What is the Difference Between a Microwave Cavity and Chamber?

[hydro2008]

hi ,

I am confused with these two terms microwave cavity and
microwave chamber . Some say that the microwave chamber
inside which food gets cooked acts like a resonant cavity
( which I believe is not true ) because resonance must best be
avoided inside the chamber .Resonance I think takes place only in
a cavity . thanks

hydro2008

 

[seycyrus]

What makes you think that the chamber is not a cavity?

 

[waht]

There is an "anechoic chamber" used a lot in testing microwave devices.

 

[hydro2008]

well , this is my simple understanding . A chamber may be an untuned cavity where reverberation or resonance does not take place. It is strictly to be avoided in order to maintain the same cooking power levels. A resonant cavity where resonance can hasten heating is used for materials R&D to estimate the dielectric loss factor of specific materials . see http://www.msi-sensing.com/microwave_cavity.htm
I might be totally wrong here . I am wondering if the chamber could be made much smaller without encountering resonance with thick steel and without any transparent window for a specific application .

 

[seycyrus]

First off, I want to differentiate between the resonance that creates the microwaves in the klystrom and the resonance that occurs in the microwave chamber.

I believe that the microwave chamber does indeed meet the requirements of a resonant cavity.

The reason that you do have dead spots inside the chamber is because most modern models have something called a "stirrer" that slightly reflect the microwaves as they leave the waveguide, slightly altering the resonant conditions inside the cavity. If you didn't do that, or didn't rotate the food, cold spots would result.

Ever notice that microwaves do not come in "all sizes"? Rather they are dictated by the wavelength used, which is in turn dictated by the economics of wavelength used in the industry. S-waves can be slightly adjusted, perhaps by a dielectric or somesuch, and thus there is a tiny variation in microwave sizes, but they do not run the gambit.

 

[hydro2008]

microwaves come in various volume sizes .I have a large convention
microwave sitting in the kitchen much larger than an ordinary microwave. Recently I read a patent about
a cylindrical space saving design for a microwave . The larger and smaller ovens use the same
2.5 Ghz frequency . The stirrer blade assembly however is present only in a few models and not all
ovens have them .

 

[seycyrus]

microwaves come in various volume sizes .I have a large convention
microwave sitting in the kitchen much larger than an ordinary microwave. Recently I read a patent about
a cylindrical space saving design for a microwave . The larger and smaller ovens use the same
2.5 Ghz frequency.

The size variation is nothing like the size variation that you see in more conventional ovens, where you can go from a chamber an inch across to huge ovens where they cure petrochemical pipes, gun barrels and such. The variation is pretty continuous.

For microwave ovens, the fundamental nature of the microwave is important to the chamber design.

The stirrer blade assembly however is present only in a few models and not all
ovens have them .

Some have rotating plates which serves the same purpose. All must deal with the problem of "cold" spots in some manner. Maybe there are other methods as well.

I've brought this up to reinforce the notion that the microwave chamber is a resonant cavity, not in the sense that a klystron utilizes resonance, but in the fact that there are reflections at the chamber walls, producing three dimensional standing waves and such.