NFC Tags and Emitters
The Near Field Communication (NFC) hardware begins with two separate physical pieces, but NFC couples these pieces to form a unit using RF energy. This figure shows the role of the emitter (a device such as a smartphone) and the tag (which could be anything the emitter wants to interact with, including another smartphone).
The combination of the two antennas and the air between them creates an air core transformer. The energy in the emitter creates an electromagnetic field that induces an electrical current in the tag. For NFC, the coupling takes place at a range between 4 cm and 10 cm. In order to work, the electromagnetic field produced by the emitter antenna must be strong enough to induce a current in the tag antenna. Otherwise, no communication can take place. Just knowing that the coupling takes place is enough information for working with NFC.
Air core transformers are notoriously inefficient, which is one reason that the coupling distance between the two devices is so limited. The tuning of the antennas to provide matching between emitter and tag helps determine the connection range of the devices. When the emitter antenna is properly matched to the tag antenna, you obtain the maximum coupling possible.
Many of the problems associated with NFC tag interactions are the result of poor coupling, influenced by the presence of external factors, such as a metal mounting surface, the presence of an elemental factor (for example, water), or the distortion of the antenna in some manner, which affects tuning.