long 
at the resonant frequency.
 
Energy can flow in out of the resonator through a hole in the cylinder wall 
or by inserting a hair pin loop in the cavity or both.  

The magnetron oscillator was the first device developed that was capable of 
generating large powers at microwave frequencies (centimeter wavelengths). 
It was the key to the development of radar transmitters during World War 
II. Since that time improved devices such as traveling wave tube amplifiers 
(TWTA) have been developed for use microwave systems, yet the magnetron 
continues in production for use in  microwave ovens. 

Early radars developed during the 1930s operated at wavelengths much longer 
than 30 centimeters (12 inches) and used transmitter tubes that were 
developed for high frequency radio broadcasting. These radars, such as the 
Chain Home system developed to protect the British Isles from German air 
raids, required very large antennas which produced very wide, inaccurate, 
radar beams. To develop an airborne radar an antenna less than 1 meter (39 
inches)in diameter was required to fit inside an airplane. To form a beam 
less than 3 degrees wide with such an antenna required that the operating 
wave length be less than 10 centimeters (4 inches) far beyond the pre WWII 
technology. For security reasons the 10 centimeter wavelength band project 
was given the code name S-band (called H2S in the United Kingdom) and later 
the 3 centimeter wavelength band project was called X-band. These names 
continue in use today.

Pictures of magnetrons and the history of their Top Secret development in 
the UK in the 1930s and 40s and subsequent transfer to the US for 
manufacturing are presented at the following Web pages:

http://chide.museum.org.uk/reme/radar.magnetron.html

To generate shorter wavelengths using conventional radio tubes required 
that the electrodes within the tube (e.g. cathode, grids, anode) be closely 
spaced so that the transit time of the electrons between  the electrodes 
were fast enough to still be in phase at the anode to generate centimeter 
wavelengths in the tuned anode (plate) resonator circuit. This close  
spacing at very high frequencies reduced the voltage that could be 
developed without arcing between the electrodes. Thus tubes operating at 
frequencies above 1000 Megahertz (30 centimeter wavelength) could generate 
less than one watt of power in the 1930s when 10 to 100 kilowatts were 
needed for radar transmitters. 

The magnetron has a central clyndrical cathode surrounded by an anode in 
the form of a thick clyndrical shell. Top and bottom plates form the 
remainder of the vacuum envelope. The top and bottom plates are placed 
between the poles of a strong magnet. A number of cylindrical cavities are 
machined in the anode forming parallel resonators. The microwave fields 
from the resonators extend into the region between the cathode and anode. A 
strong magnetic field makes the high speed electrons curve so that they 
canít reach the anode and they return to the cathode unless they are slowed 
down by giving up energy to a cavity electric field. If the electrons 
arrive at the wrong time they take energy from the microwave field and 
speed up and spiral back to the cathode. At the correct voltage and 
microwave frequency the electrons move at the correct velocity to continue 
to loose speed and give up most of their energy to the microwave field 
before the impact on the anode. In turn the parallel microwave cavities 
share energy and generate a thousand watts in our 2400 MHz microwave ovens 
for cooking and a million watts in our 3000 MHz air traffic control radars! 

Pictures and text discussing the operation and testing of magnetrons for 
microwave ovens are on the following Web pages:
http://www.gallawa.com/microtech/mag_test.html

To reduce cost, the oven magnetrons use rectangular resonant cavaties in 
place of the more expensive to manufacture cylinders. Waveguides are also 
discussed on these pages.


Best regards, your Mad Scientist
Adrian Popa  


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