Presence and data labels

A system for detecting presence of and for communicating with one or more electronic coded labels, the or each coded label including a presence signalling antenna, a label circuit and a data communications antenna, said system including:

• presence detection means for creating a presence detection electromagnetic field;
• data communications means for creating an interrogation electromagnetic field;
• wherein the or each label circuit is adapted to operate in a first mode when the electromagnetic field to which the associated label is exposed is at a first power level, said first mode being that of a linear circuit with an impedance dependent upon frequency, and the or each label circuit is adapted to operate in a second mode when the electromagnetic field to which the associated label is exposed is at a second power level, said second mode being that of a non-linear circuit with behaviour dependent upon time.