An Near Field Communication (NFC) antenna is a coil inductor, and together with capacitors to ground, they form a parallel resonant LC tank. A maximized power transfer takes place when the active and the passive device antenna have the same resonance frequency. Since the active device operates at 13.56 MHz, the passive device antenna should also resonate at that frequency. In addition to the resonance frequency, the antenna's physical size and shape also matters. Two similar shaped antennas will result in more power transfer than two very different antennas.
The following figure shows the antenna and the parallel capacitors forming an LC tank circuit. The goal of the tuning is to find the optimal value of the parallel capacitors, Ctune1 and Ctune2, so that the resonance frequency of the LC-circuit is at 13.56 MHz.
