On Wed, Apr 10, 2002 at 02:22:04PM -0400, Trei, Peter wrote:
| > If a stack of bills containing these transponders are supposed to be
| > read from afar, way beyond what a "valid bill detector" is likely to be
| > engineered to do, I'd like to see the physics worked out.
| >
| Detection range turns out to be function of antenna size - the reader's
| antenna, not the one on the transponder. So if you have a big (eg,
| doorframe size) antenna, you can do a lot better than the 'valid bill
| detector' on the countertop. There's actually a privacy win here for
| the passive tags - the returned signal strength falls with the fourth
| power of the distance.
Interesting. What does that work out to for, say, a 2 meter antenna?
(I'm not sure if this actually works out to a security win. It may be
that I can use this fast fall-off to ensure that I'm picking the right
pocket..)
Adam
--
"It is seldom that liberty of any kind is lost all at once."
-Hume
