## 4 Results and Discussion
To show the applicability of our results, we carried out an additional experiment in a controlled environment (semi-anechoic chamber). In this experiment, the power received from three different antennas at a realistic electric field strength (obtained in this study) was measured in a semi-anechoic chamber (see Figure 8) to prove the hypothesis that EMFs emitted by wireless devices in a car can be used as a new energy source for ultra-low-power consumption devices. The measurement setup contains a signal generator (SMB-100A, Rohde & Schwarz, Munich, Germany), a power amplifier (FLG-50F, Franklin, Heideck, Germany), a Stacked Log Periodic Antenna (STLP 9128 C, Schwarzbeck, Schonau, Germany) as the transmitting antenna, a receiving antenna and a power meter. A horn antenna (BBHA9120D, Schwarzbeck, Schonau, Germany), a half wave dipole antenna and a flexible wearable patch antenna [55] were used as receiving antennas that received the electric field emitted from the STLP antenna at 2.4 GHz, 2.45 GHz and 2.5 GHz. The receiving antennas were located 3 m from the STLP antenna. The E-field probe (LSProbe 1.2 Field Probe System,  
Figure 6: Control measurements. E-field measured inside the car at two locations: Location 1 and Location 2 for weekdays and weekends: (**a**) Control 2, Location 1, weekend; (**b**) Control 2, Location 1, weekday; and (**c**) Control 2, Location 2, weekday. LUMILOOP GmbH, Dresden, Germany) was placed in the location of the receiving antenna to determine the signal generator settings necessary to generate the appropriate electric field strength (for example, 1 V/m, 5 V/m, 10 V/m, and 20 V/m at 2.40 GHz, 2.45 GHz and 2.50 GHz). In this procedure, the relationship between field strength within the receiving antenna position (plane) and the forward power applied to the transmitting antenna was determined. It can be seen in Figure 9 that, at 20 V/m, the power of the output of the horn antenna, dipole and flexible wearable antenna was 2613 \(\mu\)W, 338 \(\mu\)W and 197 \(\mu\)W at 2.4 GHz, respectively. At frequency 2.45 GHz, the power was 1634 \(\mu\)W (horn), 521 \(\mu\)W (dipole) and 205 \(\mu\)W (wearable antenna). At frequency 2.5 GHz, the power was 3650 \(\mu\)W (horn), 772 \(\mu\)W (dipole) and 240 \(\mu\)W (wearable antenna). This power was much higher than the power required in [20] of 3.2 \(\mu\)W (\(-\)25 dBm) to power small devices. As can be seen in Figure 9, the required power of 3.2 \(\mu\)W can be obtained at the output of the antennas at an electric field strength between 1 V/m and 3 V/m. These results prove the hypothesis that the  
Figure 7: Comparison of results obtained in this study and a similar study [20]: (**a**) average values and (**b**) maximum values. The EMFs emitted by wireless devices in a car also can be used for ambient backscatter. Ambient backscatter focuses on enabling communication among ultra-low-power devices using ambient RF as the only source of power, without requiring any additional power sources [26]. Recent work [27] on ambient RF power harvesting demonstrated prototype ambient backscatter devices requiring the power consumption of the analog portion of the transmitter (Tx)/receiver (Rx) of only 0.25 uW (Tx) and 0.54 uW (Rx), respectively. From the results presented in Figure 9, it can be seen that the power required for the transmitter or receiver of a backscatter device is achieved at an electric field intensity of 0.5 V/m to 1 V/m, regardless of the type of the receiving antenna (horn, dipole or flexible wearable antenna). From Figure 7a, it can be seen that, in the car cabin during voice and video calls and data transmission via 2G, 3G, 4G and 5G cellular networks, the mean values of the electric field strength vary between 0.5 V/m and 1.25 V/m; therefore, they are sufficient for use for ambient backscatter.