Tri-signal electromagnetic article surveillance system

摘要

In a method for electronic surveillance of articles in a given area, one preselects first and second zones in the area respectively for surveillance and non-surveillance and affixes to the articles devices adapted to provide a first output indicative of an alarm condition upon receipt of first predetermined signals and further adapted to provide a second output not indicative of an alarm condition upon receipt of such first signals together with second predetermined signals diverse from the first signals. One transmits the first signals in the area, and transmits the second signals at least into the second zone, such that devices on articles present in the second zone provide the second output. The first signals may be microwave and modulated low-frequency signals. The second signals may be unmodulated low-frequency signals. Systems and antenna configuration are also shown.