Patent ID: 7034687

Claim:
A system for detecting the presence of a passive electromagnetic wave receptor-reradiator with signal mixing capability in proximity to a doorway, comprising: a. a first transmitter transmitting a low frequency signal in proximity to said doorway; b. a second transmitter transmitting a high frequency signal in proximity to said doorway, so that when said passive electromagnetic wave receptor-reradiator is in proximity to said doorway, said passive electromagnetic wave receptor-reradiator will emit an upper side band having a frequency equal to the sum of said low frequency and said high frequency signals, and a lower side band having a frequency equal to the difference between said low frequency and said high frequency signals; c. a receiver, including i. a frequency reducer, capable of reducing the frequency of said upper sideband and said lower side band to produce an intermediate frequency upper side band and an intermediate frequency lower side band; ii. an intermediate frequency upper side band detecting circuit tuned to detect the presence of said intermediate frequency upper side band and emit a first positive signal in response thereto; iii. an intermediate frequency lower side band detecting circuit tuned to detect the presence of said intermediate frequency lower side band and emit a second positive signal in response thereto; and iv. a logic unit configured to create an alarm when both said first positive signal and said second positive signal are present.