| 摘要 |
The method and the system are intended for monitoring the position of an object in a monitoring zone, particularly unpaid goods marked with a label in a shop in the space behind the cash register. A known system contains a radio transmitter, a passive frequency doubler and reradiator on the label and a receiver, tuned to twice the transmitter frequency, for detecting the presence of the label. The invention provides for increased security and in addition a position indication by using two or more locally separated transmitters (30, 36; 32, 38; 42, 44), the signals of which are modulated with one another in a label (40) in the monitoring zone (34). The signal reradiated by the label (40) is received by a receiver (52) and split into different intermodulation products by means of filters (60-62). A comparison (100) of the magnitude of different intermodulation products results in a position indication (102-120) and individual or several intermodulation products of predetermined magnitude switch on an alarm device (92) via a logic circuit (55). <IMAGE>
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