Watis wrote:I keep my car's proximity keys in a steel biscuit tin overnight.
I've just checked and found that, with the keys in the tin, the car can still detect them if the tin is within a foot or so of the lock.
So even that thickness of metal is not a perfect solution but hopefully sufficient when the tin is kept near the centre of the house.
Watis
Not to invalidate what you said but your proximity keys and an RFID card are somewhat different physics.
The keys are standard familiar radiated radio. The waves decouple from the source and travel away at the speed of light.
The card uses "near field" EM fields - it's more like a high frequency transformer that couples both power and data over very short distances. It is totally useless at distances over a few cm. The RFID name is something of a misnomer.