The coupling system in Fig. 2 consists of two identical
helical open circuit coils. The resonance is achieved by its
self-inductance and parasitic capacitance. Therefore, no external
lumped capacitor is needed. The coupling system is
represented by its equivalent circuit model shown in Fig. 3, in
which L is the self-inductance, C is the parasitic capacitance,
R is the parasitic resistance and Lm is the mutual inductance.
All the circuit model parameters are given in Table I.
With the help of circuit model, several parameters of the
coupling system can be derived. The input impedance of the
coupling system is