Abstract: An article surveillance system employs a label or tag containing a non-linear impedance element, such as a semiconductor diode, connected to a metal antenna loop configured to pick up two distinct radio frequency transmissions displaced on either side of a selected center frequency. The non-linear impedance element connects opposing sides of the metal loop forming a tuned tank circuit that resonates at twice the selected center frequency. Two transmitters generate separate tone modulated radio frequency and continuous wave radio frequencies equally displaced on opposite sides of the center frequency that are fed to respective orthogonally disposed dipole radiating antenna strips on opposite sides of a surveillance area. The dipole strips for different frequencies are at right angles on each side, while those for the same frequency are at right angles to one another on opposite sides to achieve cross polarized transmission of both signals within the surveillance area.