|MadSci Network: Physics|
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:
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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