The answer is magnetic coupling. Some people refer to RFID base stations as “couplers”. This
terminology is certainly quite appropriate in our case. We have indeed to consider the RFID system,
antenna plus tag, as a loosely coupled transformer, with the base station antenna acting as the primary of
this transformer. This concept is of paramount importance for the system designer. One must always
remember it. The tag AND the base station “antenna” constitute THE system, and cannot be studied
separately.