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Building Faraday room

Discussion in 'Electronic Design' started by Steve Larson, Apr 7, 2007.

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  1. Steve Larson

    Steve Larson Guest

    I would like to build a faraday cage large enough to work in. Can
    anyone please advise on the following points.

    1. Will aluminium window screen work as well as brass? Are there any
    more cost-effective materials?

    2. How much does grounding influence the degree of shielding? I would
    assume grounding is more effective for lower frequencies where
    reflection is not as much of a factor. Can anyone quantify this effect
    in Hz?

    3. Now-a-days most aluminium screen is anodized. Does the insulation
    this adds between each strand diminish its overall effectiveness in
    grounded applications?

    Thank you for any advice along these lines.

    Steve
     
  2. I suspect the main problem with ali is getting a low-impedance join due to the oxide layer.
    You really need a solid seam join, which typically means welding/soldering/brazing, which is hard to
    do with ali
     
  3. If aluminum, take chromatized one. It has a conducting
    surface. A good faraday has thw shield within each
    other joined at just one point and at the same point
    both shields are earthed. Iron is even better, by the
    way. It is better for lower frequencies.

    Rene
     
  4. MassiveProng

    MassiveProng Guest


    That's why brass is what the industry uses. The frame gets soldered
    or brazed such that all screen thread elements are grounded to the
    frame, and the frame is then attached to the rest of the shielded room
    constructs.
     
  5. Paul Mathews

    Paul Mathews Guest

    There was a really good article in Compliance Engineering or one of
    the ITEM publications a few years back comparing various low-cost
    approaches to building screened rooms. Galvanized steel actually came
    out the overall winner in the estimation of the authors, but some of
    the really low-cost options considered were nearly as good at most
    frequencies. The resistivity of the steel provides some benefits,
    IIRC.
    Paul Mathews
     
  6. MooseFET

    MooseFET Guest

    The biggest problem is at the joints. You have to be able to make a
    good joint all along where the sheets meet.

    Copper based materials can be soldered and this saves a lot of labor.
    Another option would be to put a screw every 1/2 inch but this would
    take forever.

    Grounding is an issue of the common mode voltage. You want all of the
    grounds of everything inside the shield to be at the same voltage. You
    also want all the signals and power lines that come into the sheild to
    pass through the same point and be bypassed there.

    The main reason to ground the shield is because it has a large
    capacitance to the earth. If the shield is at some AC voltage,
    currents will flow through this capacitance.

    If the shielded space is a small fraction (lets say 1%) of the
    wavelength of a signal, the shield blocks the electrostatic component
    of the radiation but less so the magnetic. For the magnetic
    component, you can think of the shield as a turn of an inductor with
    some resistance.
    I would stay away for anodized screen for this reason. You can also
    get material that is sometimes called "expanded". If you are working
    below 100MHz, if you go with aluminum, I suggest you look at it.

    Even that plastic "bubble wrap" like insulation that has a metalized
    surface will block a lot of the electrostatic field from 60Hz mains.
    If you imagine an 8 foot cube with that stuff stapled to the inside.
    A box like that lowered the electrostatic part of the 60Hz down to
    about 1mV.

    In many cases more layers of less good shielding is better. You most
    likely want at two layers that are connected to each other only at the
    ground point. This is much easier to build than 3 layers and not much
    harder than one. You need a frame to hold all this stuff up. This
    frame can be nonconductive. Both the inside and outside surfaces can
    be covered with conductive material. Dry wood works.
     
  7. Neimadre

    Neimadre Guest

    Nothing like smoking a good joint before hitting the sheets. Rock on
    dude.


    Usenet lits score:
    GIT-R-DONE!
    #20 Usenet asshole
    #6 Lits Slut
    #11 Most posting trolls/hunters/flonkers 2007
     
  8. Oooh you even adopted a transmogrified form of his e-mail as yer return
    address. Uh oh now he's really in yer crosshairs
     
  9. Neimadre

    Neimadre Guest

    I own him and he will back down, I learned from a pro.


    Usenet lits score:
    GIT-R-DONE!
    #20 Usenet asshole
    #6 Lits Slut
    #11 Most posting trolls/hunters/flonkers 2007
     
  10. As$Clown

    As$Clown Guest

     
  11. Tim Williams

    Tim Williams Guest

    What of RF?

    Not really. Iron is pretty darn resistive, making it about useless for RFI,
    and a mere screen isn't thick enough to block much EMI. (For that you need
    sheer thickness. Hope you have 2x8's for rafters!)

    Tim
     
  12. MassiveProng

    MassiveProng Guest


    Damn. The KookTard actually said something funny.
     
  13. Large dog cage?
     
  14. Meat Plow

    Meat Plow Guest

    Why is everyone to you (but not you) a:

    Kootard
    Retarded Twit
    Idiot
    Dipshit
    Pathetic
    Total Loser

    Why is it that you think you are the only rational/viable entity while
    exactly the opposite is glaringly evident?


    --
    #1 Offishul Ruiner of Usenet, March 2007
    #1 Usenet Asshole, March 2007
    #1 Bartlo Pset, March 13-24 2007
    #10 Most hated Usenetizen of all time
    Pierre Salinger Memorial Hook, Line & Sinker, June 2004
    COOSN-266-06-25794
     
  15. Guest

    http://www.lessemf.com/mag-shld.html

    This is a step up from the Al foil used in hats.

    http://people.csail.mit.edu/rahimi/helmet/
     
  16. Neimadre

    Neimadre Guest


    Still humping my posts obsesso?


    Usenet lits score:
    GIT-R-DONE!
    #20 Usenet asshole
    #6 Lits Slut
    #11 Most posting trolls/hunters/flonkers 2007
     
  17. jasen

    jasen Guest

    if you can find a way to solder/weld the seams
    cooking foil ? sheet-steel ?

    I'd be looking for a punched, expanded, or welded, mesh rather than a
    woven one.
    probably.


    Bye.
    Jasen
     
  18. Don't assume that. It's more likely to be dyed and coated with lacquer. True
    anodizing is expensive.
     
  19. Tim,
    the RF is no problem at all. The poenetration depth
    is inverse proportional to the conductivity, moreless.
    So if the 100MHz penetrate 200um instaed of 20um, who
    cares. And you're having two sheets of 1mm or so.
    I'm have to look up the numbers.

    Rene
     
  20. Phil Hobbs

    Phil Hobbs Guest

    The skin depth is proportional to 1/sqrt(freq*mu*sigma), where sigma is
    the conductivity. Iron's high permeability is a help at low frequency,
    but not by as much as one would think from this calculation.

    To think about shielding effectiveness, it's important to remember that
    the concept of skin depth comes out of a perturbation theory problem.
    The zero-order solution you start with is a perfectly conducting
    boundary, i.e. E and H are zero at the metal surface. We often think of
    RF shielding as a perturbation on the free-space problem, which leads
    to underestimation of shielding effectiveness, because it assumes that
    the fields on the metal surface are the same as the unperturbed free
    space fields, which is far from true.

    We also tend to get confused by analogies with light shields, which
    makes us underestimate the importance of keeping the shield electrically
    continuous. A truly continuous copper shield of any reasonable
    thickness will eliminate all electromagnetic radiation from inside.

    Iron is mostly important for quasistatic magnetic fields, e.g. from
    power transformers.


    Cheers,

    Phil Hobbs
     
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