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Re: [TCML] Faraday Cage Safety

On 8/11/13 3:18 PM, dave pierson wrote:




I've got some questions I was hoping you or someone else could answer
about Faraday cages:

What would be a safe mesh size to catch streamers from entering the
cage?

    Leaving?
    Any reasonable _mesh_ from 1.5" 'chicken wire on down to
    window screen.  Or an array of bars.
I've seen cages with 4" spacing work fairly well. And the wires can be fairly thin, subject to mechanical conditions, so if you're looking for a stage presentation, they can be basically invisible. 







More important would be, what mesh size should one use to have a
Faraday cage that's effective to keep interference within?

    Keeping RFI/EMI inside also depends on handling all
    wire penetrations correctly:  filters 'in' the
    'wall', 'bonded' to the wall.  This becomes more
    critical in dealing with the VHF range strays, rather
    than the 20/50/100KHz/whatever 'fundamental'.

    Planning for some debug/test/rebuild time is
    desirable..
My gut feel is that using a cage to keep TC RFI contained is not entirely practical. Making a shielded room that is decently shielded is very hard. The real question is what level are you trying to get down to?
People run wireless mics, audio and video gear, etc. with no cages. It's more about paying attention to return paths and the like. 




I only presume it has got something to to with the wavelength
of the emitting device?

     Yep.  The rule of thumb i was familiar with was openings
     to be smaller than 1/10 the wavelength.
Really, it's that the perimeter of the opening has to be small enough, relative to the wavelength. The opening is like an antenna (Babinet's principle). Short antennas (relative to wavelength) are inefficient, etc.
The other criteria is the "waveguide below cutoff" where there has to be some length to the path, in the direction of propagation. Most commonly this is done with stacks of tubes or hexagons (think of the core in honeycomb composites). Lots of little parallel waveguides, all below cutoff.
For TC frequencies, the "small antenna and hole" is more the issue, more than the WG below cutoff. 



 This is simple

     at the '50 KHz end', may take some attention at the VHF
     stray end.  A subtlety is that this measure applies to
     the single longest dimension of an opening: picture
     a solid copper room, with a solid copper door.  close
     the door and its still a 6 ft opening, unless some
     bonding/finger stock is provided.  (OK: likely
     not relevant to most cases here.)


For VHF frequencies, it might be relevant.
But really, the problem is the near fields. All those shielding principles sort of assume you've got an incident plane wave from infinite distance. Close to the coil, the fields are anything but plane waves. 


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