Help with son's Faraday Cage Science Project

Discussion in 'The Projects Forum' started by fisce, Apr 3, 2012.

  1. fisce

    Thread Starter New Member

    Apr 3, 2012
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    I am working with my son on a science project to show the principles of a faraday cage. We want to place a walkie talkie inside a cage and demonstrate that it will not receive a signal. As an initial prototype we tried an aluminum plate with wire mesh covering the radio (see attached pic). (we grounded the mesh to the plate and then to the house's ground circuit and verified connectivity with an ohm meter). The walkie talkie still worked. We tried standing an aluminum tube on the plate with the radio inside and covered it with another aluminum plate, again grounded as before; and the radio still worked. We even wrapped the radio in a towel, and then wrap the towel in aluminum foil and then attached the foil to the house's grounding rod, and the radio still worked. The radio transmits on a frequency of 460 MHz. I know a faraday cage should work, but so far all we have proven is that they do not. Any idea what we are missing?
     
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Hi fisce, have you checked your house ground? Have you tried it outside, grounded to a long metal stake pounded into the dirt? Also, I notice you use an extension cord. I wonder if the increased resistance from the extension cord could be reducing its ability to ground properly. And one more thing--Have you tried using slightly less-fine mesh (chicken wire, for example)? Just a few things to try.

    Good luck!
    Regards,
    Der Strom
     
  3. wayneh

    Expert

    Sep 9, 2010
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    Most of the inexpensive walkies are about 49MHz, I think, so you might try one of those. Lower frequency = longer wavelength = easier to block

    A great learning experience, by the way. It's not always so easy to reconcile theory and observation and schools don't teach this.
     
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  4. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Yup, higher frequencies means shorter wavelength, which means the signals can "fit" through smaller spaces. Lower frequencies means longer wavelength which means it's easier to block. Thanks for mentioning that wayneh
     
  5. rfredel

    New Member

    Jan 23, 2011
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    Hello,

    the faraday cage like you do it, will only protect against electrostatic fields. To stop highfrequency radiation you need a close box of tin, because the wavelength is to short to be stop by the wire mesh. All parts of the cage must be connected together and then grounded.
    The best example for a faraday cage is a microwave oven. Try to put the radio inside the oven, and you will not hear the signal. Attention, don't start the oven. :D

    Regards

    Rainer
     
  6. KJ6EAD

    Senior Member

    Apr 30, 2011
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    You're using an FRS/GMRS radio in the 470MHz band so the wavelength is around 24" but since your radio has a 1/8 wave antenna the holes in your cage only need to be smaller than 3". That part you've got covered using your plate and aluminum window screen. What you don't have is at least a wavelength of separation between the cage wall and the antenna but again, anything over 3" should be effective in attenuating the signal.

    Set that up and test it's range, then if it's not enough attenuation add another layer at least 3" outside the first.

    Edit: The conductivity of aluminum is inherently good but will be significantly degraded by the oxides that form on it. You could improve the conductivity of your cage by weaving in a loose grid of copper drain lines spaced 1/2" or so apart. Just use whatever size wire will conveniently fit through the mesh holes and bus them all together at the edges.
     
    Last edited: Apr 3, 2012
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  7. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    I disagree. The theory behind a Faraday Cage is that it can cancel out any electromagnetic waves of certain wavelengths (or longer), depending on the size of the mesh. Also, the wavelength is NOT too short to be stopped by the wire mesh. You can find the wavelength by dividing the speed of light by the frequency. By the way, the speed of light is 300,000,000 meters per second.

    My money is on what KJ6EAD said. That makes the most sense.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    One thing strikes my attention, it is critical that the box be completely sealed. I have no problem with the mesh, but every part of the metal on the mesh must make electrical contact with the plate. There can be no holes in the joins (corners), anywhere.

    As long as the square holes in the mesh are at least 1/4 wavelength of the RF signal, it will work fine.
     
  9. fisce

    Thread Starter New Member

    Apr 3, 2012
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    I want to thank everyone for all the help. I tried the microwave oven idea and the walkie talkie still worked. I did the wavelength calculations and the wavelength at 460MHz is .652 meters or 25 inches. This seems like a large wavelength. From my reading it appear that the hole size on a faraday cage should be less than 1/20 of the wavelength which would mean my hole size should be no greater that about 1.0 inches. The mesh I am using is significantly less than that so this should work. Since it seems the principle is conducting of fields, I am thinking that the aluminum mesh may just not be conductive enough so I am ordering some copper screen to repeat the effort with. Does everyone agree that a faraday cage, if done right, in principle will stop this wavelength, or is there some other phenomena at work and I am just wasting my time?

