The magnetron is an oscillator which is characterized by its small size, light weight, good efficiency, reasonable operating voltages, and long life. An oscillator is in turn an electronic circuit employed to produce high frequency pulses. The magnetron was the first practical high-power pulsed radar source used at microwave frequencies and it is still widely utilized today. Its main disadvantage is that it, being an oscillator, is not suitable for use in many coherent systems. It is also not well suited for generating short, high-power pulses.
The magnetron converts energy extracted from a constant electric field to a radio frequency (RF) field. The main parts of the magnetron are the cathode, the interaction region, cavities, and the output coupling. A magnetic field is applied perpendicular to the plane of the paper and a constant potential is established between anode block and the cathode. Therefore, the electric and the magnetic fields are crossed in the interaction region. If the proper electric and magnetic fields are applied, electrons travel from the cathode, coupling energy to the RF field, which is in turn coupled to the output.
Magnetron operation is strongly influenced by the load characteristics and by the applied magnetic and electric fields. The change in frequency for a change in anode current with a fixed load is called the 'pushing figure' (mHz/A). Another important parameter of magnetrons is the 'pulling figure', which is defined as the difference between the maximum and minimum frequencies of the magnetron oscillator when a phase angle of a load having a voltage/standing-wave ratio (VSWR) of 1.5 is varied through the 360-degree of phase.
Below, a schematic image of a coaxial magnetron.
Voltage-current relationships for a microwave magnetron.
