US Patent 7646303 Detector

A detector for detecting the presence of a memory tag, the detector comprising a radio frequency source operable to generate a radio frequency signal and a detector resonant circuit part connected to the radio frequency source, the detector resonant circuit part comprising an antenna,the detector further comprising a power monitor responsive to the power of a reflected signal returned from the detector resonant circuit part,the power monitor being operable to generate an output in response to the power of the reflected signal,wherein a decrease in the power of the reflected signal indicates the presence of a tag in the vicinity of the antenna.