## 3 Development of RF WPT-Enabled IoT Application
### Integration and Evaluation  
Finally, we integrated all the individual elements of the system. Before mounting them to the target environment, we tested them in a static operation case. For this, we connected the designed antenna to the developed WPT transmitter and enabled the latter. The second designed antenna was connected to the Hewlett-Packard EPM-442A power meter (Palo Alto, CA, USA), which was used to measure the power of the received radio signal. The antennas were pointed towards each other. Note that, for the reference, we repeated the very same experiment using the dipole antenna (Linux Technologies ANT-868-CW-HWR-ccc half wave center-fed antenna (Merlin, OR, USA) with a peak gain of \(-2.3\) dBi) on both ends, and using one patch and one dipole antenna. Figure 13 and Table 5 illustrate the results.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Received Power** & **Power after DC** & & \\
**(dBm/mW)** & **Conversion (mW)** & **Max Voltage (V)** & **Efficiency (\%)** \\ \hline
5/10.00 & 2.04 & 4.12 & 20 \\
0/1.00 & 0.59 & 2.21 & 59 \\ \(-5/0.32\) & 0.16 & 1.15 & 51 \\ \(-10/0.10\) & 0.038 & 0.56 & 38 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Radio frequency to direct current converter efficiency.  
The results reveal that the level of the power delivered when using directional antennas exceeded that for omnidirectional ones almost two orders of magnitude. For example, at the one-meter distance, the system equipped with patch antennas delivered 10.7 mW, while the one using dipoles--only 0.10 mW. Given the RF-DC conversion efficiency of 50-60%, the former looked promising. Therefore, we instrumented the complete application by attaching the sensor node and mounting all the components to a rotating stand. This set-up is depicted in Figure 14. Note that the WPT transmitter was placed perpendicular to the axis of shaft's rotation.  
Figure 14: Wireless sensor node application in a rotating environment enabled with wireless power transfer.  
Figure 13: Harvested energy with patch and dipole antennas.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Distance (m)** & **Patch (mW)** & **Dipole (mW)** & **Patch + Dipole (mW)** \\ \hline
2 & 3.6 & 0.01 & 0.30 \\
1.5 & 4.2 & 0.03 & 0.60 \\
1 & 10.7 & 0.10 & 0.70 \\
0.5 & 41.7 & 0.30 & 5.00 \\
0.4 & 52.5 & 0.56 & 5.40 \\
0.3 & 79.40 & 1.30 & 7.40 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Harvested energy with patch and dipole type transceiver antennas with 500 mW (27 dBm) transmission power.  
The further experiments with the set-up showed that, at the distance of up to one meter, the WPT transceiver operating with 500 mW transmit power provides sufficient energy for non-intermittent operation of the sensor node. Specifically, the sensor node could sense both the acceleration and the temperature, process the data (e.g., calculate the FFT and estimate the angular speed, and report these data twice per second over BLE advertisements). The results were received by a tablet computer over a BLE link, and can further be injected in a database or forwarded anywhere over the Internet. Note that the antenna used in our experiments was vertically polarized and thus the receiver was not receiving the energy constantly. We also confirmed that longer distances are possible, but then the operation of a sensor node becomes intermittent.