As the air gap between the antennas increases, the coupling
in between the antennas weaken, and the coupling coefficient
will be smaller. Therefore, the impedance of the circuit will
change as the air gap changes, thus affecting the power
transfer efficiency.
Fig. 3 shows the ratio of power reflection η11 and
transmission η21 , and the frequency characteristics of the
system when the air gap(g), is changed in between 100mm-
250mm. The antenna used here is a self resonating 5 turn,
150mm radius, 5mm pitch, open type helical antenna. Fig. 4
shows the relationship of the coupling factor and the air gap.