Fred said:
If there is nothing in the microwave, there is no load, or at least very
little. If there is no load, there is no power dissipated. It is
somewhat like having 120 VAC at the wall socket with nothing plugged-in.
While this is true, relative to the normal operation of the oven, I
don't think it is quite correct to characterize the unloaded oven this
way.
Here's why I say this:
If there is no additional load (an empty oven) there is still a load,
but it's mainly reactive. This means that the energy the tube generates
reflects, rather than being absorbed. There is a high SWR (Standing Wave
Ratio). When the returning comes back to the tube, it sums vectorially
with the outgoing wave. This may result in anything from zero to twice
the magnitude RF voltage. Put another way, the phase of the reflection
may be anything from an open to a short, including everything
in-between. As a result, the tube may see *either* excess voltage or
excess current.
But, there is in fact always *some* load in the system. This is what
causes real energy (heat) to be dissipated within (mainly) the tube. For
the unloaded (no user supplied loss) case, these intrinsic losses, R's,
whether from dielectric or conductive elements, will tend to see higher
voltage or current than they normally would. The power lost in these
R's, is generally related to either I^2R or E^2/R. But for the case of
doubling(say) voltage or current these loses quadruple.
As a consequence *more* power may be dissipated *within the tube* when
there is no user-supplied load than was the case with straight-and-level
operation with a normal cooking chamber load.
This increased dissipation along with the potential for increased
current and increased voltage within the tube may cause things to be
tougher on the tube. Often power devices are specified to operate into
some maximum SWR and it may be that modern magnetrons can no take
whatever is likely to be thrown their way.
Exactly what happens is probably a pretty complicated function of the
whole assembly, dimensions and such. But it isn't unreasonable to think
that the tube might get hotter, in addition to being stressed in terms
of it's voltage and current limits.
g