Method of detecting a label used in an anti-theft surveillance system

摘要

Permanently magnetized members (24, 26) of a surveillance system label (16) are disposed adjacent a ferromagnetic nonlinear element (30) for biasing the hysteresis loop of the nonlinear element (30) near the knee (40) of the magnetization curve. The nonlinear element (30) includes low permeability sections (32,34) and a high permeability section (36). The high permeability section (36) of the nonlinear element (30) is disposed adjacent a radiating dipole (28) for radiating the summation frequency of a pair of frequencies impinging on the nonlinear element (30). A memory magnet (38) with a changeable magnetism is disposed adjacent the high reluctance section (36) of the nonlinear element (30). The magnetism of the memory magnet (38) can be changed to provide either an active or inactive status of the label (16).