Re: Faraday Cage

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>



Tesla list wrote:
> 
> Original poster: "Kelly & Phillipa Williams by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <kellyw-at-ihug.co.nz>
> 
> Hi All!
> 
> I would like to take some pictures of my coil in operation, and other stuff
> such as Ocsope waveforms, etc with a digital or electronic camera. However,
> cameras with any sort of electronics in them freeze up and refuse to
> function when with 20-50m of the coil in operation, and it causes our
> cordless phone to beep haphazardly, and generally wreaks havoc with
> electronic devices.  I am fairly sure the coil is in good tune, and it is
> throwing about three 75" arcs to grounded wires at once. (crappy foil toroid
> with sparp points all over it.)  It uses a single static spark gap.
> 
> Is this correct -
> A metal Faraday cage blocks electromagnetic
> radiation (radio waves) from penetrating it into the enclosure. The
> electromagnetic radiation can get through a very thin slit in the side
> of the cage, so it is the longest diagonal or horizontal or vertical
> distance across a hole in the cage that determines whether or not radiation
> can get in.  The frequency of the radiation determines the shortest slit
> the electromagnetc wave can get through.

For propagation of an electric wave in the far field...It is the perimeter
of the aperture that is important, rather than the diameter or distance
across. TC's work at fairly low frequencies, where the wavelengths are HUGE
(hundreds of meters), so any reasonable sized aperture isn't going to cause
any troubles.

BUT...

Close up to the source (and particularly with sparks forming fast high
current transients) magnetic coupling is more of an issue, and the magnetic
field goes right through a conductive (but non magnetic) cage.  The key
here is mimimizing the area of victim loops (i.e. watch the stray grounding
paths that form loops.  An instrument (grounded through the power cord)
sitting on a grounded table or shelf(perhaps capacitively) easily forms a
loop of a square meter.


And also..

Wires that pass through the wall (i.e. say with woven mesh with wires that
go over and under) can carry significant power through the wall.

> 
> If this is all right (which I am not at all sure about), what is the
> mathematical relationship between
> frequency and slit length?
> 
> And can I make a faraday cage out of tinfoil or fine mesh such as a sieve,
> but cut a hole large enough for the 5mm by 5mm arpeture of my digital
> camera? That's 0.197 inches by 0.197 inches. So the longest slit (The
> diagonal distance across the square) is 7mm or 0.276 inches.
> Or is chicken wire fine?

Chicken wire is fine for the electric field (and will protect you if you
take a strike to the cage!)

Actually, welded concrete reinforcing mesh might work even better (no woven
effects).  The 3 or 4 inch squares are still plenty small at TC
frequencies, and your cell phone will still work inside!

> 
> My coil has a resonant frequency of 51 kHz.
> 
> I would welcome any information at all on faraday cages.

Check back in the archives... there have been a number of threads on this
in the last 6 months.

> 
> Thank you very much,
> 
> Alan Williams