## 4 Results
### Harvesting Antenna  
For energy harvesting from the Wi-Fi network at 2.4 GHz and 5.0 GHz, a fractal antenna design was investigated. Figure 11 shows the simulation and measurement results of S\({}_{11}\) of the design fractal antenna.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Nodes** & **Energy Consumption** \\ \hline Active & 528\(\sim\)858 mW \\ \hline Modern Sleep & 9.9–66 mW \\ \hline Light Sleep & 2.64 mW \\ \hline Deep Sleep & 33 \(\mu\)W \\ \hline Hibernation & 8.25 \(\mu\)W \\ \hline \hline \end{tabular}
\end{table}
Table 1: Power consumption of ESP32 module.  
Figure 10: Timing diagram of sensor node operation.  
A comparison of the simulated and measured spectra of the antenna's S\({}_{11}\) characteristic is shown in Figure 11. The black line corresponds to the measurement results, the green line represents the simulation results, and the grey area indicates the Wi-Fi frequency bands at 2.4 GHz and 5 GHz. From Figure 11, it can be seen that there is a perfect match between the simulation and measurement results at 2.43 GHz. The measured bandwidth (at the \(-10\) dB level) is 2.40 GHz to 2.49 GHz. This bandwidth fully covers the lower Wi-Fi frequency band. Not such a good agreement is found at higher frequencies. The expected theoretical reflection minimum at 5.78 GHz is shifted to 5.95 GHz, and the measured bandwidth for the upper Wi-Fi frequency band was from 5.8 GHz to 6.04 GHz. From Figure 11, it can be seen that only a small part of the upper Wi-Fi frequency band can be used. Nevertheless, at least nine Wi-Fi channels should fit into this frequency band. To match the spectral characteristics of the antennas to the frequencies used by Wi-Fi, the dielectric properties of the polymeric materials used in the substrates must be taken into account.  
Additional investigation was performed into how the working frequencies of the fractal antenna depend on its two main parameters--the size of the largest triangle and the edge ratio of other triangles. The upper limit of the parameter space is limited by the size of the PPE440 substrate. All other parameters are fixed. The simulation results are presented in Figure 12--the increase in the size of the triangles or the ratio between the triangle edges decreases the antenna resonance frequency in both bands. The dependences are close to linear. The ratio between the two frequency bands increases from 2.24 to 2.41 when \(A\) increases, and from 2.29 to 2.53 when the ratio between the triangle edges increases. This allows us more precise control of the resonance frequencies of both bands by choosing proper dimensions of the antenna and slight separate tuning of both resonance frequencies.  
For microwave rectifying, Schottky diodes SMS7630 from Skyworks were applied. At an optimal load resistance, a voltage of about 0.5 V could be expected at the output of the rectifier circuit. The voltage is too low to power microcontrollers (3-5 V needed), and an additional power control module should be connected to the power supply to charge the battery. The power management module BQ25570, manufactured by Texas Instruments, was selected as it has a low input voltage and it has a high-efficiency factor. It can be expected that when the energy-harvesting circuit is irradiated by a maximum allowable microwave power density of \(10\) W/m\({}^{2}\), the output of the generated energy harvester could provide a balanced power of 5 mW. The design of energy harvesters for other spectral ranges is ongoing. However, the initial results for energy collection from electromagnetic waves existing in the environment are promising. Other sorts of energy (solar, wind, etc.) could be harnessed to power detached sensor nodes in WSN. The SSAIL technology can be used to integrate any of these kinds of energy harvesting antennas with a Wi-Fi module using the appropriate design matching the requirements.  
Figure 11: Simulation and measurement results of S\({}_{11}\) manufactured fractal antenna.