[Premium-Rx] Faraday Cage

[Dave]

It was designed for 65 dB reduction for VHF TV station interference.  The 
windows are screened over and the power entry was via through-the-wall 
Corcom filters.

/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