# Flexible Rectenna on an Eco-Friendly Substrate for Application in Next-Generation IoT Devices
## 3 Results and Discussion
### Rectenna: Fabrication and Testing  
To investigate the performance of the rectenna (antenna and rectifier), we integrated the optimized fractal antenna with the rectifier, as shown in Figure 11. To evaluate the performance of the rectenna, we placed the fabricated prototype inside a semi-anechoic chamber at a distance of 1.5 m from a horizontally oriented horn antenna, as illustrated in Figure 11b. We connected the horn antenna to a power amplifier and a signal generator (located outside the semi-anechoic chamber). After that, we measured the DC voltage across the load using a voltmeter in the frequency range of 2.1 to 3.5 GHz for five intensities of the incident electric field (1 V/m, 3 V/m, 10 V/m, 30 V/m, and 100 V/m). We used an isotropic field probe to adjust and continuously monitor the electric field intensity in the plane of the rectenna, ensuring consistent testing conditions. To assess the sensitivity of the rectenna to variations in the orientation of the incident electromagnetic field, we repeated all measurements with the horn antenna oriented vertically and swept at an angle of 45\({}^{\circ}\). We selected electric field intensities of 10 V/m and 30 V/m to test the rectenna under realistic exposure conditions corresponding to various real-world wireless communication scenarios. For example, peak values of up to 11 V/m have been reported during IEEE 802.11n uplink transmissions [42] in an indoor environment. Inside vehicles, the electric field during GSM voice calls and 5G data transmissions exceeded 20 V/m [43]. For LTE, approximately 10 V/m are observed [44]. Also, in situ measurement of a car cabin during travel shows a maximum electric field intensity of 17 V/m [45]. Moreover, in microwave wireless power transfer systems, the electric field intensity can exceed 100 V/m [46]. Figure 12 presents the output DC power (P\({}_{\text{out DC}}\)), derived from the measured DC voltage at the rectenna output, as a function of frequency for five electric field intensities under different polarizations. It can be seen that the highest DC power of 4000 uW is obtained at the rectenna output when the intensity of the electric field is 100 V/m at the vertical orientation of the horn antenna. We can find from Figure 12b,c that when the transmitting horn antenna is in horizontal orientation or swept at an angle of 45\({}^{\circ}\), the highest DC power in the target frequency band (2.1 GHz-2.7 GHz) is 2000 uW at 100 V/m. Also, across the targeted frequency band, for the three orientations of the horn antenna, the obtained DC powers are 100 uW, 80 uW, and 50 uW, respectively, when the electric field intensity becomes 30 V/m. From the results, we can conclude that the  
Figure 11: Fabricated prototype of the rectenna: (**a**) photograph of the top and back side; (**b**) photographs of the test setup and prototype inside a semi-anechoic chamber. retenna can effectively receive signals from all directions over the frequency bandwidth of interest, making it suitable for wireless power transfer and energy-harvesting applications. Moreover, the standard deviations remained consistently low across all measurement points, confirming the excellent reproducibility of the results. This high level of repeatability is attributed to the precision of the experimental setup and the controlled conditions provided by the use of an anechoic chamber. We also evaluated the rectenna performance in an indoor environment. For this purpose, we selected a residential apartment as a representative ambient indoor setting, featuring a ceiling height of approximately 2.5 m and a total area of around 50 m\({}^{2}\). First, we measured the DC voltage at the output of the rectenna versus distance for a frequency of 2.45 GHz using a microwave generator and a dipole antenna in vertical orientation, as shown in Figure 13a. Figure 13b presents the output DC power (\(\mathrm{P_{out\,DC}}\)), derived from the measured DC voltage at the rectenna output, as a function of distance.