| 摘要 |
An electronic article surveillance system responsive to the distance to a target within an interrogation zone is provided. The interrogation zone is defined by an electromagnetic field generated with a known level and transmitted by at least one antenna. A target within the interrogation zone can be any object, such as a person or shopping cart, within the interrogation zone. The target may include an EAS marker securable to an article for passage through the interrogation zone. The EAS marker is detectable at a frequency when in the electromagnetic field, and is detected by EAS detection equipment, as known in the art. The target within the interrogation zone is detected, and the distance from the antenna to the target is measured. The level of the electromagnetic field is controlled according to the distance to the target within the interrogation zone. The output level is adjusted according to the distance to the target. The EAS system can include multiple antennas each producing an electromagnetic field that in combination define the interrogation zone. A ranging transducer is mounted near each antenna, or at opposing ends of the interrogation zone to measure the distance to a target within the interrogation zone. The output level of each electromagnetic field transmitted by each antenna can be individually controlled according to the distance from that antenna to the target. Multiple targets can be detected, and the power level of each electromagnetic field is adjusted accordingly. T he electromagnetic field can be switched off and on by detection of the target in the interrogation zone. The direction of motion o f the target can also be detected and used as a factor in alarm activation decisions.
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