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I did a project to identify cows, but I believe store antitheft is about the same: the towers are transmitters/receivers.
They transmit a radio wave to an antenna (the anti-theft device) on the store item. This provides enough energy for the device to send back its i.d. number. If the i.d. number was just purchased, it's okay. If not, an alarm sounds.
Sometimes an electronic key card will set them off. If this happens, you can get a shield for them. If this shield blocks the key card, the same material would block a store's i.d. system.
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