Faraday Cage Seams Question

Discussion in 'General Electronics Chat' started by nbc2016, Sep 19, 2016.

  1. nbc2016

    Thread Starter New Member

    Sep 19, 2016
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    Hello AACers,

    I am trying to construct a Faraday Cage to protect my EEG recordings from 60Hz noise, but I am not getting good results. I am using copper mesh screening and a gasketed conductive seal on the door, but I'm not getting much attenuation of signal inside my cage. Commercial manufacturers recommend copper screen for this type of application, so I thought I'd build my own. The amount of 60Hz noise is exactly the same if the door is open or closed! I'm a newbie to these forums, and not an EE, so please be gentle in your comments...

    I think the problem must be in the seams between the mesh, but I don't understand what the problem is. I've included an image that shows the way that I've constructed the connection point between the seams, with two possible problems in the construction. Could you please tell me which is more likely to be the problem?

    A: The two pieces of screen are connected via pressure (sandwiched between two pieces of wood). It is possible that there is not a good connection at each point, because the wood might not be perfectly square. I could use conductive tape to seal the seams (I don't know how to solder!).

    B: The sandwiching between wood is somehow incompatible with a Faraday Cage. I understand that a Faraday Cage works by having a continuous surface, but I couldn't figure out how to keep the mesh up without sandwiching between a wood frame. Will having a protrusion from the continuous surface eliminate the Faraday Cage effect (and if so, could you explain why or point me to resources on constructing a cage?).

    Thanks for your help!
    nbc2016
     
  2. kubeek

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    Do you plan to put the patient inside that cage?
     
  3. nbc2016

    Thread Starter New Member

    Sep 19, 2016
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    Yes, and it is grounded.
     
  4. AlbertHall

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    Jun 4, 2014
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    A Faraday cage provides shielding against electric fields but not magnetic fields. 60Hz current in wires produces a magnetic field so that may be your problem.
     
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  5. crutschow

    Expert

    Mar 14, 2008
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    As Albert stated, a standard Faraday cage does not shield against low frequency magnetic fields, as noted here.
    To shield against 60Hz line noise you need a cage wrapped in a magnetically soft material such Permalloy or Mu-metal.
     
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  6. nbc2016

    Thread Starter New Member

    Sep 19, 2016
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    Based on AlbertHall and crutschow's comments, would shielding nearby wires cables (wrapping in shield) help because it would minimize the magnetic fields generated?
    Also, it is typical practice in EEG research to have people inside Faraday Cages, but based on this description of the magnetic fields that would suggest it would be useless?
     
  7. crutschow

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    Wrapping the cables would likely not help much since the problem is all the mains wires in the walls of the room.

    The Faraday cage would help with other types of electromagnetic interference.
    Perhaps the Faraday cages they use have magnetic shielding also.
    I believe the EMI rooms were I used to work had solid metal magnetic walls plus a layer of copper screens to protect against both EMI and 60Hz magnetic fields.
     
  8. AlbertHall

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    Jun 4, 2014
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    Did you still need the silver foil hat?
     
  9. BR-549

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    Sep 22, 2013
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    I would try two things first. Power your equipment with battery DC.....to confirm the AC supply is not dirty.

    And remove florescent lights AND the ballasts.

    If necessary.....current loops around the space can deflect B fields.
     
  10. crutschow

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    Only to protect against questions like that. :rolleyes:
     
  11. Papabravo

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    The construction of an anechoic chamber is truly a marvel to behold. I've seen it done twice, and it is not a low budget on the cheap proposition. I think a quarter of a million is about right and that was 25 years ago.
     
  12. BR-549

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    Sep 22, 2013
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    We only need to filter 60 Hz right?
     
  13. DGElder

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    Apr 3, 2016
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    As others have said, the Faraday cage will provide little shielding from near field power line emissions which are relatively high current and low voltage sources. The interference is predominately from the 60Hz magnetic field. If you have control over the room's wiring then running the neutral/return wires next to the hot wires would reduce local sources of emissions.
     
    Last edited: Sep 19, 2016
  14. GopherT

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  15. crutschow

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    That must have been a pretty large one. ;)
     
  16. GopherT

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  17. Papabravo

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    Feb 24, 2006
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    I don't think it would hold an F-18, but it could comfortably hold a dozen people working on the tests.
     
  18. DickCappels

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    Aug 21, 2008
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    Yes, it is a good idea to solder the seams. Overlapping the screen does not assure low resistance connections. The seams don't need to have a continuous bead of solder for the frequencies of interest but you should assure that all surfaces are electrically connected at all of the joints.
     
  19. AlbertHall

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    Jun 4, 2014
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    A company I worked for had a 'screened room' where the screening was chicken wire (galvanised steel) which was used for some radio testing at around 100MHz. The ineffectuality of this was clear when we realised that the guy doing the testing was listening to an FM radio in there and getting very good reception :rolleyes:
     
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  20. Papabravo

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    That is right up there with 'Security Theater'
     
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