At 07:13 AM 11/6/2005, Tom Rauch wrote:
> > If the perimeter of the hole is < 1/2 wavelength (or so),
>then it will
> > shield fairly well. The real challenge for building a
>shielded room would
> > not be propagation through holes, but signals carried on
>wires penetrating
> > the shielding wall.
>
>I don't think "shield fairly well" paints an accurate
>picture of what it could do.
>"Might reduce magnetic flux in a small area around the loop
>axis " would be a better description.
I was assuming that the loops went around the entire room. Imagine a giant
bird cage, albeit with widely spaced bars.
And, of course, the shielding effectiveness will potentially be greatly
compromised by any of a number of construction details (like how the bars
are attached to each other).
>I can't imagine anyone doing that for lightning protection.
Perhaps not for lightning protection, but to establish an equipotential
surface around a room, I can see.
This kind of thing is moderately common in HV impulse testing work and
Tesla coil demos, where you create a Faraday cage to put test equipment or
an observer inside. The pulses are "lightning like" in that they have <1
microsecond rise times with RF spectra well up into the HF region. Even
fairly high power discharges directly to the cage bars have almost no
effect. I say almost because although I've never actually seen any damage
or hiccups, even on fairly low level signals (video), I imagine that one
could set up an experiment to detect the fields.
Naturally, nobody is stupid enough to actually stick anything out between
the bars!
Which brings up the whole problem of how useful this might be for shielding
a shack. When working with these sorts of things, I've always used battery
powered equipment, so there was no need to bring any wire into the cage
from outside.
>You'd be better off burying the copper and using it for a
>ground if lightning protection is the goal.
>
>73 Tom
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