The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field while moving past a series of open metal cavities cavity resonators.
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Electrons pass by the openings to these cavities and cause radio waves to oscillate within, similar to the way a whistle produces a tone when excited by an air stream blown past its opening. The frequency of the microwaves produced, the resonant frequencyis determined by the cavities' physical dimensions. Unlike other vacuum tubes such as a klystron or a traveling-wave tube TWTthe magnetron cannot function as an amplifier in order to increase the intensity of an applied microwave signal; the magnetron serves solely as an oscillatorgenerating a microwave signal from direct current electricity supplied to the vacuum tube.
An early form of magnetron was invented by H. Hans Erich Hollmann filed a patent on a design similar to the modern tube in but the more stable klystron was preferred for most German radars during World War II. An important advance was the multi-cavity magnetron, first proposed in by A. Samuel of Bell Telephone Laboratories.
However, the first truly successful example was developed by Aleksereff and Malearoff in Russia inwhich achieved watts at 3 GHz. The compact cavity magnetron tube drastically reduced the size of radar sets  so that they could be more easily installed in night-fighter aircraft, anti-submarine aircraft  and escort ships. In the post-war era the magnetron became less widely used in the radar role.
This was because the magnetron's output changes from pulse to pulse, both in frequency and phase. This makes the signal unsuitable for pulse-to-pulse comparisons, which is widely used for detecting and removing " clutter " from the radar display. In this form, approximately one billion magnetrons are in use today.
In a conventional electron tube vacuum tubeelectrons are emitted from a negatively charged, heated component called the cathode and are attracted to a positively charged component called the anode.
The components are normally Gan Power Amplifier Thesis concentrically, placed within a tubular-shaped Gan Power Amplifier Thesis from which all air has been evacuated, so that the electrons can move freely hence Gan Power Amplifier Thesis name "vacuum" tubes, called "valves" by the British. If a third electrode is inserted between the cathode and the anode called a control gridthe flow of electrons between the cathode and anode can be regulated by varying the electric charge on this third electrode.
This allows the resulting electron tube called a " triode " because it now has three electrodes to function as an amplifier because small variations in the electric charge applied to the control grid will result in identical variations in the much larger current of electrons flowing between the cathode and anode. The idea of using a grid for control was patented by Lee de Forestresulting in considerable research into alternate tube designs that would avoid his patents.
One concept used a magnetic field instead of an electrical charge to control current flow, click to the development of the magnetron tube.
In this design, the Gan Power Amplifier Thesis was made with two electrodes, typically with the cathode in the form of a metal rod in the center, and the anode as a cylinder around it. The tube was placed between the poles of a horseshoe magnet  arranged such that the magnetic field was aligned parallel to the axis of the electrodes.
With no magnetic field present, the tube operates as a diode, with electrons flowing directly from the Gan Power Amplifier Thesis to the anode.
In the presence of the magnetic field, the electrons will experience a force at right angles to their direction of motion, according to the left-hand rule.
In this case, the electrons follow a curved path between the cathode and anode. The curvature of the path Gan Power Amplifier Thesis be controlled by varying either the magnetic field, using an electromagnetor by changing the electrical potential between the electrodes.
At very high magnetic field settings the electrons are forced back onto the cathode, preventing current flow. At the opposite extreme, with no field, the electrons are free to flow straight from the cathode to the anode. There is a point between the two extremes, the critical value or Hull cut-off magnetic field and cut-off voltagewhere the electrons just reach the anode.
At fields around this point, the device operates similar to a triode. However, magnetic control, due to hysteresis and other effects, results in a slower and less faithful response to control current than electrostatic control using a control grid in a conventional triode not to mention greater weight and complexityso magnetrons saw limited use in conventional electronic designs.
It was noticed that when the magnetron was operating at the critical value, it would emit energy in the radio frequency spectrum. Due to an effect now known as cyclotron radiationthese electrons radiate radio frequency energy. The effect is not very efficient. Eventually the electrons hit one of the electrodes, so the number in the circulating state at any given time is a small percentage of the overall current.
It was also noticed that the frequency of the radiation depends on the size of the tube, and even early examples were built that produced signals in the microwave region. Early conventional tube systems were limited to the high frequency bands, and although very high frequency systems became widely available in the late s, the ultra high frequency and microwave regions were well beyond the ability of conventional circuits.
The magnetron was one of the few devices able to generate signals in the microwave band and it was the only one that was able to produce high power at centimeter wavelengths. The original magnetron was very difficult to keep operating at the critical value, and even then the number of electrons in the circling state at any time was fairly low. This meant that it produced very low-power signals. Nevertheless, as one of the few devices known to create microwaves, interest in the device and potential improvements was widespread.
The first major improvement was the split-anode magnetronalso known as a negative-resistance magnetron. As the name implies, this design used an anode that was split in two — one at each end of the tube — creating two half-cylinders.
