On 28 Dec 2006 13:29:34 -0800, wrote:

As a newcomer to this newsgroup, please allow me to clarify.

I do not mean to dispute your information about the stirrer blades, I
agree with you.

I am trying to get more information about what exactly is involved in
the dispersion. More than what I pick up off of the various "how does a
microwave oven work" hits that google returns.

Thank you,

Mike K.

g They are in the path between the magetron and the oven (cavity).
The microwaves reflect off the blades at varying angles (as the blades
turn).

It (the cooking chamber) is not a resonant cavity. To be that it would
have to be rather small (a wavelength of a microwave oven is in the
range of 4 to 5 inches, IIRC). There are 'hot' spots (and cool spots)
in that chamber, caused by reflections, and the interface between the
source and the oven chamber.

I hope you don't mind the discussion. And I hope you don't mind that I
address your points out of sequence.

I think you are close enough in wavelength for our discussion. But you
can set up a standing wave of half a wavelength, which would be
approximately the size of the standard cooking chamber of the average
microwave oven.

I note that there is not a large variation in microwave oven sizes.
Perhaps cooking a large turkey would be impractical due to overcooking
at the surface, but I can envision a scenario where you might want to
heat up 20 bowls of soup at a time.

I don't see this happening, that is why I think that the cooking
chamber size is dependent on the wavelength being used. And since the
wavelength of the microwave is limited by the economic considerations
of the magnetron, this restricts the range of the cooking chamber.

This is why if you have a turntable microwave without a dispersal fan
and take out the turntable (or it breaks... g) you can end up with
your dinner over cooked in some places and frozen in other spots.

I used a microwave resonance system and we turned the cavity by making
adjustments at the boundary layer between the wave guide and cavity. It
seemed to me that the dispersal fan might be method of tuning the
cavity through the various modes of the cavity, averaging out the maxs
and mins of the standing wave.

-Mike K.

Well, a few microwave oven factoids: bg

Frequency: 2450MHz (approx)
Wavelength: 4.81 inches (.122 meters)

The size of the interior is a factor of the energy (power) level for a
given (typical) location. A bigger interior would require a larger
power source (magetron) which is more expensive and presents some
technical problems.

There used to be a couple of good sites on the net where you could
see exactly this stuff. Today all we find are glossy consumer reviews,
and really simple stuff that is *USELESS* at best. g

But try to envision that there are standing waves in the cooking
interior cavity, the microwaves enter from a 'port' or window on the
(usually upper right) side, and that port's size is only a few square
inches. The waves exit from this port, and travel outwards much like a
light from a flashlight might: some spread, but not a great deal of
difusion. The result is that waves travel to the far wall and then are
reflected off that wall towards the wall with the port. A few even
manage to get back into the waveguide, but that's not important. What
is imporatnt is that those reflected waves are all out of snyc, as the
paths traveled are all different, and the differences are more than a
wavelength (see, that's why we need to look at the wavelength).

The final result is that at some places these waves reinforce each
other (they add because they are in phase) and at other places they
tend to cancel (that is they are 180 degrees out of phase). By
sticking the fan in the path of the microwaves, you change the paths,
and the locations where they cancel or add change as well.

Just for the record, the points of maximum addition or cancelation
would be less than a half wavelength, or a couple of inches. So the
changes needed to move those points is slight.

Not all microwave ovens are well designed... g No turntable, and the
fan dispersal technique is not well implemented. With those ovens
you'll find that it is necessary to rotate your meal, or move it some
(again a few inches can be enough) to get it to heat reasonably
evenly.