12.1. Physical construction of a magnetron
The magnetron is a high-powered vacuum tube that works as a self-excited microwave oscillator. Crossed electron and magnetic fields are used in the magnetron to produce the high-power output required in radar equipment. These multicavity devices may be used in radar transmitters as either pulsed or CW oscillators at frequencies ranging from approximately 600 to 95,000 megahertz.[1] The relatively simple construction has the disadvantage that the magnetron usually can work only on a constructively fixed frequency.
Magnetron
12.1. Physical construction of a magnetron
The magnetron is classed as a diode because it has no grid. The anode of a magnetron is fabricated into a cylindrical solid copper block. The cathode and filament are at the center of the tube and are supported by the filament leads. The filament leads are large and rigid enough to keep the cathode and filament structure fixed in position. The cathode is indirectly heated and is constructed of a high-emission material. The 8 up to 20 cylindrical holes around its circumference are resonant cavities. A narrow slot runs from each cavity into the central portion of the tube dividing the inner structure into as many segments as there are cavities.
Each cavity works as a parallel resonant circuit. As depicted in figure 3 by the low-frequency analog, the rear wall of the structure of the anode block may be considered to as the inductive portion (a coil with a single turn). The vane tip region may be considered as the capacitor portion of the equivalent parallel resonant circuit. The resonant frequency of a microwave cavity is thereby determined by the physical dimension of the resonator. If a single resonant cavity oscillates, then it excites the next one to oscillate too. This one oscillates at a phase delay of 180 degrees and excites the next resonant cavity, and so on.
From one resonant cavity to the next there is always a delay of 180 degrees. The chain of resonators thus forms a slow-wave structure that is self-contained. Because of this slow-wave structure, this design is also-called “multicavity traveling wave magnetron” in some publications.
Cutaway view of a magnetron
The cathode of a magnetron provides the electrons through which the mechanism of energy transfer is accomplished. The cathode is located in the center of the anode and is made up of a hollow cylinder of emissive material (mostly barium oxide) surrounding a heater. The feeding wires of the filament must center the whole cathode. Any eccentricity between anode and cathode can cause serious internal arcing or malfunction.
The open space between the anode block and the cathode is called the interaction space. In this space, the electric and magnetic fields interact to exert force upon the electrons. The magnetic field is usually provided by a strong, permanent magnet mounted around the magnetron so that the magnetic field is parallel with the axis of the cathode.
It generally consists of an even number of microwave cavities arranged in a radial fashion. The form of the cavities varies, shown in figure 4.
- slot type
- vane type
- rising sun type
- hole-and-slot type
A resonant cavity in the anode block has the function of a parallel resonant circuit: the opposite anode walls of a slot are the capacitor, the detour around the hole is the inductance (with only one turn).
The slot type, hole-and-slot type, and the rising sun type are usually machined by hobbing methods out of solid copper stock. But it can be difficult to cut soft metal (such as copper) in a lathe. The vane type is generally made up of individual vanes assembled and brazed into a support ring, therefore. The resonance behavior can already be tested and calibrated in the laboratory before the anode block is installed in the vacuum tube. The output lead is usually a probe or a loop extending into one of the resonant cavities and coupled into a waveguide or coaxial line.