Faraday Cage - Principles of Operation

[BigJohn1]

I'm trying to understand how a faraday cage actually serves to provide electromagnetic shielding.

What I've read seems to suggest that a true "Faraday cage" is a mechanism that only applies to electrostatic charges; as best as I understand it the charges applied "cage" are distributed equally along the outer surface, which creates a uniform electric field, these fields somehow cancel each other. Though, I'm not sure about that because it seems to me that theory would be strongly dependant on the actual shape of the cage...

If a true Faraday cage does only applies to electrostatics, then what is taking place inside say, an automobile struck by lightning or shield providing protection from RF radiation? Those devices are subject to changing electrical fields, what keeps those fields from propigating inside the respective car/shield? I'm leaning towards what I understand about the skin effect, is that the right track? Would the car/shield be analogous to a solid conductor? If the shield in question is not made up of solid sheets of metal, but rather is an actual cage of conductors, would my best bet be examining the parallel current flow through the respective conductors?

I'm having a lot of difficulty finding straight answers on this, any help would be greatly appreciated. Feel free to throw in any pertinent equations.

Thanks.

-John

 

[logbook]

John,
you are on the right track. The Faraday cage is from Faraday’s early experiments on electrostatics. Things like the butterfly net and the ice pail experiments showed that charge resides on the outside of conducting bodies. If the cage is like an animal cage with vertical bars it would be a very poor RF shield. An unpolarised RF beam would get in, although certain polarisations would be prevented. A cage made of netting, with welded joints, is a much better approximation to a Faraday cage and would have reasonable shielding effect, provided the maximum opening was much smaller than a quarter wavelength of the incoming radiation.

Cars are not good RF shields, as evidenced by the number of signal strength bars on your mobile phone when you are inside the car. The openings are smaller than a half wave of the incoming RF.

You are right that the currents which flow in the surface are the cause of the shielding effect for RF. The skin effect merely limits the currents to the surface, rather than the whole bulk of the material.