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Also in regards to the high voltage cage your talking about that's a completely different effect taking place. They are using the nature of electricity to take the path of least resistance going around the cage instead of the higher resistance occupants inside of it. If you've ever seen the videos of the band Arc Attack they rely on the same principles to stay safe from their tesla coils running at somewhere around 1M volts. Not the same requirements for RF energy though.
Actually in the very article your quoting if you continue reading it does discuss the importance of grounding in regards to shielding. As someone with a background in rf engineering I can say for a fact that although an ungrounded cage will provide some shielding the attenuation factor is minimal. I can fairly easily transmit in and out even on frequencies in the mhz range. Grounding the same cage makes it nearly impossible until you exceed 8-10 ghz.
There is one critical thing that you're missing. In order for a Faraday cage to work it needs a low impedance connection to ground they work by dissipation of the electromagnetic energy. You can actually build one with fine wire mesh, so long as the whole thing is electrically bonded to ground you can even have a door! As a side note a microwave is actually a Faraday cage if you find a junk one for cheep you can just cut the plug off and connect the ground wire to a copper water pipe and Bob's your uncle, ready made cage!
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