Re: Grounding

[Michael Motyka <mmotyka@lsil.com>]

Jim,

I really have to disagree about shielding and leakage.

***
Electricity runs from a high-potential point to ground. If it isn't
grounded
it radiates away into space where it can be detected. If the Faraday
Cage
isn't grounded it simply acts as a parasitic oscillator and re-emits the
signal from inside, though at a lower amplitude. Remember, charge rests
on
the *OUTSIDE* of a object so that any charge picked up internaly gets
passed
to the outside surface.
***

Consider a battery-powered spark gap inside a copper box. Lots of
beautiful wideband noise. I refuse to ground my copper box. I'm going to
suspend it from a weather balloon over Menwith Hill. 

The solutions to the wave equation inside the cavity have a real part ~0
in the exponent. The boundary condition at the inside surface of the
copper box splices together the solutions in the cavity and inside the
conductor. Which solutions, BTW, have a real component that is non-zero
so the waves are of exponentially decreasing amplitude the further into
the Cu you go ( skin depth, power loss ). 

At the inside boundary, the higher the conductivity the smaller the E
field at the surface, the shallower the skin depth and the more power is
reflected with less loss.

The amplitude of the internal Cu ( exponentially decreasing ) solutions
at the outside surface determines the extent to which the wave leaks out
because the solution is imaginary again outside. 

If it helps, the solution looks like the Schroedinger equation solution
for a particle in a potential well when at least one boundary is a
region of finite potential.

Leakage has nothing to do with being grounded since the wave can be
generated without the total charge residing on the surface of the box (
DC potential ) changing at all. What's there just moves around a little,
that's all.

I don't think it takes a whole lot of copper to do the job: the skin
depth is pretty small. A copper screen behaves much like the copper
sheet execept that it deviates as the wavelengths become closer to the
dimensions of the holes. You might say that the resistivity increases
with frequency. It leaks more.

**
The 1st rule of electrons:
 They always take the shortest path to ground.
Corollary:
 If they can't get to ground they radiate their excess energy away as
 photons.
***
Oh, for crying out loud! Where did this stuff come from?

I prefer the three laws of thermodynamics in layman's terms:
i)   You can't get something for nothing
ii)  The best you can do is break even
iii) You can't even do that

Works for my checking account too.

Regards,
Mike

BTW - I'm still at a loss to understand what the geology has to do with
RF shielding. Chaff, I think.

BTW^2 - The more conductive the TARMAC the more clutter you'll get
because it will be a better reflector. Ground or not. The incident waves
*don't care*.