How Electromagnetic Technology Works?

Question

Accepted Answer

The electromagnetic EAS system creates a low frequency electromagnetic field (fundamental frequencies between 70 Hz and 1 kHz are typically used) between two pedestals at an exit or checkout aisle. The field continuously varies in strength and polarity, repeating a cycle from positive to negative and back to positive again. With each half cycle, the polarity of the magnetic field between the pedestals changes.
In response to the changing magnetic field created by the transmitter, the magnetic field domain of the tag material abruptly "switches" as the field strength varies past a particular point, whether positive or negative, during each half of the transmit cycle. This abrupt change in the magnetic state of tag material generates a momentary signal that is rich in harmonics (multiples) of the fundamental frequency. Using electronic signal processing techniques, the system identifies that the harmonics are at the right frequencies and levels, and that they occur at the proper time in relation to the transmitter signal. If the criteria are met an alarm occurs.