# Faraday cage



## adcantor (Nov 28, 2012)

Hey Guys,

This is my first post here. I've enjoyed reading a lot of your posts, but now have a question myself. 

Has anyone tried building a trash can faraday cage? I just tried building one with aluminum mesh and can't seem to get me wifi signal to drop. I don't know where I'm going wrong. 

I was under the impression that the cage must be grounded and the trash can resource I found online doesn't mention it at all. 

I just want to get this done and move on with my prepping. Any suggestions?


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## hiwall (Jun 15, 2012)

Just use a metal trash can with a metal lid. Done.


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## Marcus (May 13, 2012)

It's not big enough to require grounding. Grounding is for room-sized cages and is used to control the heat buildup.
The frequency of gamma rays (30-300 EHz) requires a shielding mesh of <1 pm (picometers) which in all practically means a solid metal cage. http://en.wikipedia.org/wiki/Electromagnetic_spectrum
The mesh size has to be smaller than the wavelength in order to block the signal.

What would I suggest you do?
Replace the screen with aluminum foil using double-sided tape, cover the top and sides with particle board glued to the aluminum foil, then tape another layer of aluminum foil so you have a nested Faraday cage.


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## adcantor (Nov 28, 2012)

Thanks for the help. I'll try the garbage can way first, since it is less conspicuous - my wife hid my cage in the garage, so she wouldn't have to explain it to anyone. A trash bin won't be a conversational piece.


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## GrinnanBarrett (Aug 31, 2012)

If I am not mistaken you need to line the garbage can with something that does not conduct. I have seen ones lined with cardboard around sides and in bottom and in the lid. I am not an expert here so someone give me a heads up. GB


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## jsriley5 (Sep 22, 2012)

yes you do GB or at least that is the conventional and totally sensible wisdom on the subject.


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## Caribou (Aug 18, 2012)

You got it GB. If whatever you are trying to protect touches the metal of the cage the cage becomes an antenna. Insulation is critical. For small items try wrapping in bubble wrap as insulation then foil, bubble wrap, foil, and a final bubble wrap.


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## labotomi (Feb 14, 2010)

Insulating the interior is not required. The redistribution of charged particles along the interior effectively cancels out any electromagnetic field inside the container. 

I think there's some misunderstanding on how a faraday cage works. It doesn't shunt the field around the cage like an electrical conductor which is what I think many people are thinking.


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## Caribou (Aug 18, 2012)

http://www.futurescience.com/emp/emp-protection.html
http://www.emprimus.com/

Here are a couple of interesting links.


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## COMEANDTAKEIT (Dec 4, 2012)

Sorry I can't help but the idea of preserving electronics in the event of a emp is intriguing to me. Keep us posted as I would be interested in any progress.


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## LincTex (Apr 1, 2011)

I have all my "spare" HAM radio eqip, old CB radios and SW receivers wrapped in "pink" ESD-safe bubble wrap, in a galvanized garbage can, with the lid sealed with aluminum tape around the circumference, sitting on a piece of plywood on the wooden floor of a closed shipping container. I think I am OK. 

Spare NiMH and Li-Ion batteries and their chargers are stored similarly, but in an old ammo can instead. Solar panels are all over, some in use outside and some spares inside.


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## adcantor (Nov 28, 2012)

Sorry for the delay, but I made a couple using the cans and they work great. I used a radio to test it out (thanks to a YouTube video demonstration). 

I lined them with a construction paper-like material used for roofing. You can hear the radio go from a nice loud, clear signal to a bunch of white noise. Super cool. 

Thanks everyone. Let me know to you wanna see pics, but its pretty simple to find vids on YouTube.


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## popcorn590 (Aug 29, 2010)

labotomi said:


> Insulating the interior is not required. The redistribution of charged particles along the interior effectively cancels out any electromagnetic field inside the container.
> 
> I think there's some misunderstanding on how a faraday cage works. It doesn't shunt the field around the cage like an electrical conductor which is what I think many people are thinking.


