Re: Faraday Cage

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-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.
> 
> If this is all right (which I am not at all sure about), what is the
> mathematical relationship between
> frequency and slit length?

	If the slit is less than a half wavelength long, not much radiation
will get through it.  IF radiation is what's causing your problems, it's
almost certainly VHF radiation cause by ringing of the leads to the
capacitor.  Note that the wavelength in feet equals 983.5/F, where F is
in MHz.  If the ringing frequency were 100 MHz the wavelength would be
9.385 feet, and a slot shorter than say 3 feet wouldn't pass any
significant radiation.

> 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?
> 

	That corresponds to a wavelength of about 19,285 feet!


> I would welcome any information at all on faraday cages.
> 
> Thank you very much,
> 
> Alan Williams

	If your problem is the 51 kHz electric field from the coil itself, your
faraday cage should work fine with almost any hole size you choose. 
Note that a TC is a very, very poor radiator so that radiation from the
coil wouldn't be causing problems, just radiation from the primary
leads.

	Commercial "screen rooms" are used to contain or exclude signals, and
typically employ a double layer of bronze screen wire of around the same
pitch as window screen.  All input leads, power and otherwise, are
introduced through RF filters.  Such a screen room is good for of the
0rder of 60 dB+ shielding on up into the GHz range, and 100 dB+ in the
HF range.  One made of chicken wire or hardware cloth would be more than
adequate for any coil I can imagine.  It is necessary, of course, to
provide electrical bonding along all of the edges, and use suitable
filters at the input to the  cage.

Ed