Why is it that an antenna/waveguide is only connected to one cavity

[IniquiTrance]

Why is it that an antenna/waveguide is only connected to one cavity in the magnetron, such as in a microwave? Are the resonating RF waves in the other cavities wasted then?

Thanks!

Edit: Also, are magnetrons always thermionic diodes?

 

[Bob S]

Magnetrons (like the ones in microwave ovens) are coupled cavity resonant systems so although the power might flow out from one cavity, the power comes from all. The cavities are coupled by both the bunched electrons, by coupling of the cavities themselves, and by "strapping" (I think) like in the rising sun magnetron. The electron beam comes from thermionic emission by a dc biased cathode, but the power comes from the dc voltage and current input.

 

[oswaler]

I can provide a response to your questions about antennas and waveguides in a magnetron.

Firstly, let's understand what a magnetron is and how it works. A magnetron is a type of vacuum tube that is commonly used to generate microwaves for applications such as microwave ovens and radar systems. It consists of a cathode (negatively charged electrode), an anode (positively charged electrode) and a series of resonant cavities.

Now, to answer your first question, an antenna or waveguide is only connected to one cavity in a magnetron because of the way the device is designed. The resonant cavities in a magnetron are arranged in a circular pattern, with the cathode at the center and the anode at the periphery. The RF (radio frequency) waves generated by the cathode travel through the cavities in a circular motion, creating a rotating electric field. This rotating field then interacts with the magnetic field produced by permanent magnets in the device, resulting in the generation of microwaves.

The reason for connecting the antenna or waveguide to only one cavity is to ensure that the RF waves are properly directed and focused towards the desired output. If multiple cavities were connected to the antenna, the RF waves would interfere with each other, resulting in a distorted output and reduced efficiency.

To address your second question, the resonating RF waves in the other cavities are not wasted. They play a crucial role in maintaining the oscillations and amplifying the RF signal. The resonant cavities act as a feedback mechanism, continuously amplifying the RF waves as they pass through, resulting in a stronger output.

Finally, to your last question, magnetrons are not always thermionic diodes. While most magnetrons do use thermionic emission (where electrons are emitted from a heated cathode), there are also other types of magnetrons that use different methods of electron emission, such as field emission or cold cathode emission.

In conclusion, the design of a magnetron and the connection of the antenna or waveguide to one cavity is crucial for its efficient operation and production of microwaves. The resonating RF waves in the other cavities play a vital role in maintaining the oscillations and amplifying the signal, making them an essential part of the device's functionality.