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  #1  
Old 03-26-2010, 05:51 PM
code_grey code_grey is offline
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grounded and not grounded Faraday cage - what's the difference?

I guess a stupid question, but the Wikipedia article is not clear on this point. If we want to have a Farday cage to keep out radio waves and similar, does it matter if it is grounded or not?
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  #2  
Old 03-26-2010, 11:21 PM
ZenBeam ZenBeam is offline
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Not for keeping out electromagnetic radiation, no. I'm trying to come up with a situation where it would matter to something inside the cage, but can't think of one.
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Old 03-27-2010, 07:40 AM
Napier Napier is offline
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Ditto ZenBeam. In fact, I think you couldn't tell from inside a Faraday cage whether it was grounded. Said in a way that better helps bring insight to the question, I think experiments conducted entirely inside a Faraday cage would not be able to tell about a distant outside location whether objects there are connected to the cage. "Ground", after all, refers to an electrical connection to some small part of planet Earth, which itself is partly somewhat conductive (especially the oceans) but partly pretty nonconductive (the crust, rocks).
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Old 03-29-2010, 04:43 AM
Cardinal Cardinal is offline
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So the EM radiation causes an electric current that races around the cage and eventually loses its energy to heat due to resistance, and you don't need to ground the cage?
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Old 03-29-2010, 11:53 AM
ZenBeam ZenBeam is offline
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Some would be lost as heat, but most of it would scatter or reflect off of it. The EM wave doesn't have any charge, so there's no charge that you need to bleed off with a ground wire.

If you have a situation where the cage is protecting the inside from lightning or some other charge flow, then grounding could matter. Even then, the field inside will be zero, unless the current flow overwhelms the conductivity of the cage.
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Old 03-29-2010, 01:01 PM
kanicbird kanicbird is offline
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Would not the ungrounded cage re-radiate the EM wave much like how a metal detector works(The EM wave would induce a current which would induce a EM wave.)? So the cage would not allow the original EM wave through but allow the re-radiated wave unless grounded.

Last edited by kanicbird; 03-29-2010 at 01:02 PM..
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Old 03-29-2010, 01:44 PM
Leaffan Leaffan is online now
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In my many, many years of electronics manufacturing and quality auditing experience I have never heard of a grounded Faraday cage for storing, handling, or shipping electronic components.
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Old 03-30-2010, 06:04 AM
Napier Napier is offline
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The field inside the cage would be flat, and there would be a gradient of zero (unless you were also generating fields with some power source inside). I think you could look at the surface of the cage as reflective, both for radiation that originates inside it and radiation that originates outside it. You could say any wave induces a current that induces another wave, so it reflects; or you could say it was a reflector, and there is another virtual universe on the far side of it, like you can think of mirrors in optics.

There is a bit of a head-scratcher for me, though - if a wave causes a current which reradiates another wave, is the current so close to the surface that the wave is only reradiated back out on the same side of the cage wall? If the current circulates throughout the depth of the cage wall, why aren't there equal radiations coming out of both surfaces (inner and outer)? I guess the current must circulate at so shallow a depth that the radiation can only be out of the same surface. I feel ashamed, too, because I should know this, but maybe the coffee hasn't worked yet...
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  #9  
Old 03-30-2010, 06:34 AM
CalMeacham CalMeacham is offline
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A Faraday cage doesn't need to be grounded -- a FC or a solid metal container will arrange charges on its exterior to zero out any internal field.

But anyone building a Faraday Cage will almost inevitably ground it as a matter of course. I know I did when I built and repaired them at a job a couple of years ago. You typically have some electronics inside a Faraday Cage that you want protected from the environment outside*, and you want to have a ground for those electronics. You have some sort of isolator for the incoming power, and that requires a ground. So practical Faraday cages usually have a grounding strap. Besides, it's good engineering poractice -- why should you risk a discharge from a floating cage when it's so easy to eliminate?




