A passive transponder (10) includes a resonator circuit (16) to receive a powering signal thereby to provide electrical energy to the transponder (10) by inductive coupling. The resonator circuit (16) is switchable between a high Q factor mode in which an induced voltage in the resonator circuit (16) decays slowly, and a low Q factor mode in which an induced voltage in the resonator circuit (16) decays more quickly. The transponder (10) includes a power storing arrangement (24) to store at least a portion of the electrical energy obtained from the powering signal.
