Re: Faraday cages aren't perfect

In article <d7Odncz5UMPmuDLenZ2dnUVZ_sadnZ2d@xxxxxxxxxxxxxxx>, Offbreed
<offbreed_106@xxxxxxxxxxx> wrote:

> r2000swler@xxxxxxxxxxx wrote:
>
> > For whatever it is worth, I now feel that a single stage faraday cage
> > is likely to not
> > offer enough protection from a NEMP.
>
> Yeah, I expect the biggest problem is getting information and power in
> and out of the Faraday cage.
>
> But, your comments about protection of stuff stored in Faraday cages is
> certainly something to keep in mind.
>
> The answer should be intuitive. Consider: How does the Faraday cage
> protect the contents?
>
> The radio wave hits the conductor. Some is reflected, some is absorbed,
> a tiny bit goes through, if the metal is thin enough. The radio energy
> that is absorbed induces current in the metal. Two things happen with
> electrical current induced, some is converted to heat, and some is
> converted back into radio waves. Some goes out, some goes in.

Nope.

The main mechanism is exclusion of electric fields inside a conductor.
A consequence of Gauss's Law, relating the divergence to the charge. Or
one can calculate the distribution of charge on the outside and compute
the E-field inside. Same result, different viewpoints.

A very thin or "not good enough conductor" will not be a perfect
conductor, so some slight E-field gets through.

But all this stuff about how Faraday cages are inadequate is mostly
B.S.


--Tim May



>
> So, a Faraday cage does not really block radio as much as attenuate it.
> An inner Faraday cage would be needed to provide additional attenuation.
.

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