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