*Not necessarily, I realize. You might have a desktop cage that has nothing with active electronics inside, or with a battery source. But I'll bet 95%+ of Faraday cages are hooked to external power.
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Old 03-30-2010, 08:38 PM
Cheshire Human Cheshire Human is offline
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Quote:
Originally Posted by ZenBeam View Post
Not for keeping out electromagnetic radiation, no. I'm trying to come up with a situation where it would matter to something inside the cage, but can't think of one.
If an ungrounded one is hit by lightning, then the closest point to actual earth ground will probably melt from the current flowing through it, but other than that, not much difference that I can think of. Hope your chair isn't sitting on that point. Major Assburn.
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Old 03-30-2010, 08:45 PM
Cheshire Human Cheshire Human is offline
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Quote:
Originally Posted by kanicbird View Post
Would not the ungrounded cage re-radiate the EM wave much like how a metal detector works(The EM wave would induce a current which would induce a EM wave.)? So the cage would not allow the original EM wave through but allow the re-radiated wave unless grounded.
Yes, certainly, but that doesn't deliver any information about what's happening inside the cage, and only delivers information about what's happening external to the cage, equivalent to the EM reflecting off of it. In other words, they find out details about the wave that is being used to try to probe the cage. Ground it, and they would get nothing at all. Totally useless. Even the reflected wave is useless, since it only carries information about the EM beam used to probe in the first place. Again, useless.

Last edited by Cheshire Human; 03-30-2010 at 08:47 PM..
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  #12  
Old 03-30-2010, 08:57 PM
Cheshire Human Cheshire Human is offline
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Originally Posted by Leaffan View Post
In my many, many years of electronics manufacturing and quality auditing experience I have never heard of a grounded Faraday cage for storing, handling, or shipping electronic components.
The conductive plastic bags you ship them in are Faraday cages, although ungrounded. Not very good ones, either. But, it would take some serious high-voltage to fry the components inside, and the goal is safe shipping, not high-voltage/current resistance. They don't NEED to be good ones.
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Old 03-30-2010, 09:05 PM
Cheshire Human Cheshire Human is offline
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Quote:
Originally Posted by Napier View Post
The field inside the cage would be flat, and there would be a gradient of zero (unless you were also generating fields with some power source inside). I think you could look at the surface of the cage as reflective, both for radiation that originates inside it and radiation that originates outside it. You could say any wave induces a current that induces another wave, so it reflects; or you could say it was a reflector, and there is another virtual universe on the far side of it, like you can think of mirrors in optics.

There is a bit of a head-scratcher for me, though - if a wave causes a current which reradiates another wave, is the current so close to the surface that the wave is only reradiated back out on the same side of the cage wall? If the current circulates throughout the depth of the cage wall, why aren't there equal radiations coming out of both surfaces (inner and outer)? I guess the current must circulate at so shallow a depth that the radiation can only be out of the same surface. I feel ashamed, too, because I should know this, but maybe the coffee hasn't worked yet...
You are essentially right. They work as reflectors, with the "perfection" being directly related to the conductivity of the cage material. Gold would be best, but even that is not a "perfect" reflector. That said, aluminum, or even steel window screen is "good enough" to prevent any significant information about what's going on inside to be detectable outside the cage, even using steel. But put some sort of detector almost in contact with the cage, and some information will leak, for the reason you mentioned. Best material to make a Faraday cage out of would be a super-conductor. Too bad, we ain't got any. So put THICK concrete walls between the actual cage, and anyone's detector. It should work.

Last edited by Cheshire Human; 03-30-2010 at 09:06 PM..
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  #14  
Old 03-30-2010, 09:10 PM
ZenBeam ZenBeam is offline
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Quote:
Originally Posted by Napier View Post
There is a bit of a head-scratcher for me, though - if a wave causes a current which reradiates another wave, is the current so close to the surface that the wave is only reradiated back out on the same side of the cage wall? If the current circulates throughout the depth of the cage wall, why aren't there equal radiations coming out of both surfaces (inner and outer)? I guess the current must circulate at so shallow a depth that the radiation can only be out of the same surface. I feel ashamed, too, because I should know this, but maybe the coffee hasn't worked yet...
If you only consider the fields due to the currents on the surface of the cage, those currents radiate into the cage, but the fields they radiate are opposite of the incident field without the cage present. The (total) electric field is the sum of the incident field and the field due to the currents, and this total field is zero in the conductor and inside the cage. For perfect conductors, this current would be on the surface. For real, good conductors, there's some penetration, but the current is still near the conductor surface.
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Old 03-30-2010, 10:41 PM
engineer_comp_geek engineer_comp_geek is offline
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Quote:
Originally Posted by Napier View Post
"Ground", after all, refers to an electrical connection to some small part of planet Earth, which itself is partly somewhat conductive (especially the oceans) but partly pretty nonconductive (the crust, rocks).
The earth isn't that bad of a conductor. If you just stick the ends of a multimeter into the soil you aren't going to get a very good reading, but power systems back in the old days sometimes used the earth as their return wire (saved quite a bit on wiring costs). In some areas the earth's conductivity isn't quite as good as in others, and in some areas how well the earth conducts depends on how wet or dry it's been in the last few months. That's why we don't do it that way any more, but those systems mostly worked. Even today we rely on earth being a good conductor so that we can use it for our safety ground on residential power systems.

