[TowerTalk] Faraday Cage

[Jim Lux]

-----Original Message-----
>From: warrenwolff at aol.com
>Sent: Jun 1, 2008 10:03 AM
>To: xdavid at cis-broadband.com, sanorm at columbus.rr.com
>Cc: towertalk at contesting.com
>Subject: Re: [TowerTalk] Faraday Cage
>
>Hi again folks,
>
>Wow, I surely do stir up a hornet's nest when I pose some of my 
>questions to this group.
>
>This subject of a Faraday?Shielded Room came up when I was
>trying to get this new house as protected as possible.? The almighty
>dollar interfered with some of my thoughts, this being one of them.


This is not unlike the problem of designing electronic equipment to keep noise inside.  There's several aspects to the problem, and if you're covering a wide range of frequencies, there's lots of things to think about.

Looking at the websites of companies that do EMI shielding, or who make shielded rooms, will be fairly informative.

But, in short..

On holes/apertures.  There's two ways to look at this: 1) as "waveguide below cutoff" and 2) as an antenna.  The WG below cutoff analysis implies that the "length" of the hole is long compared to the cross section. You'll see this in schemes using honeycomb material for vents and windows.  For this mode of propagation, the loss increases VERY fast below cutoff.  But the more common analysis treats the aperture as an antenna.  By Babinet's principle, a conductive shape in a nonconductive environment is the same as the identical nonconductive shape in a conductive medium.  So a slot that's 1/2 wavelength long and 1/100th wavelength wide looks just like a dipole antenna that's 1/2 wavelength long.  If you "light up" a dipole with an incident wave from one side, it will radiate it out the other side.

You  can build a frequency selective surface this way, using either holes in a metal skin or metal shapes on a non-conductive skin.  You can also make a very low or high loss at a frequency with multiple layers.. much like designing a Yagi.

This is where the "perimeter of the hole must be <1/2 or <1/10th wavelength" sort of rule comes in.. Just like antennas, a loop that's a 1/2 wavelength in circumference works fairly well, one that's 1/10th works not so well, and one that's 1/100th works pretty inefficiently.  

Consider the window on your microwave oven, with an array of 1/10" diameter holes (call it 2mm, or a circumference of 6-8 mm).. compare that to the wavelength of the 2.45 GHz you're shielding (about 120 mm).. so it shields fairly well.

http://en.wikipedia.org/wiki/Babinet's_principle


---

Now lets look at the real issues for shielding EMP or HF.. They're at much lower frequencies.. wavelengths in the 10s or 100s or even 1000s of meters.  Obviously, holes aren't an issue.. even your front door is a tiny fraction of a wavelength at the 1MHz frequency of that lightning impulse.

The real issue is magnetic shielding.  Here, the shielding principle is a bit different.  The changing field outside the shield induces a current in the shield wall.  If the wall is thick enough (many skin depths) and conductive enough, then the field from the induced current exactly cancels the external field, so there's no net field inside.  THings with very high mu (iron, etc.) help (because the physical thickness is very much greater than skin depth), hence the use of mu-metal and similar things.  The metal needs to be magnetically very soft and not saturate (hysteresis and nonlinearity results in imperfect cancellation)

You can use wires to simulate that conductive wall.  The overall conductivity (i.e. the mesh density) determines the shielding effectiveness.  And, of course, with wires, you have an orientation sensitivity (to the point that you make microwave polarizers with a grid of wires).

Worst of all.. the wall only shields that component of the magnetic field that is parallel to the wall.  If the field is perpendicular to the wall, it doesn't induce any current, so there's no shielding effect (for instance, you can build a loop antenna in a shielded box, and it works just fine. Ditto for isolation transformers that have shielded windings)

For signals arriving from a distance, you might be able to assume that the field is plane and perpendicular to the ground, but if you're in the near field (say, because the field is coming from a nearby lightning strike, or the current from a lightning strike flowing through some conductor.. like the cable from grounding rod to ground).. that shield isn't going to do a whole lot.


So, the whole protection strategy is one of reducing vulnerability, rather than shielding.  That is, make the victim loops small in area (twisted pair is your friend) so any fields don't induce big voltages.  If you're really paranoid, avoid magnetic components that aren't self shielded (i.e. a rod core inductor is a fine magnetic field sensor, a toroidal core inductor isn't).

Mostly, though, make sure all the equipment goes up and down together.  

For those interested in a detailed analysis of the effectiveness of rebar cages and the like for lightning and EMP, there's some studies from Lawrence Livermore Labs on explosives magazines.
See, for instance, "Lightning Protection Certification for High Explosives Facilities at Lawrence Livermore National Laboratory" ,by Todd J. Clancy, Charles G. Brown, Jr, Mike M. Ong, and Grace A. Clark , IEEE Ant 
Prop Soc Intl Symp 2006, 9-14 July 2006

If you google "Clancy Lightning LLNL" you'll get some press releases etc, with contact info.


Jim, W6RMK