>How conductive does plastic really get? Most
>conductive plastics I have seen have a few k ohms of resistance from a
>typical point to another point, say, a centimeter away, only good for
>bleading away static charges. I have a hard time believing that this
>would affect RF fields much at all.

>I don't think regular polycarbonate is conductive. It contains carbon, but
>conductivity is totally dependent on the configuration of the electrons in
>the molecules, which depends on how the atoms are bonded, etc., not just
>what element you are talking about.

>Sean

Conductivity is not the key here, but a certain bulk resistivity at AC. If
that is equivalent to the characteristic impedance of the plastic (i.e.
the complex components of Z at F0 in that material) then the material is
the best possible absorbent. Much better than metal which reflects
everything back in to bounce around and exit through the fan gills and
power cords. This is why absorbent sponges are often used inside RF proof
cases. They attenuate the energy bouncing around inside and dissipate it.
I suppose that you could make the case out of 'sponge' and live to see a
FCC test.

Since the characterisitc impedance of the plastic is fairly high (and
depends on frequency) the required resistivity for perfect match is also
relatively high. F.ex. certain PVCs exhibit very high DC resistance and
absorb RF above ~30MHz like a sponge (and get hot and melt in higher
fields).

RF (300kHz-10MHz) melting of large plastic (or what passed for plastic
then) parts for machining/forming purposes has been known since the 1920s.
Look up dielectric heating.

In practice for cases and shields the resistivity is lower than 'best' and
some energy is reflected back in in an effort to minimize transmission.

Think adapted transmission line (or blackbody), not wall/mirror.

Peter

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