As a former radio engineer, i feel i should point out a couple of things. First, most of the information presented is correct, shielding is provided by a Faraday cage and the openings in it have to be much smaller than the wavelength of radio you are interested in shielding.

But the devil is in the details. The cage, or box has to be electrically continuous all the way around, otherwise it will leak. This means that any opening (door, lid etc) has to have an electrically conducting lip in order for the cage to be continuous. So ammo cans may not be a good choice, they only have a rubber seal. Shiny new paint cans might be better. If you look in radio gear, especially old HF rigs, you will notice that some sections of the chassis are bolted on with screws every one inch or less. This is for electrical continuity, not strength. Some may even have beryllium-copper finger stock around the openings to improve the conductivity.

Second, i'm pretty sure that the cage itself doesn't need to be grounded in order for shielding to be effective. As mentioned in earlier posts, the charge redistribution in the conducting cage is what provides the shielding by cancelling the field internally. One does not need to add or remove charge to make this happen (the reason for the ground).

What might be an interesting experiment is to take your cell phone and wrap it completely in aluminum foil, making sure that the seams are folded tight. Then try calling it to see if the RF from cell site can penetrate the shield. I would bet that this works regardless of whether the aluminum foil is grounded or not. Then you could poke holes of various sizes to see how big or small it has to be to let the RF in. It's also probably not a good idea to leave the phone in there for too long, it's probably trying to transmit at full power to contact a cell site...

As far as communicating after a nuclear bomb EMP, i would use tin cans and a length of string. <img src="/images/graemlins/smile.gif" alt="" />