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When both were charged to the same voltage the system worked like the original model. But by slightly altering the voltage of the two plates, the electron's trajectory could be modified so that they would naturally travel towards the lower voltage side. The plates were connected to an oscillator that reversed the relative voltage of the two plates at a given frequency.
At any given instant, the electron will naturally be pushed towards the lower-voltage side of the tube. The electron will then oscillate back and forth as the voltage changes. Gan Power Amplifier Thesis the same time, a strong magnetic field is applied, stronger than the critical value in the original design.
This would normally cause the electron to circle back to the cathode, but due to the oscillating electrical field, the electron instead follows a looping path that continues toward the anodes. Since all of the electrons in the flow experienced this looping motion, the amount of RF energy being radiated was greatly improved.
And as the motion occurred at any field level beyond the critical value, it was no longer necessary to carefully tune the fields and voltages, and the overall stability of the device was see more improved.
Unfortunately, the higher field also meant that electrons often circled back to the cathode, depositing their energy on it and causing it to heat read article. Gan Power Amplifier Thesis this normally causes more electrons to be released, it could sometimes lead to a runaway effect, damaging the device.
The great advance in magnetron design was the resonant cavity magnetron or electron-resonance magnetronwhich works on entirely different principles. In this design the oscillation is created by the physical shaping of the anode, rather than external circuits or fields.
Mechanically, the cavity magnetron consists of a large, solid cylinder of metal with a hole drilled through the center of the circular face. A wire acting as the cathode is run down the center of this hole, and the metal block itself forms the anode. Around this hole, known as the "interaction space", are a number of similar holes "resonators" drilled parallel to the interaction space, separated only a very short distance away.
A small slot is cut between the interaction space and each of these resonators. The resulting block looks something like the cylinder on a revolverwith a somewhat larger central hole.
Early models were actually cut using Colt pistol jigs. The parallel sides of continue reading slots act as a capacitor while the anode block itself provides an inductor analog.
Thus, each cavity forms its own resonant circuit, the frequency of which is defined by the energy of the electrons and the physical dimensions of the cavity. The magnetic field is set to a value well below the critical, so the electrons follow arcing paths towards the anode. When they strike the anode, they cause it to become negatively charged in that region.
As this process is random, some areas will become more or less charged than the areas around them. The anode is Gan Power Amplifier Thesis of a highly conductive material, almost always copper, so these differences in voltage cause currents to appear to even them out. Since the current has to flow around the outside of the cavity, this process takes time. During that time additional electrons will avoid the hot spots and be deposited further along the anode, as the additional current flowing around it arrives too.
This causes an oscillating current to form as the current tries to equalize one spot, then another. The oscillating currents flowing around the cavities, and their Gan Power Amplifier Thesis on the electron flow within the tube, causes large amounts of microwave radiofrequency energy to be generated in the cavities.
The cavities are open on one end, so the entire mechanism forms a single, larger, microwave oscillator.
A "tap", normally a wire formed into a loop, extracts microwave energy from one of the cavities. In some systems the tap wire is replaced by an open hole, which allows the microwaves to flow into a waveguide. As the oscillation takes some time to set up, and is inherently random at the start, subsequent startups will have different output parameters.
Phase is almost never preserved, which makes the magnetron difficult to use in phased array systems. Frequency also drifts from pulse to pulse, a more difficult problem for a wider array of radar systems. Neither of these present a problem for continuous-wave Gan Power Amplifier Thesisnor for microwave ovens.
All cavity magnetrons consist of a heated cathode placed at a high continuous or pulsed negative potential created by a high-voltage, direct-current power supply.
The cathode is placed in the center of an evacuatedlobed, circular chamber. A magnetic field parallel to the filament is imposed by a permanent click. The magnetic field causes the electrons, attracted to the relatively positive outer part of the chamber, to spiral outward source a circular path, a link of the Lorentz force.
Spaced around the rim of the chamber are cylindrical cavities. Slots are cut along the length of the Gan Power Amplifier Thesis that open into the central, common cavity space. As electrons sweep past these slots, they induce a high-frequency radio field in each resonant cavity, which in turn causes the electrons to bunch into groups. This principle of cavity resonator is very similar to blowing a stream of air across the open top of a glass pop bottle.
A portion of the radio frequency energy is extracted by a short antenna that is connected to a waveguide a metal tube, usually of rectangular cross section.
The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar. The Gan Power Amplifier Thesis of the cavities determine the resonant frequency, and thereby the frequency of the emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load impedancewith changes in the supply current, and with the temperature of the tube. Where precise frequencies are read more, other devices, such as the klystron are used.
The magnetron is a self-oscillating device requiring no external elements other than a power supply.
A well-defined threshold anode voltage must be applied before oscillation will build up; this voltage is a function of the dimensions of the resonant cavity, and the applied magnetic field.
In pulsed applications there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode voltage must be coordinated with the build-up of oscillator output. Where there are an even number of cavities, two concentric rings can connect alternate cavity walls to prevent inefficient modes of oscillation. The modern magnetron is a fairly efficient device.
In a microwave oven, for instance, a 1.