[Premium-Rx] Re: Faraday Shield.....To Be, or Not to Be?? Need advice, please!

[James C. Garland]

>
>
>I've done it for a large space.  It's a recording studio in RF laden NYC. 
>We used Corcom through the wall power filters.  Here are some photos:
>
>http://www.wfmu.org/~dathedj/Faraday/Faraday.html
>
>/Dave
Hi Gang,
Although very simple faraday cages offer protection against lightning 
strikes, screening RF requires considerable care in room design.  The 
photos in the above link, for instance, show a room that, because of the 
large windows, would be very ineffective at screening RF at frequencies 
much above the AM broadcast band.  A room intended for RF screening would 
either have no windows, or windows consisting of a double mesh of copper 
screen.

A common way to test RF screened enclosures is with a portable FM radio. 
Strong local FM stations cannot be heard in a well-designed and constructed 
enclosure.  I recall a test I did years ago in a screened room in my 
physics research lab at Ohio State. The room was made of solid 
particle-board panels, coated on each side with soft steel (for magnetic as 
well as electric field shielding), and held together with specially 
designed metal clamps, with screws spaced every two inches. There were 
literally thousands of screws holding the room together. The "windows" were 
small rectangular openings, with two layers of copper mesh. The light 
fixtures used special cold-cathode fluorescent bulbs and, of course, all 
power leads into the room were through large RFI filters. The door was a 
very heavy metal lever-operated structure that looked like something out of 
a submarine, except that it was lined with RF "weatherstripping."

After the room was completed, I carried an FM radio into the room and shut 
the door. Then I walked around all the seams with the radio, listening for 
any signal leakage through the joints. Occasionally I'd hear something, 
which meant torquing down the clamp screws a little tighter. Just cracking 
the lever on the door would let the FM band come sailing in.  As an 
experiment, I stuck a 4" length of insulated hookup wire through the window 
mesh, and the FM band came up to nearly full strength.

Screening low frequency electromagnetic waves is much easier than screening 
the FM band, which is why AM reception is poor while driving through a 
metal bridge. For really effective screening (say, >100 dB attennuation) 
the size of any opening (and slits are the worst) should be less than about 
1% the wavelength of the radiation one is trying to keep out.

Regards,

Jim Garland W8ZR