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

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In magnetrons, antennas or waveguides are typically connected to only one cavity to efficiently extract microwave energy while minimizing losses. The resonating RF waves in other cavities are not wasted; they contribute to the overall power generation through coupling mechanisms, including electron bunching and cavity interaction. The design allows for effective energy transfer from all cavities, even if only one is directly connected to the output. Magnetrons are indeed thermionic diodes, relying on thermionic emission from a biased cathode to produce an electron beam. This setup ensures that the power generated is a result of both the DC voltage and current input.
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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?
 
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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.
 

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