Microwave Tubes

It was discovered in 1939 that a toroidal cavity made of conductive material called a cavity resonator surrounding an electron beam of oscillating intensity could extract power from the beam without intercepting the beam itself. The oscillating electric and magnetic fields associated with the beam ‘’echoed’’ inside the cavity, in a manner similar to the sounds of traveling automobiles echoing in a roadside canyon, allowing radio-frequency energy to be transferred from the beam to a waveguide or coaxial cable connected to the resonator with a coupling loop. The tube was called an Inductive output tube (IOT).

The Magnetron Tube

Magnetron tubes forms the heart of every microwave oven, generating hundreds of watts of microwave RF energy used to heat food and beverages.

Magnetron tubes are different from the inductive output tubes; whereas the Inductive output tubes use a linear electron beam, the magnetron directs its electron beam in a circular pattern by means of a strong magnetic field as shown in the diagram below:

The cavity resonators are used as microwave frequency ‘’tank circuits’’, extracting energy from the passing electron beam inductively.

In all microwave-frequency devices using a cavity resonator, at least one of the resonator cavities is tapped with a coupling loop: a loop of wire magnetically coupling the coaxial cable to the resonant structure of the cavity, allowing RF power to be directed out of the tube to a load. In the case of the microwave oven, the output power is directed through a waveguide to the food or drink to be heated, the water molecules within acting as tiny load resistors, dissipating the electrical energy in the form of heat. In magnetron operation, the magnetic flux, runs perpendicular to the plane of the circular electron path.

Related: Radio Frequency (RF) and Microwave Spectral Analysis