Re: Dave, one more question (please!!!)

"george" <nowhere@xxxxxxxxxxxx> writes:

Oh good...someone who understands microwaves <g>. I have a question for
you. Why is it that some microwaves (mine, for instance) have a metal rack
in them and they are just fine with that...but all the microwaves I've seen
(mine included) arc if some foreign metal object gets in (like a tiny bit of
foil seal at the top of a jar)???

Warning: I'm not an RF engineer, just a radio hobbyist. But this is my
understanding of how metals interact with radio waves, particularly
microwaves:

When radio waves approach a conductor like metal, there are currents set
up in the conductor at the frequency of the incoming RF energy. If the
metal was a perfect conductor suspended from a perfect insulator, the
pattern of these currents and their amplitude would result in perfectly
mirror-reflecting the incoming signal, with no loss in signal power and
no heating of the metal. The stronger the RF energy, the larger the
currents induced in the metal and the higher the voltage present at the
edges of the piece of metal, but as long as we assume perfect conductor
and perfect insulation around it, the metal reflects the microwaves more
or less like a mirror reflects light (including diffraction effects
around the edges).

But things are never this simple. Real metals have resistance, and the
induced current loses some of its energy in heating the metal. With
thick metal, resistance is still low, losses are small, and the metal
can easily absorb the waste heat. On the other hand, really thin metal
(like gold foil trim around the rim of china plates, or the aluminum
layer in a pressed CD disc) has much higher resistance, gets very hot
very fast, and melts or evaporates.

Also, sharp points produce high voltage gradients in the air near them,
sometimes enough to break down the air and cause an arc. Two pieces of
metal near each other can exaggerate this. If the metal is near a
half-wavelength of the radio waves in some dimension, that increases the
voltage because of standing waves in the metal.

Now, your metal rack is probably made of heavy metal, with ribs that
run in one direction only. All the joints are welded. The corners are
probably rounded. It's supported on plastic insulators that keep the
rack a safe distance from the metal walls. And the ribs are probably
spaced far enough apart to let microwaves pass through, rather than
reflecting them. So it's designed not to create problems.

On the other hand, the metal foreign object probably has sharp corners,
and much less capacity for carrying current, so it will melt itself or
cause an arc in the air near it.

Dave
.

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