    Thanks
     
  10. #12

    Expert

    Nov 30, 2010
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    I stopped a cell phone from ringing by wrapping it in aluminum foil 4 layers thick with a large fold over at each end. Nothing was grounded.

    I also put the cell phone in a prefab aluminum box and it still answered. Then I put that aluminum box in another aluminum box and that stopped the cell phone. None of this was grounded.

    Point? Grounding isn't mandatory. It's the number of tiny leaks that allow a signal to get in. If your first layer leaks, another layer will often fix the problem.
     
    Last edited: Apr 5, 2012
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  11. fisce

    Thread Starter New Member

    Apr 3, 2012
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    Oh I like the multiple layer option. I will keep that in mind. I am looking at copper screen material and one product has the following specs, but I am not sure what I am looking for here.

    FIELD TYPE
    USC-44 TYPICAL ATTENUATION
    Magnetic​
    15 dB at 1 kHz, 40 dB at 15 kHz, 70 dB at 100kHz
    Electric​
    >80 dB at 1kHz, 100 dB from 15 kHz to 10 MHz
    Plane Wave​
    100 dB at 400 MHz
    >85 dB at 1 GHz
    >75 dB at 2 GHz
    Microwave​
    60 dB at 10 GHz

    Since I am working at 460 MHz, can anyone interpret these numbers and tell me if this material is any good?

    Thanks
     
  12. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Remember that if you are testing a walkie- talkie with maybe kilometres of potential range at just a few metres, the system has some tens of decibels of dynamic range in hand. Considering the inverse square law, that means that to be effective your cage may need to attenuate something like a million times, maybe more.

    The attenuation provided by any real-world screen can never be perfect, and achieving large attenuation requires care, hence the costly arrangements needed for things like screened chambers for RF measurements.

    As well as making the best screening cage that you can, could you try using a receiver with a bit less margin, such as a broadcast radio tuned to a not-too-powerful signal? If a walkie-talkie has to be used, you might also try to get one which is not too powerful, perhaps the kind that kids use, rather than anything with any claims to long range. Good luck!

    Edit: if you can't initially get the screen good enough to work at very close range, try finding out what the actual range is, as a guide to whether different screens are better or worse. Remember that doubling the distance should drop the signal four times.
     
    Last edited: Apr 5, 2012
  13. wayneh

    Expert

    Sep 9, 2010
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    That's a good example of how size matters. Your attenuation should be in the 85-100 dB range, according to those specs.

    You know, sensitivity could be one of your issues here. Your walkie talkie might work fine with another nearby transmitter even when the cage is knocking down the signal 100-fold. The microwave really should have reduced the signal a LOT. A little scary it didn't do more, but it might take very little signal for the radio to appear to work.

    And getting back to theory for a second. If the cage is porous (not solid metal), doesn't each pore become a point source for the signal? Yes, the original signal is severely reduced, but not eliminated?
     
  14. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Posts 11 & 12 let a lot of air out of my balloon. I was under the impression (I think so was fisce) that all you had to do was build a wood frame and stretch some wire mesh over it and you had a "blocks anything" faraday cage. Glad I read through this thread.

    I didn't want to highjack fisce's question so I made a new post. I have a related question
     
    Last edited: Apr 5, 2012
  15. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Yes, those numbers are much worse than my "guesstimate" clearly the cage has to attenuate a lot. As for the microwave, I am not sure that you should be too worried about a bit of leakage at maybe a fifth of its design frequency. In particular, I doubt whether the coupling of the door to the cabinet will be much good down there, so that there could be appreciable leakage through it.
     
  16. BackyardBrains

    Member

    Aug 26, 2010
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    We are designing a similar experiment with Faraday cages as a teaching tool: See our write up here. I tried blocking a cell phone with our cage, and it did not work as well. I then wrapped a cell phone tight with our cage mesh without gaps and the cell phone still worked. I posted a question oh here to see if I was reading the equations wrong (which I wasn't). So I was still not quite sure how a Faraday Cage works. Best of luck to your son. Its a cool project.
     
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