[LINK] Correction!

[rchirgwin at ozemail.com.au]

Linkers,

Since I've propagated an inaccurate view of RFID, I ought to "own" the 
correction.

First, thanks Geoff Ramadan for the technical references. These have 
yielded information I should have known, but didn't. >blush<

Second. I've always cited the inverse square law as a distance limiter 
in RFID. Only partly right: the inverse square law (that is, power falls 
by the square of the distance from the source) is mis-applied, when 
we're talking about the passive tags that are the source of most of our 
discussion.

The passives - that is, the "WalMart mandate" tags, so to speak - work 
on inductive coupling. To oversimplify that explanation: instead of a 
"radio transmitter/receiver" relationship, these tags act as two halves 
of a transformer coil. Current in one coil produces current in the other.

This kind of coupling is >vastly< more distance limited than radio. 
Instead of falling by "distance ^2", inductive coupling follows 
"distance ^6" as its falloff. 1W at 1 metre becomes 10uW at 10M. Under 
such circumstances, remote scanning is in the realm of fantasy.

Of course, covert near-field skimming is still feasible, and none of 
this cancels the genuine concerns about back-end data matching. But 
don't worry about alfoil suits...

RC