Re: Questions (fwd)

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

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 

Indeed, Simon is correct... 

Some practical caveats with respect to shielded rooms/cages/etc (not
entirely applicable to "cage of death types of demos"):
The idealized case assumes a perfect conductor (it doesn't have to be any
particular shape) with no holes.  Oddly, such things are tough to come by
(how do you get the power in (batteries?) without drilling a hole in the
conductor.. so we approximate....

Distinguish between conducted and radiated emissions... The typical TC is a
conducted emission problem, not a radiated emission problem (the frequency
is low, the "antenna" is small, the radiation efficiency is poor).  There
are signficant VHF and UHF emissions from a TC, but they aren't real
strong.  Unfortunately, the typical victim receiver is pretty sensitive
(nanovolts)

A cage made of something with holes will shield frequencies that have
wavelengths signficantly longer (lower frequency) than the perimeter of the
holes.  Perimeter, not diameter, is the important thing.  A slot 6 feet
long and 1/8" wide (like at the door jamb) is just as bad as a round hole 4
feet in diameter!

Avoid conductors that pass from inside to outside through holes, slots, etc.

Real screen rooms use a flat springy gasket that lays over the gap (not in
the gap).

Any conductor that penetrates the wall will provide a path to carry EMI in
or out.  Good practice is to put the line filter at the cage wall.  A
classic filter will be a circuit with a C on the inside, and an L passing
through the wall.  The inner C provides a path for the EMI to flow to the
INSIDE of the wall. The L provides a high impedance to the EMI flowing
through the wall. The real problem is that real L's have some parasitic C,
which lets some of the emi flow through the L to the outside world. A pi
circuit with a C on both sides is commonly used, but, can actually make
things worse... The EMI that leaks out through the L will be coupled
through the outer C to the outside wall of the cage which makes a fine antenna.

Wire mesh that is woven (like window screen) is much worse than wire mesh
that is welded (like hardware cloth).  The strands in the woven mesh go
back and forth between the two sides.  Chicken wire is sort of in the
middle... At least it's cheap, and if you use a pair of separatedlayers
(say, stapled to opposite sides of 1x2 lath, it works pretty well. Be
careful how you connect the inner and outer layer (if at all...)

Aluminum foil covered insulation board (Celotex) works remarkably well and
is cheap.  you want to trim the aluminmum from the edges, butt the panels
together, and use aluminum tape to join the panels.  Again, watch out for
conductors that pass through gaps!. Not only that, but it is a good thermal
insulator, so it's a good way to insulate that garage/shop/etc while making
a screen room.

Doors are always a problem...the problem is to get a continuous conductive
joint without having any metal go through a gap. flat gaskets and spring
stock are popular, but expensive.

Tesla list wrote:
> 
> Original poster: "Simon Yorkston" <quantumx-at-mail.ozemail-dot-com.au>
> 
> 
> > 2) What exactly do Faraday Cages do? I've heard that they virtually
> > eliminate
> > electromagnetic and radio frequency interference, and if this is true,
> > I'd
> > like to find out how to build a nice-looking one for my coil since RF
> > grounding is somewhat out of the question (this is going to be a smaller
> > coil
> > being run mainly indoors away from any suitable RF ground, and I'll be
> > transporting it to lots of places which would require a small setup
> > time).
> 
> Knowing that electricity in this case is based on an inverse-sqaure law, eg
> force=1/dist^2, if you work it out, anywhere inside an hollow enclosed
> object no electric force exists. So, in essence, by having your TC inside
> an enclosed object [or semi-enclosed, such as a cage, or chicken-wire etc]
> you cancel out any electric forces to the outside..
> 
> Do any other members want to back me up on that? [If I paid enough
> attention in physics, I think I'm right :P]
> 
> Best luck in coiling, and remember, take your time - that's probably the
> best advice..
> 
> Simon