Abstract: A method and apparatus to detect a plurality of different security tags are described. A method to detect security tags may include establishing an interrogation zone using at least two signals operating at different frequencies; monitoring the interrogation zone to detect a plurality of security tags, with a first type of security tag responsive to at least a first signal at a first frequency and a second type of security tag responsive to at least a second signal at a second frequency; determining whether to generate an alarm if a security tag is detected by receiving a third signal including a combination of the first and second signals from a first security tag of the first type and filtering the third signal to remove frequency components of the first signal; and generating the alarm in accordance with said determination.

Abstract: 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.