Re: faraday cage questions (fwd)

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Original poster: Steven Roys <sroys@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Wed, 27 Dec 2006 06:06:57 -0800
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: faraday cage questions (fwd)

At 09:10 PM 12/26/2006, you wrote:
>Original poster: Steven Roys <sroys@xxxxxxxxxx>
>
>
>
>---------- Forwarded message ----------
>Date: Tue, 26 Dec 2006 02:50:08 -0800 (PST)
>From: davep@xxxxxxxx
>has to be easily collapsable for when im done experimenting and
> > need
> > Mesh size:  Is it safe to assume that any frequency with a wavelength
> > greater than the mesh opening will be completely attenuated?
>       The Rule of thumb was longest opening shorter than
>       1/10 wavelength, at shortest wavelenght.  The details can
>       be dug out by looking at the math for 'slot antennas' and
>       the next comment.
>
>       This One Is The Hidden Gotcha:
>         The governing dimension is the SINGLE Longest
>         dimension. Thus, a door, all metallic, with
>         unbonded edges is an opening 6' 'open'.  Bonding
>         between edges must observe the '1/10 wavelength rule.'

The usual guide line is to work with the perimeter of the hole, 
rather than the diameter.  That takes care of square holes, long 
skinny slots like Dave mentions, etc.

---  However, your real problem is probably NOT radiated high 
frequency stuff where electric fields are the dominant propagation 
medium (which the screening would help) but lower frequency stuff, 
where the primary thing to worry about is the magnetic field.

When the top load sparks, you're getting a big current transient as 
the top load discharges to the ground, and that current transient is 
what you worry about.  Likewise, the Marx will do a similar thing.

The goal is to minimize the area enclosed by the current path.  That 
is, provide a separate "RF" return path for sparks and discharges 
that does NOT go through the cage.  If your coil sparks over to the 
cage, the current is flowing in the cage, and the cage will radiate EMI.

You might check the websites of companies making EMI/EMC gasketing 
material (Chomerics, for instance).