Abstract: In self-service stores, pay desks are usually arranged in a row side by side. Anti-theft detectors are often arranged in conjuction with the pay desks, and comprise coil loops generating magnetic fields. The coils are usually fed with a common alternating current. The result of this is that an alarm-generating object at one pay desk can give an alarm at the detectors of an adjacent pay desk. The invention is based on feeding the coil loops in alternate systems with one alternating frequency and the ones in between with a deviating frequency, whereby detection isolation between the systems with the same frequency is achieved.