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Faraday Cage Shielding



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From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Saturday, June 20, 1998 4:33 PM
To:  Tesla List
Subject:  Re: Faraday Cage Shielding


> 
> ----------
> From:  The Meyer Family [SMTP:meyer-at-clove-dot-net.au]
> Sent:  Friday, June 19, 1998 6:54 AM
> To:  tesla-at-pupman-dot-com
> Subject:  Faraday Cage Shielding
> 
> Hello,
> I am writing to ask about shielding my coil from my other electronics
> equipment. I have a shed up the back of the house which is usused so far.
> To make a faraday cage would I be able to line the concrete floor with
> chicken wire, making sure there is a connection between the walls and the
> floor, then put a big earth on the whole shed??
> Then in theory the shed should become a big Faraday cage, am I right????
> Would this be effecient?

Yes, this would make the shed a Faraday cage. You probably don't have to
put the wire on the concrete floor, since presumably it is already at
ground.  You'll also need to do the ceiling.

however, here is the problem, any wire that penetrates the cage (including
the twists in the chicken wire) provides a path for RF energy to go in or
out.

How are you going to get power into the room? Lights can be outside the
shielding and shine through, but your AC power for the coil needs to be
filtered at the wall with the metal case of the filter bonded all around to
the wall.

For actual RF emissions, you can have pretty good sized holes in the cage
without destroying the shielding. A good rule of thumb is to keep the
perimeter of any hole less than 1/2 wavelength. If you are looking to
shield at less than a MHz, that means that the hole has have a perimeter
less than 150 meters (300 m = 1 wavelength at 1 MHz).  Your chicken wire
would work fine at everything up to around 1 GHz (if it were copper, that
is).

At lower frequencies, capacitive coupling is more of a concern than actual
propagated interference. For this, you need to have more solidity, because
your shield has to actually act like a complete plate.  

 Real screen rooms use special screening that is like hardware cloth (the
non woven kind) that is spot welded at each intersection. Reinforcing mesh
might also work.  The other problem is that steel isn't a particularly good
conductor, and the lower the conductivity, the worse it is for shielding.

You might take a look for web sites of companies that make screen rooms for
ideas. http:/www.chomerics-dot-com might be one place to start.