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Originally Posted by CalMeacham View Post
But I'll bet 95%+ of Faraday cages are hooked to external power.
That's probably true, for exactly the reasons you mentioned. Most of the time you are protecting or isolating some sort of equipment, and that equipment needs a power source, and that power source needs to be protected and/or conditioned.

Most purpose built "Faraday cages" that you buy are also made out of copper screen instead of solid metal. As long as the holes in the screen are smaller than the wavelength of the EM frequencies you are trying to protect against, screen works just as well as a solid piece of metal.

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Gold would be best
Why do you say that? Copper is a better conductor than gold is. So is silver.
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Old 03-31-2010, 12:17 AM
Cardinal Cardinal is offline
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It is? I thought satellites and such used gold connections for electronics.
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Old 03-31-2010, 10:20 AM
3waygeek 3waygeek is offline
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It is? I thought satellites and such used gold connections for electronics.
More for corrosion resistance than anything else -- gold doesn't tarnish/oxidize, but copper & silver do.
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  #18  
Old 03-31-2010, 11:35 AM
gaffa gaffa is offline
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Wendy Carlos' New York City recording studio is enclosed in a Faraday cage. She sandwiched brass screen between two layers of wallboard.
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  #19  
Old 03-31-2010, 12:27 PM
meanoldman meanoldman is offline
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Not really intended as a hijack: Does aluminum or steel house siding act like a Faraday cage? I'd think the roof would still be open to EM (usually) but wouldn't the siding still block a lot of signal anyway?
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Old 03-31-2010, 01:12 PM
engineer_comp_geek engineer_comp_geek is offline
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If the siding completely covered the house then yes, it would make a Faraday cage, but a Faraday cage can't have holes in it. Since a house has things like windows and doors you'll still get radio signals into the house. Cell phones and radios and the like will usually work fine inside of a house with metal siding.

In recent years, metal frame homes with metal roofs have become more popular, and one of the complaints about them is that you don't get decent radio reception inside of one. A lot of steel frame buildings are also close enough to a Faraday cage that you can't get decent cell phone reception inside of them. My cell phone doesn't usually work in my office, but if I walk over to the break room (which has a window in it) I can get good reception.

A car's body (as long as it is metal) also acts like a somewhat imperfect Faraday cage, which is why a car protects you from lightning. Cars aren't perfect Faraday cages, and there have been cases of lightning coming in through the windows, but overall they are fairly safe, and certainly are much safer than being outside in a thunderstorm.
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Old 03-31-2010, 01:22 PM
CalMeacham CalMeacham is offline
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Neither a house nor a car makes a really good Faraday cage. From experience, I can aver that holes to the outside world can be insidious and elusive. While repairing a broken Faraday Cage -- whose walls were double thicknesses of copper mesh -- I carried my cell phone and a radio into it and turned them on. They still worked perfectly, despite the fact that I was almost completely encased in apparently seamless mesh. Carrying them around I soon found out where tears and breaks in the mesh were, and was able to repair them. When I was finally finished, my radio didn't work any more, and neither did the cell phone. But it wasn't easy to find and repair all the holes -- some of which were linear rips in the mesh.

By comparison, the windows in a metal-covered house or a car are enormous. If a tear so small you can't see it can let signal leak in, any gap as big as a window you can easily see through is hopeless, and the "Faraday cage" with that big a gap will leak signal like a sieve.
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Old 03-31-2010, 04:36 PM
Napier Napier is offline
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Quote:
Originally Posted by engineer_comp_geek View Post
The earth isn't that bad of a conductor. If you just stick the ends of a multimeter into the soil you aren't going to get a very good reading, but power systems back in the old days sometimes used the earth as their return wire (saved quite a bit on wiring costs).
Isn't the old power system method reliant on an enormous impedance shift for transmission? That is, at least today, transmission systems work with impedances maybe a million times higher than end users. I mean, my appliances consume power at 120 volts, but there are long distance transmission lines operating at over 120,000 volts, and my share of the load is drawing 1000 x fewer amps at that voltage too.

If you tried to transmit 120 volt power for appliances using dirt for the return, it probably wouldn't even work an inch away from the power station.
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Old 04-01-2010, 07:58 AM
Squink Squink is offline
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grounded and not grounded Faraday cage - what's the difference?

Sigh, the grounded Faraday cage may not go to the Prom this weekend.
The not grounded Faraday cage may go to the Prom this weekend, but if it comes home at 3AM, with a Hickey, it will quickly find itself grounded too.
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