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.Saturable ferrite layers, fusible links, magnetic switches and the like are described associated with the labels or tags to allow for deactivation. Various deactivation devices are disclosed including radio frequency generators for burning out the nonlinear capacitor or diode.