An anti-theft surveillance system capable of detecting the presence of tags affixed to merchandise. The tags contain a diode attached to two dipole elements. When the tag receives alow frequency signal and a high frequency signal, it will emit two side bands. The system includes alow frequency transmitter, a high frequency transmitter, and a receiver. The two transmitters place alow frequency signal and a high frequency signal in proximity to the guarded area (typically a store doorway). When a tag is near the doorway, the receiver will receive the upper side band and lower side band. The receiver reduces the frequency of the upper and lower side band signals to form an intermediate frequency upper side band and an intermediate frequency lower side band. These two intermediate frequency signals are then fed into two separate detector circuits. One circuit detects the presence of the intermediate frequency upper side band and the other circuit detects the presence of the intermediate frequency lower side band. Only if both side bands are detected is an alarm created.