## 3 Fabrication of Class-E Amplifier and Experiment Verification
The MMs was located between the \(Rx\) coil and the \(Tx\) coil and the position of the MMs changed along with the distance between the \(Rx\) coil and the \(Tx\) coil accordingly. Firstly, we changed the distance between the load loop and the \(Rx\) coil to achieve the optimal load, and then changed the distance between the drive loop and the \(Tx\) coil to attain the needed \(Z\)in of the Class-E amplifier. The LCR meter was to make sure that the output resistance, \(Z\)in, was equal to about 10 \(\Omega\) by varying the distance between the two coils. Figure 12 shows the optimal load by changing the distance between the load loop and the \(Rx\) coil in the four-coil WPT system without MMs. \(d_{tl}\) denotes the distance between the load loop and the \(Rx\) coil. \(d_{tr}\) represents the distance between the \(Tx\) coil and the \(Rx\) coil. When the \(d_{tr}\) was fixed at a certain value, the \(d_{tr}\) was varied to achieve the maximum efficiency. It can be seen that as the system transfer distance, \(d_{tr}\), increased, the optimal distance of \(d_{tl}\) increased. Changes of the coil distances resulted in variations of the coupling coefficient. This experimental phenomenon is in good agreement with the previous analysis. Table 4 lists the measured power results of the WPT system with the MMs. The magnitudes of the input current is \(I_{1}\), and the input and output voltages are \(V_{1}\) and \(V_{2}\), respectively. \(V_{1}\) and \(I_{1}\) were obtained from the DC power source. \(V_{2}\) was measured by an oscilloscope. The load resistance, \(R_{L}\), was 10 \(\Omega\), and the output power \(P_{L}\) was calculated as the radio of the square of the output voltage, \(V_{2}\), to the load resistance, \(R_{L}\). The system power efficiency was derived by the expression, \(\eta=P_{L}/V_{1}I_{1}\). This table lists a set of measured data and manifests that the system output power can reach 5w approximately of 20.11% efficiency when the transfer distance was 200 cm. Figure 11: Experiment setup of the WPT system with the MM slab. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Inductance** & **Resistance** & **Parasitic** & **Q (Unload)** & **Additional** & **Resonant** \\  & **(uH)** & **(\(\Omega\))** & **Capacitor (pF)** & **@2.80 MHz** & **Capacitor (pF)** & **fre. (MHz)** \\ \hline Drive coil & 1.76 & 0.1 & — & 110 & \(1.835\times 10^{3}\) & 2.81 \\ \hline \(Tx\) coil & 49.6 & 1.64 & 10.9 & 532.1 & 50 & 2.80 \\ \(Rx\) coil & 49.1 & 1.63 & 10.5 & 531.8 & 50 & 2.81 \\ Load coil & 1.75 & 0.1 & — & 109 & \(1.835\times 10^{3}\) & 2.80 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The electrical parameters of the four coils. Figure 13 shows the measured results for the optimal loading position of the MMs in the WPT system. \(d_{tr}\) represents the distance between the \(Tx\) coil and the \(Rx\) coil and \(d_{tm}\) denotes the distance between the \(Tx\) coil and the MMs. The distance of the \(Tx\) coil and the \(Rx\) coil was fixed to a certain value, varying from 1 m to 2 m, as labeled in the upper right area of the figure. The MM slab was moved 10 cm apart between the \(Tx\) coil and the \(Rx\) coil at the certain \(d_{tr}\), as shown in the x-coordinate of the figure. It can be seen that the transfer efficiency decreased when the \(d_{tr}\) increased. Furthermore, the transfer efficiency reached the maximum when the MMs slab was placed in the middle of the two coils approximately and the MM's optimum loading position increased with the enhancement of the transmission distance generally. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **V\({}_{1}\) (V)** & **V\({}_{2}\) (V)** & **I\({}_{1}\) (A)** & **n (\%)** & **P\({}_{L}\) (W)** \\ \hline
100 cm & 10.5 & 7.14 & 0.62 & 77.90 & 5.10 \\
120 cm & 11.4 & 7.16 & 0.61 & 73.64 & 5.12 \\
140 cm & 14.3 & 7.16 & 0.62 & 57.03 & 5.13 \\
160 cm & 15.1 & 7.14 & 0.71 & 50.92 & 5.10 \\
180 cm & 15.73 & 7.16 & 1.05 & 30.87 & 5.13 \\
200 cm & 19.25 & 7.15 & 1.31 & 20.11 & 5.12 \\ \hline \end{tabular}
\end{table}