## 4 Measurement Results  
The experimental setup is designed and implemented to validate the simulated results obtained in Section 3. The \(L_{1}\), \(L_{2}\) and \(L_{3}\) are fabricated using PCB technique from copper with the same height and variable widths. As shown in Figure 8a,b, \(L_{1}\), \(L_{2}\) and \(L_{3}\) are fabricated from copper with a width of 1.9 mm, 1.5 mm, and 0.3 mm respectively and a height of 0.07 mm. The PTE of the 3-coil inductive link through the air and tissue mediums are measured using two experimental setups shown in Figure 8c,d.  
Figure 8: Fabrication of **(a)** External coil; **(b)** Implantable coil; **(c)** Experimental setup for measuring the PTE; **(d)** Experimental setup for measuring the PDL.  
The first method is to measure \(S_{21}\) parameter through the air and tissue mediums by connecting \(L_{1}\) to port 1 and \(L_{3}\) to port 2 of a vector network analyzer (VNA). The ports 1 and 2 of the VNA are connected to an impedance of 50 \(\Omega\) each. The distance between the external and implantable coils is set to 10 mm at air and tissue mediums. The PTE of the 3-coil inductive link through the air at perfect alignment, lateral misalignment, angular misalignment and a combination of lateral and angular misalignment are 41.7%, 18.2%, 15.4% and 21% respectively. To check the PTE in tissue environment, a boneless pork sample with 10 mm of thickness is placed as a medium between the external and implantable coils. At perfect alignment, the PTE through pork sample waned to 30.8%.  
We have measured the link efficiency using VNA in order to compare it to the simulated transmission coefficient (\(S_{21}\)). However, we cannot measure the actual amount of power received by the receiver coil using VNA. Thus, a second method as shown in Figure 8d is used to measure the received power. This method is used to measure the actual amount of power transferred from \(L_{1}\) and \(L_{2}\) to \(L_{3}\). The RF power generator is connected to the impedance matching (IM) network to create a matched frequency between the external and implantable coils. Generally, the capacitances are varied to tune the resonance frequency to operating frequency. However, in practical applications, it is not possible to vary the values of the capacitance. So, in this paper, we have used IM network to tune the resonance frequency to the operating frequency. Auto tuning the IM network allows the design to achieve the maximum PTE in the air or tissue medium as IM network tunes till it achieves the minimum power reflected. The minimum power is reflected only when the resonance frequency of the coil is equal to the operating frequency. Therefore, the resonance frequency can be tuned to operating frequency in the air and tissue mediums.  
At 13.56 MHz, the impedance network sends an input of 1 W received from the RF generator to the external coil and the output on the implantable side is measured in terms of voltage and current through an oscilloscope. At a distance of 10 mm from the external coil, the implantable coil at perfect alignment received power of 347 mW and 266 mW (in terms of PTE: 34.7% and 26.6%) in the air and tissue mediums respectively. It can observed that the PTE measured is less than \(S_{21}\). The proposed work is compared with the conventional works in terms of PTE with respect to coil size, operating frequency and distance between the coils. The results are summarized in Table 4.