Relative coupling coefficient | Frequency where the TX coil voltage and current phase difference becomes zero | How to find the RX resonant frequency by observing the phase difference of the TX coil |
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_{critical} | _{a}, _{b} (two different positive frequencies) | The lower frequency where the phase difference becomes zero. |

_{critical} | _{a}=_{b} (only one positive frequency) | The point where the phase difference is the lowest. |

_{critical} | - (two different complex frequencies) |