A nonlinear radar system includes a first antenna configured to generate and emit an incident signal having a first frequency, a transponder tag attached to an existing portable object, wherein the transponder tag includes an electromagnetic nonlinear element configured to be detected by the incident signal. Upon being detected by the incident signal, the transponder tag is configured to transmit a reflected return harmonic signal having a second frequency different from the first frequency, wherein the reflected return harmonic signal is an on-off keying (OOK) signal. A second antenna is configured to receive the reflected return harmonic signal having the second frequency. The transponder tag includes a conductive layer in electrical communication with the electromagnetic nonlinear element including a Schottky diode.