[TowerTalk] Faraday Cage

[Pat Barthelow]

I had an interesting experience with Faraday Cages, Screen Rooms at the Navy School, Monterey, in 1986.
We needed one in an Oceanography Computing Research Lab, and the Public Works department took a bunch 
of stored, bolt together Screen Room panels  that had wooden frames inside, and  double surfaces of fine copper mesh screen.  The panels were 
6 each, forming a cage about 15ft long, by 8 ft by 8ft.  They had large filters of the AC mains that filtered incoming and outgoing RF 
to the cage.  The cage panels  had long, bolted clamps that pressed the 
panels together on the edges, insuring  firm (?) contact between panel edges of the fine mesh screen.  The panels wee
old, in storage for some years, and the copper screens had a uniform patina of brown, copper oxide.

The door must have been expensive, precision constructed,  with a brass/copper structure, and close fitting phosphor-bronze finger stock firmly compressed around  the complete door frame when it was closed.  
As a first check of effectiveness after they bolted the screen room together, I  took a battery powered AM/FM table radio inside and closed the door, and it played almost perfectly, Both AM and FM,  with  little attenuation.   HUH?!  We worked for some time to get it RF tight, and found that the devil is in the details on assembly...

Turns out that the "patina" on the copper, and simple clamping the frames together, produced very leaky joints, that had to be worked on
extensively (cleaned and tightly clamped) to get them to be RF sealed. The door frame surfaces also had to be burnished to bright metal to get a good RF tight seal.

73, 
All the best,
Pat Barthelow    (916) 315-9271
Jamesburg Moonbounce Team
aa6eg at hotmail.com



> To: xdavid at cis-broadband.com; sanorm at columbus.rr.com
> Date: Sun, 1 Jun 2008 13:03:25 -0400
> From: warrenwolff at aol.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.
> 
> I was suspecting that a bit less than 1/4 wave at 2 meters would have
> been sufficient for such a room.
> 
> Now David has dug up a reference:
> 
> 
> - Aperture
> Aperture
> An opening in a shielded enclosure that may become the source of leakage for electromagnetic energy. This is
> 
> most common in joints, seams and penetrations such as switches, connectors, and lamps/LEDs. Typically, the
> 
> maximum dimension of an aperture should be less than 1/20 wavelength of the highest frequency of interest in
> 
> order to avoid emissions.
> 
> Interesting, to say the least.
> 
> The comments re: air conditioning resurrected an old experience(s).? When I was an OIC of a STRATCOM
> Receiver Site in Hawaii, we had a all copper screen room.? As Program Manager of a Comm System in
> Kuwait, we also had one.? In both cases, we did not close the door unless absolutely necessary because
> of poor air conditioning.
> 
> Then it hit me; my project engineer from USAF days has a PhD in Physics.? Think I will
> ask him if 1/20 or 1/10 is overkill.? The idea of something less than the tightly closed
> copper mesh room still intrigues me - - IF the spacing is not too small.? Doorways and
> A/C ducts probably pose the biggest problem to a "cheapo" room.
> 
> I appreciate all of the interest and the well-founded references provided so far.
> 
> Warren; W7WY
> 
> 
> 
> 
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