As a general matter, the underlying concepts involves in electromagnetic field surveillance systems have been proposed heretofore by a number of different persons over a considerable span of time, dating back to at least about 1970 or before, at about which time the early work of Dr. Edward Fearon began to be published and seen in patents, etc. Actually, the work of Fearon was itself built upon technological phenomena published as long ago as 1934, by the French citizen P. A. Picard, to whom French Pat. No. 763,681 was issued in 1934, such patent describing the perturbation effects produced by Permalloy and other such low-coercivity materials upon an alternating electromagnetic field. Based upon these early investigations of Picard, a number of persons have heretofore proposed the use of the technological phenomena involved for surveillance purposes, to detect attempted surreptitious movement of objects and articles past a point of egress or entrance if such objects or articles have hidden upon them a "tag" or "marker" of Permalloy or the like.
Much effort has been spent heretofore to refine and improve the basic systems proposed by early workers such as Fearon, for example, who himself is named in a number of prior patents for such improvements or enhancements. Generally speaking, most of the prior work has been done with the objective in mind of either improving the nature of the detection tag or marker device or to improve the detection and signal-analysis circuitry, primarily for the purpose of increasing the sensitivity or selectivity of the system so as to avoid erroneous alarm signals while at the same time missing detection of as few tags or markers as possible. In the latter connection, reference is made to previously-filed co-pending U.S. patent application Ser. Nos. 358,299, now U.S. Pat. No. 4,535,323 and 358,383, now U.S. Pat. No. 4,524,350 and 364,264 by the present inventors and/or co-workers, which are commonly owned herewith.
In the aforementioned prior work, little has been indicated as to refinements and improvements in the interrogation field-generating apparatus and methodology, apart from describing generally the large physical size of the field-generating inductance coils themselves and noting that the same should be made part of a resonant LC circuit which has seemingly always been referred to as a parallel-connected or "tank" circuit, the resonant frequency of the tank circuit being rather generally selected, typically from the standpoint of merely producing the generally-desired interrogation field frequency. Apart from this, one of the few known publications or disclosures referring to particular methods or techniques involved in field generation or field characteristics is a prior U.S. Patent to Richardson, No. 4,300,183, which refers to the technique of alternately changing the phase relationships of the currents used to drive the resonant LC tank circuits, so that the resultant direction of the flux in the interrogation field was made to alternate in direction, thereby increasing the likelihood of detecting marker strips or the like which were physically oriented in varying directions, some of which might produce a relatively slight or negligible change in the interrogation field and thus obscure or preclude detection. In actuality, such phase alternation had already been known and utilized prior to that time, but generally speaking the concept is a valid one and as stated represents one of the few improvements made in the area of field generation or field characteristics over the many years in which the interrogation systems generally have been considered and/or used.