# Miniature Coil for Wireless Power and Data Transfer through Aluminum +
## 3 Through Metal Scenarios
### Stainless Steel  
Figure 9 shows the measured results of harvested power versus input power for stainless steel plates of different thicknesses. Stainless steel is a material that is less conductive than aluminum. The conductivity of stainless steel and aluminum is \(1.38\times 10^{6}\) S/m and \(25\times 10^{6}\) S/m, respectively [29]. Hence, the skin effect is reduced and power can be transferred more effectively through stainless steel [30]. For a 0.88 mm-thick stainless steel plate, the harvested power is almost 700 mW. If the plate thickness is increased to 1.49 mm, more than 400 mW can be harvested. When using a 2.99 mm-thick stainless steel plate, 300 mW can be harvested, which is nearly double the harvested power through a 1 mm-thick aluminum barrier.  
Figure 10 shows the power transfer efficiency results for the same configuration. By using stainless steel, efficiency can be tripled, even if the thickness of the plate is 2.99 mm. Efficiency increases further with thinner plates. For stainless steel, power transfer efficiency can go from a minimum of 6% to a maximum of 14% for thicknesses from 2.99 to 0.88 mm, respectively. Figure 10 also includes the power transfer efficiency results for aluminum plates of different thicknesses. As the aluminum plate thickness increases from 1.5 to 4.98 mm, the efficiency decreases from results 1% to 0.04%, respectively. For the thickest aluminum plate, 4.98 mm, less than 0.04% efficiency is obtained.  
The skin depth (\(\delta=1/\sqrt{\pi f\mu\sigma}\)) [31] of stainless steel and aluminum at 2 kHz is 9.57 mm and 2.25 mm, respectively. Therefore, the E-field \(\left(E=E_{0}\,e^{-d/\delta}\right)\) attenuated into the metal faster for aluminum than stainless steel. In other words, at the same distance into metal, the electric field amplitude in stainless steel is much higher than that in aluminum. As a result, harvested power through stainless steel is higher than that through aluminum for the same metal thickness.  
Figure 11 describes the measured harvested power with respect to frequency at an input power of 3 W for stainless steel. In this experiment, the frequency of the input signal is swept from 1.5 kHz to 2.5 kHz while load \(R_{L}=4\)\(\Omega\) and parallel capacitances \(C_{tx}\) and \(C_{rx}\) are fixed at 24 \(\upmu\)F. The results indicate that harvested power starts to decrease above 2 kHz due to the skin effect of the metal plate. In addition, above 2 kHz, the circuit in Figure 4 is no longer operating at resonance. To achieve maximum harvested power, the resonance principle should be maintained [32].  
Figure 9: Harvested power versus input power using stainless steel plates of different thickness. Operating frequency is 2 kHz.  
Figure 12 compares the harvested power through different stainless steel plates relative to the 1 mm-thick aluminum. The harvested power through a 2.99 mm-thick stainless steel plate is three times more than through a 1 mm-thick aluminum. As the thickness of the stainless steel plate decreases to 0.88 mm, the harvested power increases 7 times more than that through 1 mm-thick aluminum.  
Figure 11: Harvested power versus frequency using stainless steel plates of different thicknesses. Input power is 3 W.  
Figure 10: Power transfer efficiency versus input power using stainless steel and aluminum plates of different thickness. Operating frequency is 2 kHz.