Abstract: In a transponder adapted to be attached to an object to be identified, an interrogator for interrogating the transponder utilizes an antenna system including first and second antennae with the first antenna being formed as a loop and the second antenna formed in a figure-8 configuration, and being coplanar with the first antenna. A source of power provides first and second transmission signals to the first and second antennae, respectively. A phase shift is introduced between the first and second transmission signals such that the first and second antennae create an electromagnetic interrogating field in a desired region with the electromagnetic field having a non-zero value at all points within the desired region for activating the transponder regardless of the orientation of the transponder. At least one of the two antennae may be arranged to function as a receiving antenna to receive signals emitted by the transponder.

Abstract: A memory suitable for use as a responder, comprising:(a) receiver means for receiving a transmitted signal S1;(b) generator means (76) for generating a carrier component of a transmittable signal S2;(c) logic means (L.sub.2) for enabling signal S2 to be in mathematical relationship with signal S1, said logic means comprising:(i) at least one memory means (60) for storing code;(ii) at least one modulation means (65) for modulating said carrier component of signal S2, the modulating being under control of at least a portion of said stored code, such that signal S2 is in coded mathematical relationship with signal S1; and(d) transmitter means (T.sub.2) for transmitting signal S2.