| 摘要 |
A system (10) produces an alternating magnetic field in a detection zone between a pair of doorways (346, 348) to produce harmonic signals from a marker (330) when the marker (330) is activated and located in the detection zone. Each of the doorways (346, 348) is provided with a trapezoidally shaped coil which is in a resonant circuit. The coils in the doorways (346, 348) are driven at periodic time periods with in-phase currents to produce aiding magnetic fields perpendicular to doorways in the detection zone. At alternate time periods the coils in the doorways are driven with out-of-phase signals which produce opposing magnetic fields perpendicular to the doorways (346, 348) and aiding magnetic fields parallel to the planes of the doorways. The magnetic fields thus produced in the detection zone cause the marker (330) when activated to produce harmonic signals despite the orientation of the marker (330) as it passes through the detection zone.
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