In article , TC wrote:
It is possible - since both RF tags and Oyster use radio waves for power.
The two fundamental differences are that an RF tag has a resistor and oyster
a smartchip - and a RF tag has a spiral loop and the oyster a single loop*.
The RF tag idea is that the resistor changes the radio frequency (which gets
retransmitted) and then this is picked up by the sensors (I guess in a
similar way to a TV van). We have a lot of trouble at work with simple loops
of cable (ie. stuff we actually sell!) setting off the alarms occasionaly -
so I guess the resistor is not required and anything can produce the right
frequency - so it might be possible.

I can confirm that Cotag security building passes will interfere with the
operation of an Oyster card. Cotag is an RF based system, and it appears that
if a Cotag pass is in the same wallet as an Oyster card, both cards gets
activated, and the it appears that both respond about the same time, leading
to corrupt signals being received by the gate. The gate reads
"Seek assistance". I found out that having pulled the oyster card from the
wallet, and try again, and it works fine.

Most (not all) Cotag readers appear to be a little more tolerant of
'interference' from other contactless cards.

--
Simon Hewison