All cavity magnetrons consist of a hot filament (cathode) kept at, or pulsed to, a high negative potential by a high-voltage, direct-current power supply. The cathode is built into the center of an evacuated, lobed, circular chamber. A magnetic field parallel to the filament is imposed by a permanent magnet. The magnetic field causes the electrons, attracted to the (relatively) positive outer part of the chamber, to spiral outward in a circular path rather than moving directly to this anode. Spaced around the rim of the chamber are cylindrical cavities. The cavities are open along their length and connect the common cavity space. As electrons sweep past these openings, they induce a resonant, high-frequency radio field in the cavity, which in turn causes the electrons to bunch into groups. A portion of this field is extracted with 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 sizes of the cavities determine the resonant frequency, and thereby the frequency of emitted microwaves. However, the frequency is not precisely controllable. This is not a problem in uses such as heating, or in some forms of radar where the receiver can be synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other devices such as the klystron are used.
The magnetron is a fairly efficient device. In a microwave oven, for instance, an 1100 watt input will generally create about 700 watts of microwave energy, an efficiency of around 65%. (The high-voltage and the properties of the cathode determine the power of a magnetron.) Instead of a magnetron, transistors can be used to provide microwave power; transistors typically operate at around 25 to 30% efficiency. Transistors are used in roles which require a wide range and/or stable range of frequencies. Thus the magnetron, having a higher efficiency, remains in widespread use in roles which require high power, but where precise frequency control is unimportant.