Patent ID: 7528699

Claim:
A transponder for communication with an interrogator which transmits a main carrier, the transponder comprising (a) an antenna which receives the main carrier, and (b) a circuit connected to the antenna, the transponder effecting a predetermined modulation of the main carrier by changing an impedance of the circuit, as viewed from the antenna, to generate a reflected wave for responding to the interrogator, wherein: the circuit includes a switching element operable to modulate the main carrier received by the antenna from said interrogator, and an impedance element connected to the switching element; and the impedance element has an impedance determined so as to provide a difference of 90 degrees or higher between a phase of a total impedance which is a sum of an input impedance of the antenna and an impedance of the circuit, at a frequency of the main carrier, when the switching element is in an on state, and that when the switching element is in an off state, wherein an amount of an electric current to be applied to the switching element is determined such that an absolute value of the total impedance of the input impedance of the antenna and the impedance of the circuit at the frequency of the main carrier when the switching element is in the on state is substantially equal to that when the switching element is in the off state.