Abstract: Sensor-emitter labels or tags containing a two-terminal nonlinear capacitor, e.g., a semiconductor diode, directly connected to antenna means are applied to articles for purpose of surveillance. A transmitter coupled to an antenna establishes an electromagnetic wave field above about 100 MHZ, and preferably about 915 MHZ, within a surveillance zone. Introduction of said label or tag into said zone causes reradiation of a different signal distinguishable from the signal produced by said transmitter and otherwise occupying said zone. Various embodiments are disclosed capable of reradiating a second harmonic signal. A receiver associated with said transmitter and tuned to the reradiated signal detects the presence of said label or tag and activates a signal or alarm. Various tracking arrangements between transmitter and receiver are disclosed for accommodating frequency drift in the transmitter.