Distance Adaptation Method for Magnetic Resonance Coupling Between Variable Capacitor-Loaded Parallel-Wire Coils

A distance adaptation method for magnetic resonance coupling between coils is proposed. It is known that transmitting and receiving parallel-wire coils provide longer distance of highly efficient transmission when compared with single-wire coils. Instead of matching the coil impedance to the port impedance at the coil feed point, the ratio of the currents on one wire to the other of the parallel-wire coil is manipulated by a loading scheme of variable capacitors in the proposed system. Such a current manipulation gradually changes the coil operation equivalently from the parallel-wire coil to the single-wire coil. This enables us to obtain highly efficient transmission continuously in a wide variation of the coil distance. Impedance and quality factor of the proposed system are discussed with the equivalent circuit. Current distributions of the parallel-wire coils are exploited by the method-of-moments-based software FEKO. A prototype transmission system exhibits insertion loss less than 0.9 dB in a range from 0.2 to 0.7 m at 11.25 MHz.