I've been playing around with some contactless bank cards and an RFID reader app on my phone. As expected, if I wrap the card in foil, the reader no longer detects it. But I was surprised to find that if I place a layer of foil on a flat surface, put the card on top of it and the phone on top of that, it also fails to detect the card. Why does placing a barrier behind the card prevent communication?
The metal is detuning both the tag's antenna and depending on how close the phone is, the phone's RFID antenna too. When a piece of metal is placed in the near field area of an antenna it becomes coupled to the antenna and it's resonance frequency drops, the impedance decreases (causing a large signal loss) and the bandwidth widens (Q decreases). In an RFID tag, these little radios probably have very little transmitter power and a small reference ground plane. Both of these things make detuning the system very hard to have enough signal to communicate.
This is a very common problem in antenna design as many people around the world witnessed with iPhone4 as demonstrated in this video. Anything conductive can do this. So your hand with water in it, detunes your cell phone. Designers have to be aware of these cases and make sure the design is robust enough to overcome some of these conditions.