Roger Mills wrote:


If you carry it around, keep it in a lead sheath!

Silver is best, then copper, but aluminium foil seems quite good, as
tested with Oyster cards, which use the same near field communication
standards. Lead is likely to be relative non-conductive. Sandwiching
them between two Oyster cards may also trigger a multiple card error.

You can get foil lined card sheaths, and even foil lined card wallets
(e.g. google "rfid blocking wallet").

The foil probably detunes the circuit, as well as screening it.