A method of identifying an article of interest includes providing one of a plurality of RF antennas each having a non-linear element which may be a rectifying diode and being resident at a different frequency on the article of interest. One RF antenna is interrogated with RF energy of a first frequency and converts the interrogated RF energy into a reflected energy of a different frequency. The reflected energy is sensed and on the basis of the different frequency determining if a specific antenna is present. The non-linear element may be a diode and is preferably a variable non-linear element. In another embodiment, the reflected RF energy is responsive to a change in ambient physical conditions such as pressure or temperature. Corresponding apparatus is provided.