MICROWAVE REFLECTIVE TRANSPONDERS

摘要

MICROWAVE REFLECTIVE TRANSPONDERS This invention relates to a transponder for transmitting coded data to an interrogating transmitter/ receiver and to monitoring apparatus including the transponder. The transponder can be used for security purposes, vehicle identification and the remote sensing of information relating to animals. The transponder includes an element of which a reflection or transmission property for an electromagnetic signal at a first frequency may be varied. A coded signal is applied to the element so that when it is illuminated with energy of that frequency it reflects or transmits energy with that frequency but modulated with the coded signal. Power is provided for the transponder responsive to electromagnetic energy that is at a second frequency different from the first frequency.