A question about microwave ovens

[hkBattousai]

https://en.wikipedia.org/wiki/Microwave_oven#Principles

Cooking chamber of a microwave oven is a Faraday cage, so no radiation can leak out. Then what happens to the radiated power if I run the microwave without food in it? Is the power wasted to the ground by the Faraday cage, or does something else happen?

Another question, what happens to a microwave beam, if it reaches to the cage/case before penetrates the food? Is it wasted/grounded or reflected back into the chamber? If it is reflected, does the occurence of reflection depend on the angle of the incident wave to the case?

 

[FvM]

I think most Magnetron (microwave oven) user manuals tell not to use it without load, e. g. a glass container filled with water.

Microwave is multiple reflected at the walls in normal operation, a smaller part is always reflected back to the source, causing additional lossses. Viewn from the generator, this effect can be described as a standing wave ratio (V.S.W.R.). With most of the MW power reflected, the Magnetron source gets overheated. However, home appliance devices are designed to stand a high V.S.W.R. operation for shorter dwell times, but not permanently. Hopefully, the thermal protection switch at the source responds before permanent device damage occurs.

Higher power industrial microwave generators usually have additional protection circuits to prevent from possibly dangerous modes of operation. At higher power levels umatched load can also cause electrical arcs, in the waveguide or inside the magnetron.

 

[hkBattousai]

Some of the reflected waves return to the source (magnetron), the rest keep reflecting until they too reach to the magnetron. In progress, magnetron absorbs almost all of the power, and because of the standing wave high power levels occur inside the chamber.

Did I understand it correctly?

 

[FvM]

Yes. High standing wave ratio means higher field strength at some locations of the waveguide and oven room. You can imagine the oven room as a cavity resonator. Without losses (a load), it has high Q and high voltages from little input power. But the levels are not an issue with a home Magnetron normally. Maximum V.S.W.R. can also be found as a parameter in magnetron tube datasheets to specify the allowed reflected power.

 

[kspalla]

I think they will have circulator to protect the system from the reflected power.

 

[rami.masadeh]

it must has a reflectors as a protection,

Added after 2 minutes:

it must has a reflectors as a protection,