sensors without batteries

Steve Schear s.schear at comcast.net
Sun May 21 16:42:30 PDT 2006


At 03:14 AM 5/21/2006, Eugen Leitl wrote:
>Smith explains that Intel's sensor devices use off-the-shelf components: an
>antenna to send and receive data and collect energy from a reader, and a
>sensor-containing microcontroller -- a tiny computer that requires only a
>couple hundred microwatts of power to collect and process data.
>
>The antenna harvests this power directly from the radio waves emitted by an
>RFID reader. When a tag comes within range of a reader, the reader's radio
>signal passes through the antenna, generating a voltage that activates the
>tag. The tag is then able to send information to the reader through a process
>called backscattering, in which the antenna essentially reflects a
>data-encoded variation of the received radio signal.

One of the first uses of backscatter or passive transmission was when the 
Russians bugged the U.S. embassy in Moscow in the 1960s using resonant 
'nails'.  The heads of the nails (no larger than the standard variety) were 
actually hollow with two resonant cavities (I think at non-harmonic 
frequencies) formed by a 'wall' and covered by a metal diaphragm.  The nail 
shaft was an antenna. The nails had been placed just below painted 
surfaces. Sound pressure caused the diaphragm to alternately cover and 
uncover the cavities leading to a change in resonance at audio frequencies. 
Nearby, a microwave transmitter operated by Russian agents beamed energy 
into the embassy.  They could listen in on conversations by detecting the 
changes in reflected power from nails.  From reported stories it took quite 
a while to discover these babies, but then a gain it might not have and the 
embassy security people used them to run 'false flag' operations.

Steve 





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