Re: Faraday Cages....

["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>


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Friday, February 23, 2001 5:40 PM
Subject: Faraday Cages....


> Original poster: "davep by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<davep-at-quik-dot-com>
>
> Tesla list wrote:
>
> > Original poster: "Gary Johnson by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" ><gjohnson-at-ksu.edu>
>
> > >Doesn't a Faraday cage only work if it's a COMPLETELY ENCLOSED room?
>
> That way, it is 'guaranteed' to work.  Lesser solutions may
> be acceptable, depending on conditions/applications.  Caution
> (as has been noted) is in order.
>
> >> I don't mean airtight (but there is a specific maximum size hole for
> >>a given frequency protection limit)?
>
> Some have been cited, which seem plasuible.  The one i used to
> use/hear was:
> No opening with a side longer than 1/10 pf a wavelength.
> Thus, a cage for VHF work has a very differnt door than one for
> Tesla coiling.
>
> As also noted, its the single longest dimension of an opening
> with 'matters'.  Thus, a metal door, with gasketing, but
> NOT gasketed along one side (or with failed gasketing) is the
> same as a six foot hole.  And no door, but a gap of 6 feet
> between fasteners, is the same.
> (6 feet is likely fine for the usual coiling freqs (100KHz =/-)
> 'tis useless at VHF....)
>

Perimeter is the key.. If you have a narrow slot, but the perimeter is on
the order of 1/2 wavelength (i.e. the slot is 1/4 wavelength long and very
narrow), you'll get significant leakage.  The slot can be any shape, round,
linear, etc. because the EM wave will propagate along the conductive
surface.

If you really are hard core, you make the surfaces not a perfect conductor,
but lossy. The amount of loss is best determined empirically.  (It's akin to
reducing radar cross section for stealth... little details make a big
difference..)

>
> This can work, depending on details.  One problem with a transformer
> NOT designed as an isolation is stray capacitive coupling from one
> side to the other.  In the Coiling case, this can lead to stray
> where it is not expected, nor wanted.

Precisely the problem.. Of particular importance in medical applications,
where small leakage currents can be lethal for someone who has catheters in
their heart, for instance.  For TC apps, I don't think it is as big a deal.
Even if there is 100 pF between primary and secondary, the impedance at 100
kHz is going to be be pretty high, and as long as the path to ground is much
lower impedance, that's where the power will flow.
>
> The other key (as, i believe has been mentioned) is how
> wires/penetrations are handled.  Stray RF WILL ride in/out
> on ANY wiring, unless it is handled appropriately going thru
> the screen.
make that any conductor... conduit is particularly troublesome, because it
is hard to make a suitable connection to the wall of the room on both sides
that works well..

>
> > Steel sheet metal on four walls and ceiling has little effect on
reception.
>
> How are the edges handled?
> Pro screen rooms are routinely built of galvanized sheet steel.

All the ones I've seen have been either sheet steel (often with aluminum
molding between panels, which to me is asking for corrosion, but hey, that's
the way they did it) or with foil covered insulation board and copper tape.
I did once work with a copper screen room, but it didn't work particularly
well.  Screen is particularly insidious, because if it is woven, the wires
going back and forth between sides provide a fine path for leakage.
>
> I should expect it to work fine (for coiling) IF the edges and
> penetrations are well handled. (filtered at/in the screen
> wall boundary.).  (I've a lovely 1893 article thereon, which i can
> NOT find..

It's easy to test... knock together some 2x4's and have at it with the
staple gun.


I think that radiated fields (which is what screen rooms are for) are the
least of your problems.  Worry about magnetic fields, and transients (also
magnetic fields).  I once spent several months struggling with isolating
60Hz (and harmonics) interference from EEG recordings with all manner of
shielding, iso transformers, all sorts of grounding. What finally worked the
best was NO shielding, and careful attention to wiring layout (lots of
twisted pairs and NO ground loops).  You want to minimize the area of any
"victim" loops.

EMP and lightning protection are the areas to research... After all, what
you are doing is basically making lightning.....not building radio
transmitters.