Contactless bank cards

Peter Tomlinson pwt at
Wed Nov 17 07:17:43 GMT 2010

Tony Naggs wrote:
> On 16 November 2010 10:37, Francis Davey <fjmd1a at 
> <mailto:fjmd1a at>> wrote:
>     Actually my first worry on seeing these things advertised was
>     something entirely legal. Along the lines of an unobtrusive sign
>     saying "entrance fee £5" or something like that. Auto charge people as
>     they walk in (does contactless have that range? Or will it) and then
>     have plausible deniability for a criminal charge. Obviously some
>     customers will complain and have a reasonable argument for restitution
>     of the sum taken, but who cares.
> Contactless credit cards are very similar to Oyster cards, and standard
> readers have range of 2cm to 5cm or so. The radio power from the reader
> is powering the chip in the card. Actually the electric field carries the
> power, and as this follows an inverse square rule extending the range
> significantly is hard (requiring lots of copper and current).
I always understood that using a coil in the card means that it is the 
magnetic component that transmits the power (a relatvely large area coil 
is used in nearly all ISO 14443 cards such as Oyster, the bus passes, 
and the contactless interface of these new style bank cards). In normal 
use, once you get away from very close proximity with the reader's 
aerial coil [1], I also understand that the power that can be 
transferred effectively falls off with the 4th power of distance [2] [3].


[1] When very close, the model is that of an electrical transformer, 
albeit a rather lossy one.

[2] I'm not here considering those atypical configurations suggested for 
snooping - e.g. beaming power to the card, detecting its response by 
monitoring the electrical field component.

[3] There are two parts to the standard interface designs: in both 
terminal and card, a coil is used for both transmit and receive 
functions. The card transmits by changing the load that it imposes on 
the received field; the terminal receives by detecting the changing load 
on the transmitted field.

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