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Faraday Cage


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#1 chinmayrshah

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Posted 22 February 2012 - 03:39 AM

How does Faraday Cage effect to communication? If the cage can take up an external charge on its own, will it hinder in transmitting signals from inside? Can it hinder a 1-20GHz communication band if designed to resist few hundred GHz communication?

Thanks in Advance!
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#2 swansont

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Posted 22 February 2012 - 12:54 PM

Generally speaking it will inhibit communication at frequencies at or below the design point, and will stop the signal going in either direction. So if it stops 100 GHZ, it should stop anything below that. The mesh would allow higher frequencies to pass through, since they have smaller wavelengths, but the longer wavelengths will still be blocked.
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#3 Klaynos

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Posted 22 February 2012 - 01:10 PM

There are cases of anomalous transmission through subwavelength holes though.

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#4 chinmayrshah

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Posted 23 February 2012 - 02:24 PM

So what about the info which is described in this link?

http://www.intelsat....lar-weather.asp

If the satellite parts are protected by multi-layer coverings inducing Faraday Cage effect, how can they transmit signals either way? One more question. If the IntelSat is using Faraday Cage effect in its assemblies does that mean the solar flares are of radiations of not more than few GHz?

I have also seen in the Faraday Cage link of Wikipedia, where they quote:

"Effectiveness of shielding of a static electric field depends upon the geometry of the conductive material. In the case of a non-linearly varying electric field, and hence an accompanying varying magnetic field, the faster the variations are (i.e., the higher the frequencies), the better the material resists penetration, but on the other hand, the better it passes through a mesh of given size. In this case the shielding also depends on the electrical conductivity of the conductive materials used in the cages, as well as their thicknesses."

This creates a bit confusion to me now. Can you please solve it?

Thanks in advance
Chinmay :)
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#5 Klaynos

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Posted 23 February 2012 - 05:20 PM

So what about the info which is described in this link?

http://www.intelsat....lar-weather.asp

If the satellite parts are protected by multi-layer coverings inducing Faraday Cage effect, how can they transmit signals either way?


I read it as only certain parts of the device are in a Faraday cage, so the electronics I'd assume and then the antenna are out side of it. Otherwise you could envisage a cage with a window with a metamaterial/plasmonic filter so that only your comms frequencies are transmitted but I'd imagine that was a little heavy to be worth putting in.

One more question. If the IntelSat is using Faraday Cage effect in its assemblies does that mean the solar flares are of radiations of not more than few GHz?


It depends on the precise design of the cage. I would assume for a Faraday cage to work across a large frequency range it would need to be solid (so not a mesh) and of a material which acts as a perfect electrical conductor (PEC) over as larger range as possible. As soon as you need to put any connector through (to say send a signal to your antenna) you will damage the integrity of the cage.

I have also seen in the Faraday Cage link of Wikipedia, where they quote:

"Effectiveness of shielding of a static electric field depends upon the geometry of the conductive material. In the case of a non-linearly varying electric field, and hence an accompanying varying magnetic field, the faster the variations are (i.e., the higher the frequencies), the better the material resists penetration, but on the other hand, the better it passes through a mesh of given size. In this case the shielding also depends on the electrical conductivity of the conductive materials used in the cages, as well as their thicknesses."

This creates a bit confusion to me now. Can you please solve it?

Thanks in advance
Chinmay :)


A Faraday cage must be made out of "metal" (really I believe a PEC) but what counts as a metal changes depending on the frequency range you are considering. I work in the very far infrared, where many semiconductors can actually be considered as metals. This depends on a materials permittivity and permeability. A PEC has an infinite imaginary permittivity, many materials can be approximated as PEC at microwave (GHz) frequencies. This falls down the closer to the plasma frequency of the material you get.
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#6 chinmayrshah

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Posted 23 February 2012 - 06:08 PM

Can we have a Faraday cage filtering a very specific wavelengths? Can we synthesize such a cage which can allow (a two way comm) only for a fixed band of signals, separating out the others? How are the satellites protected from the solar flares then? Isn't it advisable to put the entire satellite in a Faraday cage till the solar flares get diminished and then restart them after that?
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#7 DrRocket

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Posted 23 February 2012 - 06:45 PM

This will get out of hand and people will be talking past one another unless there is agreement on what one means by a "Faraday cage". There is the ideal Faraday cage of adademic classes on electromagnetism. There is also the much less ideal Faraday cage as used in engineering applications -- complete with non-ideal conductors, penetrations and imperfect seals.
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#8 Klaynos

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Posted 23 February 2012 - 07:18 PM

This will get out of hand and people will be talking past one another unless there is agreement on what one means by a "Faraday cage". There is the ideal Faraday cage of adademic classes on electromagnetism. There is also the much less ideal Faraday cage as used in engineering applications -- complete with non-ideal conductors, penetrations and imperfect seals.


This is likely the internal problem I'm having thinking about this...

Can we have a Faraday cage filtering a very specific wavelengths?


I don't think it would be a Faraday cage, you can build a device that absorbs one frequency but not others, a hollow glass sphere would be an example of this. It allows through optical light but other frequency ranges are blocked depending on the glass used.

Can we synthesize such a cage which can allow (a two way comm) only for a fixed band of signals, separating out the others?


I could envisage something where your mesh has a passband, but the bandgap, the region blocked would not cover the rest of the electromagnetic spectrum.

How are the satellites protected from the solar flares then?


By using Faraday cages around the things most likely to break from hitting them.

Isn't it advisable to put the entire satellite in a Faraday cage till the solar flares get diminished and then restart them after that?


That sounds like an engineering nightmare. And very expensive.
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#9 DrRocket

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Posted 24 February 2012 - 03:50 PM

This is likely the internal problem I'm having thinking about this...


The ideal version of a Faraday cage comes from the simple fact that one sees in an introductory electomagnetism class that there is a zero internal electric field for a closed comductive surface on which is placed a static charge.

That is then extended from the DC case to dynamic fields under the assumption that the surface remains closed and there is zero relaxation time for any disturbance to the charge distribution. Note that thus far nothing has been said about frequency (in the vernacular we are dealing with "DC to daylight".)

Once you dispense with these unrealized idealizations, enter the real world, and introduce real materials, with real relaxation times, real resistivity, real seals and real penetrations, etc., frequency of the fields of interest and details of the construction of the "cage" become very important, as do the tolerances imposed on shielding effectiveness as a function of frequency. You have now entered the world of electromagnetic compatibility engineering, and it gets complicated fast.

A cardboard box is a fairly good shield against visible llight. It is pretty useless against radio signals or x-rays.
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#10 swansont

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Posted 24 February 2012 - 06:18 PM

A cardboard box is a fairly good shield against visible llight. It is pretty useless against radio signals or x-rays.


True, but I think one has to acknowledge that a Faraday cage is a specific type/method of shielding, distinct from the action of a cardboard box.
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#11 DrRocket

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Posted 24 February 2012 - 06:58 PM

True, but I think one has to acknowledge that a Faraday cage is a specific type/method of shielding, distinct from the action of a cardboard box.


Of coourse. But it illustrates my point that what constitutes a practical Faraday cage is a function of the specific application and one's immediate concerns.

A Faraday cage is not so specific as some might think, but in practical applications is a rather qualitative description. That is part of the point. In fact, since a true Faraday cage has no penetrations, I can think of no examples of such in "real life". It is not black and white, just lots of shades of gray. So when you are told that something is a "Faradat cage" it is a good idea to take a look and see what the term means in the particular case at hand.
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