As the gap in between the resonators change, the coupling coefficient
k between the resonators will also vary. This causes a
change in the impedance of the system, which leads to a change
in resonance frequency and the power transfer efficiency.
Fig. 3 shows the frequency characteristics of the power
reflection η11 and transmission ratios η21 of the system as
measured by a VNA and calculated using (5) and (6). The
gap g between the resonators was varied from 6 to 31 cm. As
shown in the figure, when the gaps are small and the coupling
is strong, there exist two resonance frequencies that achieve
a maximum power transfer efficiency. As the gap becomes
larger, the resonance frequencies move closer to each other and
eventually merge into one. Then, if the gap gets even larger, the
maximum efficiency will drop.