On 9/27/13 8:05 PM, Hans Hammarquist wrote:
Gentlemen,
I am on the process to build myself a (small) free-standing shack
next to my tower. I am thinking (which usually is dangerous) that I
should "dress" my shack in aluminum foil, making it into a large
Faraday Cage/shielded room.
By connecting the foil to the tower and make sure all wires entering
my shack enters through a (relatively) small window I should be able
to make the ultimate lightning protection.
What can go wrong with this? The foil will be connected through-out,
walls, floor, ceiling. All the feed-through connectors, surge
protectors etc will also the connect to the foil. I will using a wide
aluminum strip to connect the tower to the foil. Yes, the tower has
nine (9) gounding rods, so I believe that is OK.
What are you trying to protect against? Lighting hitting something
inside the shack? Or suppressing the radiated RF energy from the stroke?
From a "Faraday cage" standpoint, you don't need to have continuous
sheet. A very coarse mesh ( 1 foot spacing) will work as well. If
you've ever seen the cage demos at High Voltage shows (e.g. Boston
Museum of Science) the bars on the cage are fairly far apart.
If you want a shielded room, you need to figure out what frequencies you
want to shield from AND whether you are looking at electric or magnetic
fields. For the latter, what you really want is some magnetic material
(iron, nickle, mu metal, etc.) for the shielding.
And, of course, once you bring ANYTHING through a hole in the shield
wall, you've essentially ruined the shielding effect (unless whatever is
going through the hole is filtered.
As a practical matter, aluminum foil doesn't work very well on a large
scale: it's hard to make a good connection with no gaps (the surface
oxide doesn't help). You'd do better with welded mesh (not woven,
because with woven mesh, the wires go from one side to the other) or
with sheet metal. Galvanized steel would probably work fairly well, is
cheap, and is easy to clamp/bolt at the seams.
If you look at the FEMA designs for storm shelter/safe rooms, that might
give you some ideas for construction methods. They're not worried about
being RF tight, of course, but it's a decent starting place. FEMA P-320
"Taking Shelter from the STorm: Building a Safe Room for your Home or
Small Business"
http://www.fema.gov/safe-room-resources/fema-p-320-taking-shelter-storm-building-safe-room-your-home-or-small-business
Any feedback?
73 de,
Hans - N2JFS _______________________________________________
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