A magnetron is a device which transforms the electrical energy into electromagnetic energy, in the form of Micro-onde.

History

It was developed at the end of the years 1930, in particular by the team of Maurice Ponte with CSF to provide to the Radar incipient a powerful radioelectric source (several hundred Watts) and centimetric wavelength, therefore frequencies raised for the time of 300 MHz with 3 GHz (decimetre waves) and beyond 3 GHz (centimetric waves).

The oscillators with tube S used before were unable to provide such powers (from where an insufficient range of the radars), at these high frequencies (from where a weak angular discrimination).

Moreover, the use of the centimetric waves allowed the reduction of the size of the antennas and thus the construction of radars embarked in the boats initially then the planes. The Normandy will be thus the first steamer equipped with a radar mainly to detect the icebergs.

The technological advancements brought to the magnetron (cavities) provided to combined technological advance which played a paramount role in the exit of the Second world war.

Principle of operation

The magnetron is a vacuum tube without grid, with a central cathode, heated by a filament, and a mass and concentric anode in which several cavity resonators are dug. An axial magnetic field, generally created by two permanent magnets at each end of the tube. The course in spiral (because of the magnetic field) of the electrons is done at a frequency granted to the cavities résonnantes.
The magnetron being car-oscillating, it allows simple assemblies, as in the microwawe ovens. The powers available are about a few kw uninterrupted (MW peak) with 3 GHz and hundreds of Watts (of the hundreds of kw peak) to 10 GHz. Magnetrons are available up to 35 GHz (Ka band). To obtain these powers a tension of several thousands of volts is necessary.

On the other hand, the characteristics of the produced wave (phase in particular) are not easily controllable what limited its employment a long time. The introduction of locking by injection allowed a large projection in this field. It thus became possible to extend the use of the magnetron, definitely cheaper than the other devices ultra high frequencies.

Use

Nowadays, the magnetron has two principal uses:

• the Radar where it is competed with by the Klystron, the Carcinotron, the SIGNAL (Travelling wave tube) and from now on semiconductors.
• the Microwawe oven. According to the little story, it is a technician radar having noted that its “mess tin” posed beside the transmitting antenna was hot, which is at the origin of the invention of the microwawe oven. One also quotes the pigeons which fell cooked after being last near the antennas of the first English radars.

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