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Faraday cage not working

  1. Mar 11, 2008 #1
    Dear readers!

    I wanted to arrange a little demonstration for my relatives with my cellphone inside a Faraday cage. Of course I expected no EM waves to be present inside the cage, which means that my cellphone wouldn't ring when being called. Well, we repeated the experiment several times, placing the box at different points and (a few times) it did.

    As a "cage" I used a closed box made of some kind of thin metall. Unfortunately I don't know what it is, but it attracts a magnet.

    My question is why it didn't work. Do you think that the conductivity of the material in question wasn't high enough, thus causing the charges not to arrange themselves quickly enough in positions suitable for neutralizing the external field? Could there be an other reason? Do you know any metalls not suitable for constructing a good Faraday cage?

    Thanks in advance for any help,
    Angelos
     
  2. jcsd
  3. Mar 11, 2008 #2

    mgb_phys

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    Probably gaps in the metal cage - how does it seal? Basically if it's not watertight - it's not RF tight!

    Cell phones are very good at picking up a weak signal.
     
  4. Mar 11, 2008 #3

    Andy Resnick

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    We use Faraday cages made from simple metal mesh, nothing fancy- they look rather sloppy, actually. But as mgb_phys points out, cellphones are very, very sensitive.
     
  5. Mar 11, 2008 #4
    Have you earthed it?
     
  6. Mar 11, 2008 #5
    Thank you all for your quick replies!

    I am not sure if it was earthed. Before the procedure it was placed on my metall heater. Isn't the choice of potential 0 arbitary? What does that affect?

    The box has no visible openings, but, as it closes with a simple cap it isn't likely to be watertight - I can't test it now, since it has to be kept dry. The whole thing suprises me. From all that I've read I always believed, that it didn't matter wether there were openings in the cage (it had to be a cage, not a box). After all they say that a flash can't hit a car and a car isn't a closed box.

    But of course a car consists of much more metall, thus providing more charge carriers that can arrange thamselves properly and than again the conductivity of the material and its charge carrier density might be essential (?).

    As a matter of fact, although the cellular rang, it recieved messages notifying me of a call, as if I was out of network range. The cage, therefore, seems to have had some - strange - effect.

    Thank you for your patience!
     
  7. Mar 11, 2008 #6
    The choice of 0V is arbitrary however for electrostatic shielding, ie what happens in a faraday cage to work fully it requires it to be connected to ground. Ground is, by definition, a source of infinite charge...this ability means that the metal can be charged with whatever polarity at what ever magnitude it needs to totally oppose in coming signals, this is of course a simplified version of how electrostatic shielding works.

    I would have thought that the metal heater would have been earthed, whether it was gas, electrical or part of a central heating system.

    Also ...
    This is incorrect, you can make a perfect faraday cage with mesh which doesn't have to be water tight, of course it all depends on the frequency you are attempting to 'block' as to the maximum allowed aperture.
     
  8. Mar 11, 2008 #7

    mgb_phys

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    Yes a mesh much smaller than lambda will offer reasonable shielding - it's more of a statement of the effectiveness with which microwave photons can squeeze through small gaps in carefully made RF shields.
     
    Last edited: Mar 11, 2008
  9. Mar 11, 2008 #8

    DaveC426913

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    Excuse my ignorance, but what is the frequency of a bolt of lightning versus the frequency of a cellphone carrier? And what is the gap in the car versus the gaps in your cage?

    Surely this is key?

    Your microwave oven has a grid on it that's in the milimeter range because microwaves are in the centimeter range.
     
  10. Mar 11, 2008 #9
    As far as I'm aware lightening is DC - so there is no frequency..... Microwave radiation - used in the GSM phone network in the uk is about 1.4GHz if my memory serves me correctly.
     
  11. Mar 11, 2008 #10

    DaveC426913

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    you're talking about electrical pulse frequency - as opposed to electromagnetic frequency.
     
  12. Mar 11, 2008 #11

    mgb_phys

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    Lightning will put off a whole range of frequencies from thermal emmision of the plasma to long wavelenght RF from spark-gap effects.
    Whenn it hits a car it's pretty much a DC current so you only have to worry about the electrical binding between panels - aircraft makers go to great lengths to ensure that there is no insulating gapo between panels that lightning could jump across and create a spark.

