The coupling factor (k) is a characteristic value that indicates how closely circuits or resonant circuits are coupled to each other. It plays a decisive role in wireless energy transmission using the resonance method, as in WiTricity with the HR-WPT method and Wireless Resonant Energy Link(WREL), because it has a direct influence on the energy transmission between the energy transmitter and energy receiver. The higher the coupling factor, the better the energy transmission.
The better the coupling between the transmitter and receiver coils, the higher the coupling factor. The coupling is very high when a high proportion of the magnetic flux from the transmitting coil flows through the receiving coil.
The coupling factor can take values between 0 and 1. If the coupling factor is 0, the receiving coil is not flooded by the magnetic flux of the transmitting coil. The transmitting and receiving coils are completely independent of each other. With a factor of 1, the full magnetic flux of the transmitting coil flows through the receiving coil. The coupling factor depends on the coil size and on the distance between the two coils. Typically, coupling factors are between 0.3 and 0.6. Transformers in which the coil windings are wound over a material with high permeability have coupling factors of >0.9.