A transponder circuit comprises a loop (4) for inductively coupling with an interrogator (2) to transmit a binary coded message to the interrogator (2); the loop (4) forms part of a tuned circuit which is tuned to a first resonant frequency Fc and which is detunable, by an associated modulator, to a...http://www.google.com/patents/US5055835?utm_source=gb-gplus-sharePatent US5055835 - Track to train communication systems
