[e2e] Re: a problem...

[David P. Reed]

Passive RFIDs reflect energy when stimulated by incident energy, with 
the reflected energy bearing some relationship to the incident energy's 
structure.

For example, trivial kinds of passive RFID include a set of differently 
tuned antennas with diodes in them.  The effect of the diode is 
reflection of a frequency-doubled version of any wave that resonates 
with the tuned antenna.   A transmitter sweeps a sgnal across the 
frequency range of interest, and listens with a sweep at double the 
transmitter rate, reading a 1 at each frequency where an antenna 
reflects energy.

Note that this becomes less and less practical the longer the distance 
between source and passive device.

You can do the same thing with optical gratings coupled to the receiving 
end of a fiber.