Article detection system with near field electromagnetic wave co

Details Article detection system with near field electromagnetic wave co Article detection system with near field electromagnetic wave co

1975-06-27

1977-04-05

Caldwell, John W.

Communications: electrical

Systems

Call station

340258C, 343867, 325365, H01Q 2100, G08B 2100

Patent

active

040165531

ABSTRACT:
An electronic theft detection system includes antenna clusters connected to electromagnetic wave transmitter and receiver means. Each antenna cluster comprises a group of spaced apart parallel loops connected in phase opposition or bucking relationship. Electromagnetic waves emitted from the transmitting cluster are detectable close by, i.e., at distances substantially less than one wavelength, but not at far distances due to cancellation effects; and, conversely, electromagnetic waves incident upon the receiving antenna cluster are sensed by it only if they originate from sources close to the cluster, i.e., substantially less than one wavelength.

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patent: 3868669 (1975-02-01), Minasy

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