    GSM mobile phones use 800/900Mhz (33cm) or 1800/1900 Mhz (17cm)
     
  13. Mar 11, 2008 #12
    The Faraday cage effect usually refers to static fields or low frequency fields etc. I.e. if you place a Faraday cage in a static field then the electric field inside the cage will equall zero (this does not apply to magnetic fields though). In the case of Em waves its a bit different. An EM wave will penetrates into a conductor by a certain amount and wil thus pass some of the EM wave through a metalic sheet (although greatly reduced). The amount of penetration is a function of the conductivity of the metal. If the conductor is a super conductor then the field will not penetrate at all. I hope this helps. I suggest that you do some searching/reading on the topic of the skin effect.

    Best wishes

    Pete
     
  14. Mar 11, 2008 #13

    Ben Niehoff

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    Earlier it was mentioned that any gaps in the cage need to be less than the wavelength of the radiation to be blocked. So, look up the operating frequency range of your cellphone carrier (I believe they are on the order of a few gigahertz, but I could be wrong). Divide the speed of light by the frequency to get the wavelength, and then make sure any gaps in your cage are about 10 times smaller than that.

    Pulling some numbers out of a hat, I calculate that the gaps in your Faraday cage should be no bigger than a few millimeters in diameter. But I don't know if my hat contains the right numbers.
     
  15. Mar 11, 2008 #14

    mgb_phys

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    For cell phones the wavelength is either 17cm /33 cm. Idealy you want a mesh < 10% of this and a decent conductivity - it get complicated with skin effects at these frequencies. Chicken wire where the wires are galvanised and then made into a mesh doesn't work very well because the conductivity between wires where they touch is low.
     
  16. Mar 12, 2008 #15
    Thank you all for your interesting replies. They show, how a supposedly simply thing can become exciting, if we try to really understand it.

    I figure that the openings in the cage are not the problem. Any gaps resulting from the ordinary cap not closing tight would be smaller than a millimetre (can't see any) and the frequencies used by mobile phones in Greece are the above.

    It is possible that the box was not earthed properly. The heater, being part of a central heating system, was connected to a mass of metall, yet it is painted and paint might be an insulator. I am lookng for something very big and metallic to repeat the procedure.

    Having read the posts, I suspect, that the key lies in the FREQUENCY of the wave involved. Might it be, that the electrons are too slow to arrange themselves in a way shielding the field? Could a greater conductivity be the key? We tried the whole thing with a big kettle that didn't attract my magnets (probably Al, gaps smaller than 1cm) and no shielding occured.

    I will search for the skin effect. Thanks again.
     
  17. Mar 12, 2008 #16
    Buy one of those plugs used for anti-static straps which connect a wrist strap to the earth pin of a socket (I know these exist in the UK, don't know about else where!) that should definitely give you a good ground connection - or ue the grounding point on things like oscillators, a bench PSU or a oscilloscope.
     
  18. Mar 15, 2008 #17
    Hai Angelos,
    Faraday Cage Main principle is GAUSS Law. if the charges are uniform around the surface of the object (Sphere) then the electric field inside the object is zero. this works well only if the selected object is Sphere because charges over the sphere is well distributed then any other shapes.
     
  19. Mar 15, 2008 #18
    I'm not sure that you are completely correct, a coaxial cable is an example of a Faraday cage type arrangement and it's not a sphere but it works very well. A faraday cage is just a common name for electrostatic screening, and while spheres may be the best at this other shapes, such as cylinders are not far behind them.
     
  20. Mar 19, 2008 #19
    Thank you all!
     
  21. Mar 19, 2008 #20
    Sorry for delay.
    First i apologies for what i said (it works well ONLY FOR SPHERE). Yes, what you are saying is correct. it works well for coaxial cables too. actually the concept is the surface must be an equipotential surface. In your case there are two possibilities to fail. First one is you are keeping your mobile inside the closed surface(object). That means you are keeping charge inside. so it must have some field inside i hope you know if charge exist then the field exist (it is slightly like vandicraft mechanism). Second is, if the surface is not properly shielded then the fast changing(high frequency) waves can get through. if it is a static fields(radio wave) they are weak it can be easyly blocked.

    The Gauss who finds the electric field inside the closed surface is zero, he used sphere for his equipotential object.
     
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