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Optimal Faraday cage

Discussion in 'Electronic Design' started by Edward Quinn, Feb 25, 2010.

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  1. Edward Quinn

    Edward Quinn Guest

    I need a quiet environment for operating test instruments. Needs to be
    as close to full spectrum as practical, and about 1.5 metres square or
    equivalent internal space.

    From what I have read, this requires a combination of shielding types.
    With cost in mind, what is state-of-the-art homebrew for this?

    A few ideas:

    1. Aluminum foil on plywood box. Edges sealed with self-adhesive
    aluminum tape. I assume a solid surface is more effective than screen
    or mesh.

    2. Corrugated steel water tank. Thick metal may be better for lower
    frequencies(?) Corrugations may provide deflection(?)

    Of course, the unit would be earthed. For vents, etc. I would fit a
    curved section of flexible aluminum ducting a used in domestic
    heating/air con installations, with a metal screen over the opening.

    In addition, I would apply a thick paint with iron filings to the
    outer surface.

    Any feedback or analysis regarding the above would be appreciated.

    Ed Quinn
     
  2. Joerg

    Joerg Guest

    For similar purposes I have a large thick corrugated carton that is
    covered with copper foil. Draped around the edges so the lid makes a
    tight seal. So far any sort of radio goes totally silent in there.

    A word of caution: In certain weather conditions removing and closing
    the lid can generate an impressive blue spark, often accompanied by a
    succinct *BANG*, so be careful. Maybe keep it grounded together.
     
  3. Phil Allison

    Phil Allison Guest

    "Greegor"

    Wouldn't the aluminum foil be passive to the
    magnetic component?


    ** Aluminium or other non-magnetic foils will have no effect on a static or
    slowly varying ( ie 50 /60Hz) magnetic field.

    A static electric field is defeated because the charges on the surface of
    the foil become re-arranged so as to cancel any static field INSIDE the
    enclosure.

    A radio frequency wave is very much attenuated inside the enclosure too -
    the surface charges respond to the wave's electric field and MOVE in synch
    with the wave thereby creating a magnetic field that opposes the magnetic
    part of the arriving wave.


    ...... Phil
     
  4. Fingerstock will help a lot, but the door and the frame needs to be
    tightly matched with each other, so if the door i damaged, you can not
    just replace the door, but also the frame must be replaced that fits
    tightly with the new door.
     
  5. Glen Walpert

    Glen Walpert Guest

    The carbon loaded foam can be a useful absorber inside the chamber.

    For a cost effective functional shielded chamber you should start with
    Ralph Morrison "Grounding and Shielding" and Henry Ott "Noise Reduction
    Techniques In Electronic Systems". Ralph specifically addresses shielded
    chambers and Henry provides a good complementary discussion of shielding.

    One of the first requirements is conductive seams, so as others have
    noted aluminum is out (unless you can weld all the seams). Steel sheet
    metal is fairly cheap, easily cut, bent and nailed to a wooden frame, and
    then the seams can be soldered. But the devil is in the details and the
    details best found in the above books and perhaps a look at shielding
    gasket suppliers literature.
     
  6. GregS

    GregS Guest

    Only for magnetic interference. All you need is low Z for electrostatic.
    The most common are home brewed aluminum screening with aluminum frame.
    Sometimes steel is used but aluminum is better for the frame. Most I see are for
    lower frequencies.

    greg
     
  7. GregS

    GregS Guest

    A space withing a room is nice. You can build a good sized outer room
    using aluminum backed drywall with steel studs. I have seen a very large room
    fitted with 1/4 inch steel plate on the bottom and edges, to reduce
    magnetic field from a MRI below. I could get a steel nail to move around
    on a string pretty good while walking above that small MRI unit.
    Of course and small vibration of electronic devices could induce
    electrical noise at the vibration frequency.

    greg
     
  8. Make whatever box you (he) want(s), and remember to fill each seam,
    during mating, with silver filled, electrically conductive epoxy like
    what they use to attach IC chip dies to their substrate like: EPOTEK
    H20E

    http://www.epotek.com/SSCDocs/datasheets/H20E.PDF
     

  9. 'They' have chambers big enough to put an Osprey or even a C-17 into.
    The 'cones' are like 3.5 feet tall, and it is like the area of a couple
    football fields. NOTHING gets in OR out. Every signal read within there
    originated in there. And it is nowhere near Vegas. :)
     
  10. Reminds me of a l-o-o-o-n-g time ago when we at Philips had the only
    large kiln around capable of holding a mu-metal shield about 1.5 m
    diameter and 1 m high, wanted for some scientific experiment by a
    defence department. They provided the shield, and told us the
    temperature profile they wanted for annealing, in pure nitrogen. No
    problem, it got done on night shift, when we were not using that kiln
    for production. Only problem was that someone got the instructions
    mixed up, and fired in pure oxygen. Was a bit of a surprise when they
    opened the kiln, and found a small pile of powder.
     
  11. Tim Williams

    Tim Williams Guest

    Wow, I hope that person got...fired (use your imagination as to how)!

    Tim
     
  12. Pobably cost them the price of the item plus a penalty as well.

    Several tens of $ks sounds likely.
     
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