[Gllug] ist that right that the Oyster card got RFID on it?

[Christopher Hunter]

On Mon, 2007-01-15 at 23:26 +0000, Juergen Schinker wrote:

> no you can be scanned without knowing-you don't know how far the radio
> frequency reaches - you don't know how far special equipment can scan you

I've done a LOT of work on RFID, and can tell you how far it works:  

The usable range for an RFID tag is limited by the physical dimensions
of the device.  Most of them work around 13 MHz, which implies a
wavelength of around 22 metres.  The antenna element on the tag is
therefore MUCH less than any sensible proportion of a wavelength, and is
thus very inefficient.

Useful maximum range (for coherent data) is roughly 5 cm if you use the
highest power reader, but most common readers have a range of about 2
cm. 

If you're really concerned about the location of your Oyster Card being
scanned everywhere, keep it in a tinfoil sleeve!  This will reduce the
effective range to zero!

Chris


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