On 10/27/06, Gerhard Fiedler <lists@connectionbrazil.com> wrote:
> Vasile Surducan wrote:
>
> > One simple example: turn on an empty microwave oven.
> > Because of reflections inside the heating chamber the thermal
> > protection from the magnetron will be OFF in less than 3-4 minutes
> > (100% continous power).
>
> One thing I've always wondered. Pretty much every microwave instruction
> says not to put metal into the oven -- yet the whole oven is made of metal.
> I know that of course metal reflects rather than absorbs, but it shouldn't
> do any harm (at least not as long as there is enough other material in the
> oven). Or am I missing something?

Absorbed waves into  high dielectric loses material = heating, only
small refelections goes back to the magnetron (which is designed to
work with large VSWR)
Stationary waves into an empty cavity = huge reflections heating the magnetron.
A piece of metal (with considerable dimension compared with wavelenght )
will modify the microwave cavity (aka oven). If you're lucky (and the
oven has a good microwave spreading system) the piece will be from
time to time in the maximum electric filed, producing sparks. The
plasma around sparks acts as a variable and uncontrolled load. The
VSWR will be modified dramatically with every discharge and finally
the thermal protection will stop the magnetron's supply.
That's all which usually is happening.

Vasile
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