Patent Document ID: 4535323
Application ID: 06358299
Patent Status: 1

Claim One:
1. In a method of detecting the presence of a particular marker member within an electromagnetic interrogation field established between a pair of mutually spaced portal side members, wherein said electromagnetic field is made to alternate at one or more nominal frequencies and the field is monitored by at least first and second receiver means each disposed to access the field from a different side position at said portal to detect the presence of signal indicia introduced by such a marker in response to its exposure to the alternations of the field, and wherein said receivers each produce an electrical signal representative of the frequency content of the alternating field at least in the area of proximity nearer to that receiver, the improvement for use in detecting said marker which comprises the steps of: producing a first composite electrical signal for marker-presence analysis by summing representations of the said electrical signals produced by said first and second receivers, to thereby increase detection sensitivity; and producing a second composite electrical signal for use in marker-presence analysis by differencing representations of the said electrical signals produced by said first and second receivers, to reduce common-mode noise or other undesired signal characteristics in that composite electrical signal.