Is the Faraday Cage invulnerable?

  1. I was wondering how protective a Faraday Cage actually is, can it be overcome by sending more amperage onto the exterior or does that not matter? Also what would cause a Faraday cage to be ineffective, like would a cord powering the device inside render the cage useless? Thanks for your help!
     
  2. jcsd
  3. atyy

    atyy 10,632
    Science Advisor

    A Faraday cage perfectly shields its inside from static electric fields outside. So if the outside electric fields change fast enough, they can penetrate the cage.

    If the power cord radiates, then it can indeed generate electric fields inside the cage.
     
  4. Wait, so it can or cannot penetrate the shield, you described it as shielding perfectly and also being able to penetrate. How could it penetrate? What do you mean by "outside electric fields change fast enough" and by the power cord radiates? (im very new to this and do not understand most technical jargon, so please explain your answer so the laymen like me can understand you) :) thanks!
     
  5. berkeman

    Staff: Mentor

    Welcome to the PF.

    Here is a link to a typical shielded enclosure company that will help you see what can be done:

    http://ramseytest.com/index.php

    You use metallic shielding to shield against E-fields and RF EM fields. You use ferrous shielding to shield against magnetic fields.

    Because you often need "feedthroughs" to get power and signals through the walls of the shielded enclosure, you need to treat those feedthroughs with filtering to keep external fields from coupling into the shielded enclosures.

    Is your question for general interest, or do you have a particular application in mind? There are lots of considerations to designing effective shielded encloures...
     
  6. atyy

    atyy 10,632
    Science Advisor

    It can penetrate by changing fast enough. So the faster it changes, the more it penetrates. You can see this in the link berkeman provided:

    The STE2200 on that site is described as:

    Isolation
    -100dB @ 1GHz
    -90dB @ 3GHz
    -80dB @ 6GHz

    So you can see that the higher the frequency (ie. how fast the field changes), the less shielding the STE2200 provides.
     
  7. berkeman

    Staff: Mentor

    I think the lower attenuation at higher freqencies for those enclosures is because of the seams and apertures. For an ideal Faraday cage, I don't think there would be a change in attenuation versus frequency. But an ideal Faraday cage is not a practical thing -- who can use something with nothing going into or out of it?
     
  8. atyy

    atyy 10,632
    Science Advisor

    Yes, I mixed up real and ideal Faraday cages. Actually I had in mind that X-rays and gamma rays can penetrate metals because they aren't perfect conductors, but probably in the GHz range mentioned in the STE2200 specs, that's due more to the seams and apertures as you mentioned?
     
  9. So, I read that an airplane acts as a Faraday Cage and protects passengers form lightning strikes, but if that is true how do airplanes receive cell signal and WiFi? Also, there is no reason I ask this other than to satisfy curiosity. Would a computer act as a Faraday cage because of the metal encasing around it? Would it survive an electromagnetic pulse?
     
    Last edited: Dec 30, 2012
  10. berkeman

    Staff: Mentor

    Via external antennas.
     
  11. So what could be done to strengthen a Faraday Cage? What makes it more resistant to high frequency waves?

    Also, is there anything that would be different in designing a shield for electromagnetic pulses vs radio waves?
     
    Last edited: Dec 30, 2012
  12. Why is it the wavelength of the wave that determines whether or not it can penetrate a Faraday Cage, instead of the amplitude?
     
  13. nsaspook

    nsaspook 1,137
    Science Advisor

    Do some web searches on "TEMPEST" "EMSEC".
     
  14. atyy

    atyy 10,632
    Science Advisor

    In general the electric field inside a metal enclosure depends on both the amplitude and the frequency of the external field. However, for electric fields of the same amplitude, different frequencies penetrate differently. The basic idea behind the Faraday cage is that when the external field is applied, electrons are free to move into new positions. Within the metal, the electrons in their new positions produce an electric field that cancels the external field. Each time the external field changes, the electrons must move to produce a new cancellation. In a real metal, the electrons take some time to move to the new positions. If in that time, the electric field changes before the electrons reach their new positions, then the external field will not be cancelled. This is one reason an electric field that changes fast enough will not be shielded in real metals.
     
  15. Mobiles are still banned on most aircraft so unlikely they would provide an external antenna.

    I believe the windows are big enough to prevent the aircraft being a faraday cage at mobile phone frequencies.
     
  16. berkeman

    Staff: Mentor

    Interesting. I haven't flown for a while, but I thought I remembered cell phones working once at altitude. I could be wrong though.

    While on the ground that would probably be true. Once in the air, the antenna patterns of the cell towers would not cover airplanes.
     
  17. If that is true then how do I get cell signal on airplanes? And some airplanes do provide WiFi.
     
  18. berkeman

    Staff: Mentor

    How do you think? If you worked for the airlines as an engineer, and they asked you to design a system to provide cell and WiFi connectivity on airborne airliners, how would you do it?
     
  19. Well, I would guess that the WiFi signal was coming from a satalite with a router inside the plane, and cell signal... I don't know if that could come from a satalite as well and have an antenna that brought signal inside the plane? that would be my best guess i really don't know.
     
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