Re: Damages to Electronic Equipment (chicken wire screen effectiveness)

To: tesla-at-pupman-dot-com
Subject: Re: Damages to Electronic Equipment (chicken wire screen effectiveness)
From: "Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Wed, 16 Feb 2000 11:13:02 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme


> 
> Chicken wire is good for arcs since it does prevent them from going
> further.  I wonder what the skin depth of that wire is like and if the
very
> high frequency 10MHz, 100MHz, and GHz+ noise frequencies will be
conducted
> or just radiated off it.  Maybe they will be absorbed???  I am not sure.
> Perhaps your setup can answer this for us.  Looking forward to seeing
what
> you find.
> 

I do have some practical experience using chicken wire and hardware cloth
(the former is joined by twisting the wires, the latter by welding without
weaving) as shielding and/or microwave reflectors.

First off, neither works particularly well for magnetic field shielding,
and for low frequencies (kHz) this is really what you are worried about.
The saving grace is that the absolute magnitude of the magnetic fields is
fairly low, and it drops off fast as you move from the coil (after all, the
whole idea is to couple the mag field of the primary more or less to the
secondary).  The exception would be, as Terry notes, a big arc from the top
load to a grounded object some distance away (which might be closer to your
victim equipment). In this case you have a high peak current and a large
area loop, which makes a fairly effective EMP radiator.

For higher frequencies (MHz and above), the key factor is the size of the
holes, seams, and/or slots.  Holes that have a perimeter smaller than, say,
1/10 wavelength, are of negligble importance, providing that no wire or
other conductor penetrates the hole.  If your mesh is woven (like window
screening) or twisted (like chicken wire), the over and under nature of the
strands provides a path through the holes.  Seams at the edges are the most
likely place that problems will occur (especially if the mesh is wrapped
around a structural member), because you can get a fairly large slot
(perimeter wise) without realizing it.

A really good source for information on this sort of thing is the web sites
of the people who make EMI shielding and gaskets.  Chomerics
(http://www.chomerics-dot-com/ I think) (a division of Parker, now, I think)
would be a good source.  If you get a copy of a magazine called "Compliance
Engineering" (it refers to Electromagnetic Compliance), the ads will give
you much grist for the mill.