| 摘要 |
<p>A surveillance system is based upon generation and detection of phase shifted harmonic signals responsive to transmission of a reference signal at a fundamental frequency in a detection zone. Phase shifted harmonic signals may be generated by markers (40) comprising a core (42) of soft magnetic material and an electrically conductive material (44) at least partly surrounding the core. Phase shifted harmonics are not accidentally generated by biassed ferrous alloys, the cause of most system failures.</p> |
