Abstract: Two different frequencies are generated within an interrogation zone. The receiver portion of the system determines the presence of a predetermined marker tag within the interrogation zone by the sensing and processing of a ratio of sideband signals generated by the interaction of the two different frequencies and the predetermined marker tag. The system is greatly immuned to false alarms due to the method of processing the sensed sideband signals. Falsely generated signals are nulled and subtracted out in the receiver portion of the detection system.

Abstract: Apparatus and method for identifying the presence in a pre-established location of an object having preselected electrical or magnetic properties. By applying two periodically varying electro-magnetic fields of the same frequency to the pre-established location, the presence of the object is determined by unique perturbations of the electro-magnetic fields produced by the object. The applied electro-magnetic field is generally nonuniform in the pre-established location to maximize the interaction between the object and the electro-magnetic field. A pick-up coil is utilized to sense the resulting field perturbations. A magnetometer can be optionally incorporated in a sensing system to inhibit erroneous indications produced by magnetic objects.

Abstract: Apparatus and method for detecting the presence, in an inspection area, of a marker tag having preselected properties. Transfer of articles from a protected area can be monitored by coupling a marker tag with each article and by including an inspection area in all transfer paths. In a theft detection system, the coupled marker tag can be removed or made inoperative in some manner for authorized withdrawal of an article from the protected area.The marker tag is comprised of a small, thin strip of magnetic material. A time-varying magnetic field is provided for interacting with a marker tag located in the inspection area. The magnetic field, which includes components having frequencies f.sub.1 and f.sub.2, has a time-dependent spatial orientation which permits interaction with a marker tag having an arbitrary orientation in the inspection area. The magnetic field components of frequency f.sub.