I hate to say this but with my little electrical classes and experiance, if you have a cage Faraday type grounded and an electical field touches it the electricity will take the least line of resistance through the ground going around the ourside of the cage touching nothing inside, because the eletricity will not enter the cage if it has an easier way to ground. Insulating the items inside the Faraday cage is essential, because if the item is touching the cage the electricity will go through that item because you have allowed another path for the electricity to go through. If nothing else is touching the item inside the cage, the electricity will exit again through the cage to the ground. By the way it does shunt electricity if that charge will go around the outside to the ground. Just the same as the man standing on the big ball and has electricity surrounding him, that thye used to dislplay on shows. Essentially that is a faraday cage, where the electricity does not surge through the man since he is protected by the cage and not grounded, same as the items inside a faraday cage, the electricity travels around the oustide of the cage, because the items inside are not grounded and protected from the eltro-magnetic discharge of the solar flare.


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## Marcus (May 13, 2012)

popcorn, 
You're confusing a potential (charge) with electricity (the flow of electrons.) Think of a battery. If it isn't connected to a circuit, there will be no electrical flow. Yet it still has a potential. With an EMP, the charge on the cage is a surface charge. One of the effects of the roughly 50,000 V/m from an EMP is a buildup of heat which is why large Faraday cages need grounding. But inside the cage, there is no potential. Immediately after an EMP, I would use a grounding strap of some sort before opening the cage so that the charge would safely dissipate. But the potential will naturally dissipate over time to the Earth.


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## popcorn590 (Aug 29, 2010)

Marcus said:


> popcorn,
> You're confusing a potential (charge) with electricity (the flow of electrons.) Think of a battery. If it isn't connected to a circuit, there will be no electrical flow. Yet it still has a potential. With an EMP, the charge on the cage is a surface charge. One of the effects of the roughly 50,000 V/m from an EMP is a buildup of heat which is why large Faraday cages need grounding. But inside the cage, there is no potential. Immediately after an EMP, I would use a grounding strap of some sort before opening the cage so that the charge would safely dissipate. But the potential will naturally dissipate over time to the Earth.


So if the faraday cage is a total enclosure, and grounded to a good ground the items in the cage should be protected from an EMP and a large CME (ours is in the basement). I gather it is like an old TV tube, before we worked on them you have to disapate the chage in the tube since is did retain it for a while.


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## labotomi (Feb 14, 2010)

popcorn590 said:


> I hate to say this but with my little electrical classes and experiance, if you have a cage Faraday type grounded and an electical field touches it the electricity will take the least line of resistance through the ground going around the ourside of the cage touching nothing inside, because the eletricity will not enter the cage if it has an easier way to ground. Insulating the items inside the Faraday cage is essential, because if the item is touching the cage the electricity will go through that item because you have allowed another path for the electricity to go through. If nothing else is touching the item inside the cage, the electricity will exit again through the cage to the ground. By the way it does shunt electricity if that charge will go around the outside to the ground. Just the same as the man standing on the big ball and has electricity surrounding him, that thye used to dislplay on shows. Essentially that is a faraday cage, where the electricity does not surge through the man since he is protected by the cage and not grounded, same as the items inside a faraday cage, the electricity travels around the oustide of the cage, because the items inside are not grounded and protected from the eltro-magnetic discharge of the solar flare.


Faraday's experiment showing this phenomenon was conducted with a metal bucked on a wooden stool that prevented it from being grounded.

Electricity doesn't try to flow to "ground". It flows from high potential to low potential. Ground is the low potential in electrical distribution systems because they're designed and built for that to be the case. Some electrical systems are ungrounded (more reliable). In that case there is no actual ground. Touching a conductor live conductor in this case wouldn't provide a path for current to flow because the "ground" isn't a part of the circuit.

*for anyone wanting to nitpick, I realize that capacitance can provide a situation where current would "flow" in high frequency AC, but that's beyond this topic.


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## Marcus (May 13, 2012)

popcorn590 said:


> So if the faraday cage is a total enclosure, and grounded to a good ground the items in the cage should be protected from an EMP and a large CME (ours is in the basement). I gather it is like an old TV tube, before we worked on them you have to disapate the chage in the tube since is did retain it for a while.


The issue with keeping a permanent ground attached to a Faraday cage is that it is possible that whatever is acting as the ground (like a metal building or metal pipe) will also act as an antenna.

As Labotomi has already pointed out, in a CME type event, the concern is with long runs of wire (power lines) rather than the effects of the gamma radiation as in an EMP. You don't need a Faraday cage for a CME. You just need to make sure whatever you wish to protect is unplugged from the power plug.


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