Re: Faraday Cage

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

Original poster: "Jeff Kamla by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jeffk-at-sinclair-dot-net>

Hi  Ed  Allen  & all


Very good advice on faraday cage construction, I eagerly took
it all in,  thanks !  I have one question about one part of your
answer, maybe I'm missing something but what is 19,285 feet
about ?  I couldn't figure out how that related to anything ???

 Thanks

Jeff......




----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Friday, April 20, 2001 6:28 PM
Subject: Re: Faraday Cage


> 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
>
>
>
>