# A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications
## 2 NFC Energy Harvesting
### Tag ICs with Energy Harvesting
Therefore, the loading effect between tag and reader coils becomes significant when the distance decreases [43; 45] resulting in detuning. Moreover, as shown earlier (e.g., Figure 6), the presence of metal under the tag can decrease antenna inductance and detune the tag, thus increasing \(H_{min}\). Other strategies for reducing \(H_{min}\) involve increasing the tag antenna area or the number of turns. However, the increase in tag area forces an increase in reader antenna area in order not to degrade the coupling factor. In order to reduce the loading effect, it is important to choose the inductance value. Small inductance values, for instance, yield lower inductance voltages and require larger \(Q\), while large inductance values require lower effective \(Q\) (larger number of coils, reduced current in the coil, and the resulting load effect is lower). However, from Equation (5), higher inductance results in smaller capacitances; therefore, the tolerances due to the layout fabrication and tuning capacitance must be reduced. In fact, less inductance (and more capacitance to result in equal resonance frequency) allows achieving higher chip currents, of course at the expense of increased loading and detuning of the reader [43]. Unfortunately, \(H_{min}\) depends on parameters that are often not provided by the IC manufacturer, such as \(U_{min}\), harvesting power consumption, or other parameters that depend on the antenna and chip impedance, such as \(Q_{T}\). \(H_{min}\) is independent of the reader used. The experiment carried out in Reference [42] showed how the minimum H-field can be obtained for a tag design with the target current consumption. Figure 9 compares the measured \(H_{av}\) and the harvesting output voltage generated by two mobiles (Huawei G8 and Xiaomi Mi Note 2) as a function of the tag-to-mobile distance. Although mobile Model 1 generated higher power and the read range was wider, the threshold value was approximately the same \(H_{min}\) = 1.1 A\({}_{RMS}/\) m. In this experiment, the NFC IC (M24 LR04E-R) was loaded with the microcontroller (Attiny85 from AVR) and the sensors, in order to take into account the nominal current consumption (about 900 \(\upmu\)A) under normal operation. It can be seen that the harvested output remained almost constant throughout the read range before the IC deactivated this output. If the application requires an NFC antenna to be very close to a metal plate or printed circuit board (PCB) electronics, a thin ferrite foil can help isolate the antenna from the metal [46; 47; 48]. In the case of the reader, it can also reduce interference from mobile circuits. Ferrite material can conduct the magnetic flux multiple times better than free air. The effect of the ferrite increases the antenna inductance by a factor \(\mu_{\textit{rf}}\) (the definition of \(\mu_{\textit{rf}}\) is analogous to the relative effective permittivity to take into account the increase of the capacitance on an inhomogeneous transmission line). The analysis performed earlier is valid if \(\mu_{0}\) is replaced by \(\mu_{0}\mu_{\textit{rf}}\) in Equation (11). The change in inductance leads to detuning of the tag in comparison with the case of air; therefore, tuning capacitance must be adjusted. Two types of ferrite foils are available on the market: polymer absorber sheets and sintered ferrite sheets. The former has higher losses, and the effective permittivity \(\text{Re}(\mu_{r})\) is on the order of 20-60. The latter achieves higher \(\text{Re}(\mu_{r})\), on the order of 100-190, and fewer \(\text{Im}(\mu_{r})\) losses, typically 5-10 at 13.56 MHz (e.g., MHL12060-000 from Laird). Ferrite foils can be \(\pm 15\)-20% the tolerance of \(\mu_{r}\), which translates to tolerance in the antenna inductances. From Equation (11), after taking into account the correction in the effective magnetic permeability, we should expect \(H_{min}\) at the resonance frequency to remain unaltered compared with the antenna in the air.