# A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications
## 2 NFC Energy Harvesting
### Tag Antenna Design Considerations
Another factor that can reduce wireless power transfer is the detuning of the tag antenna when the mobile's metallic parts are close to the tag. An example of a tag antenna was presented in order to study the effects of detuning [42]. Here, the antenna was designed using a two-dimensional (2D) full-wave simulator (Keysight-Momentum). To enable reading with a mobile NFC reader, a 50 \(\times\) 50 mm loop antenna with six turns on a 0.8-mm-thick FR4 was chosen. The trace had a width of 0.7 mm and the gap between traces was 1 mm. By measuring the DC resistance, inductance, and resistance at the resonance frequency, the antenna model shown in Figure 3b could be obtained [42]. Figure 6 shows the inductance and quality factor of the tag antenna for several distances to a ground plane that simulates the mobile case. Agreement between the simulations and measurements is good. The measurements were taken by measuring the parameter \(S_{11}\) of a test antenna connected by means of a SubMiniature version A (SMA) connector to a vector network analyzer (VNA). The antenna impedance (\(Z\)) can be obtained from parameter \(S_{11}\) as a function of frequency for different antenna-to-metal distances. The antenna quality factor is obtained from \(Q=\text{Im}(Z)/\text{Re}(Z)\) at the operation frequency. An important reduction in inductance due to the induced image currents and an increase in losses due to the metal are observed. Figure 5: (**a**) Simulated coupling factor as a function of the radius of loop antennas \(r_{2}/r_{1}\) for different axial distances. (**b**) Optimum ratio of the radius as a function of the normalized axial distance \(x/r_{1}\). The resonance frequency of the tag can be adjusted using Equation (5) if antenna inductance, IC capacitance, and parasitic antenna capacitance are known. A practical procedure for adjusting the tuning capacitance (\(C_{tuning}\) in Figure 3) can be conducted with a vector network analyzer. A test antenna (another prototype of the same antenna or a simple wire loop soldered to an SMA connector) is connected to port 1, and the S\({}_{11}\) parameter is measured with the tag close to the mobile. The distance between the test antenna and the tag must be large enough (e.g., 1 cm) to avoid coupling between the antennas. After that, the tuning capacitance can be changed to tune the resonance frequency to the frequency of operation. Figure 7 shows the \(S_{11}\) parameter measured for a tag adjusted in the air with \(C_{tuning}=22\) pF and detuned due to the proximity of a mobile with a metallic case (model Huawei G8) at several distances between 1 and 12 mm. In order to solve this effect, the tag can be tuned again to 13.56 MHz for the desired distance by increasing tuning capacitance. Due to the power limitation of a standard VNA (often 20 dBm), the excitation field in this test is smaller than for a real reader. A modified VNA set-up with an external amplifier and a reflectometer was used in Reference [43] to characterize the tag under similar power conditions to the actual operation with a reader. IC impedance (especially the equivalent resistance) is, therefore, higher than for a real situation. The resistance of the chip typically decreases from \(R_{IC}=5\) k\(\Omega\) to 1 k\(\Omega\) under high power excitation when the tag is very close to the reader [43]. The quality factor of the whole transponder \(Q_{T}\) at the resonance frequency (which should not be confused with the antenna's \(Q\) factor) can be derived from the parallel equivalent circuit of the antenna:  
\[Q_{T}=\frac{1}{\frac{R_{a}}{2\pi f_{r}L_{a}}+\frac{2\pi f_{r}L_{a}}{R_{IC}}}= \frac{R_{T}}{2\pi f_{r}L_{a}}, \tag{6}\]  
where \(R_{T}\) is the tag's total equivalent resistance, calculated by  
\[R_{T}=\frac{R_{IC}R_{pa}}{R_{IC}+R_{pa}}\approx R_{IC}. \tag{7}\]  
The equivalent parallel resistance \(R_{pa}\) can be derived from the series antenna resistance:  
Figure 6: Simulated and measured antenna inductance (**top**) and quality factor (**bottom**) as a function of the distance to a ground plane [42].