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I keep hearing that for Faraday cages but its not clear to me why.

Why should it be grounded?

The cell signal in the Faraday cage causes a mirror signal on the cages inner surface and the cage’s conductivity ensures that the potential is zero elsewhere in the cage.

Why do I need the ground? Is it a consequence of not having a perfect conductor?

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The case itself becomes an antenna if it's not and just re-resonates the signal.

Does it though? At what frequency? In the limit of f=DC this is trivially not true.

Also, if that were the case, an airplane couldn't be made into a Faraday cage.

Also, the cage+Earth system would itself be an antenna.

If you need ground for a topologically imperfect cage, maybe… but I doubt it.

I think (but I don't know!) this is like the legends surrounding engine torque car guys believe. Electrical engineers putting everything to ground because they got burned

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