A passive transponder for use in an interrogation/transponder system comprises a substrate having a substrate surface defining a path of travel for surface acoustic waves; a launch transducer element arranged on the surface for converting interrogating signals into surface acoustic waves which propagate...http://www.google.com.au/patents/US4605929?utm_source=gb-gplus-sharePatent US4605929 - Surface acoustic wave passive transponder having optimally-sized transducers
