Analysis of antenna coupling in near-field RFID systems

A simple formulation has been presented for computing the coupling coefficients between two antennas that are placed in the near field of each other and are arbitrarily oriented. Although the formula is complicated to some extent, it could be regarded as a near-field counterpart of the Friis transmission formula. Based on the presented formula, the near-field coupling phenomenon has been investigated, and the result thus obtained may be useful for the design and optimization of the near-field RFID systems.