The Transfer of Power in Near Field Communication
NFC antennas really do act as air core transformers and not truly as radiating sources. The low frequency of 13.56 MHz used by NFC means that the wavelength is about 22 meters (72 feet) long. To radiate the energy effectively as an antenna, the antenna length would have to be 11 meters (36 feet). You aren’t going to fit a 36-foot antenna onto a smartphone, much less a tag.
The radiation efficiency of an NFC antenna is essentially 0, which means that you won’t transmit anything in the traditional sense of the word.
What NFC is really using are loop-inductor antennas, as shown here. Yes, they’re called antennas, but you’re talking about a specific kind of antenna, one that appears as a looped coil. The use of alternating current produces a continuous magnetic field from the smartphone. When that field gets close enough to the loop-inductor antenna in the tag, it induces a current in the tag loop-inductor. Inducing a current in the tag loop-inductor creates a coupling between the two antennas.
Now that the tag loop-inductor also has a current running through it, it also produces a magnetic field. What happens next is that you achieve mutual coupling of the two antennas, creating an air core transformer.
When the smartphone disrupts the current to the smartphone loop-inductor, the magnetic field collapses. The reader loop-inductor antenna still has a current running through it, however. As the reader loop-inductor antenna magnetic field collapses, it induces a current back into the smartphone loop-inductor antenna that the smartphone reads.
The reader chip has modified the induced current from the reader loop-inductor antenna, so the smartphone sees a modulation change that reflects the data that the reader wants to transmit to the smartphone.
In general, the more turns of wire you can use to create the loop-inductor antenna, the better it performs. More turns means a larger magnetic field, which in turn means more induced energy. The number of turns is designed to make the antenna work best for this particular application.
The presence of metal (such as a ground plane on a PC board or a metal shield) near the wire also influences the magnetic field. Depending on the device, you may find that there is metal closer than you’d like. For example, the back of many smartphones contain metal. You must make the distance between the loop-inductor antenna and the metal as great as possible in order to improve the size of the magnetic field.
This is why you often see a sheet of ferrite between the loop-inductor antenna and the back of a smartphone.