An electromagnetic wave transmitted from an interrogator 1 is converted into a single pulse (P1, Figs. 3, 7) by circuits 6, 7 in a code responder 4. Delay elements 9a - 9z in a delayed pulse generator 8 delay the single pulse (P1) to generate a series of pulses (P2, P3, P4...) deviated in time from each other. Selected ones of the delayed pulses are combined according to the state of switching elements 12a- 12z in a coder 11 to form a serial bit code. Each pulse in the serial bit code causes the impedance of a circuit 15 to change from an infinite value state, in which a carrier wave transmitted from interrogator 1 is absorbed by responder 4 with no reflection, to a zero value state in which the carrier wave is reflected by responder 4.; The single pulse (P1) may be produced by a demodulator 7 in response to an interruption in the interrogator's carrier wave, or only when the carrier wave is modulated by a specific code detected by a SAW correlator (16', Fig. 6) in the responder. The code produced by coder 11 may be fixed or may vary in dependence on external factors. For example the coder switches 12a - 12z may be temperature responsive bimetallic or semiconductor elements, or light responsive elements, or vibration sensors responsive to external force.