# A Dual-Polarized Omnidirectional Rectenna Array for RF Energy Harvesting Harvesting
## 2 Rectenna Array Design
### Antenna Design
The length of each RF cable is 0.2 m, and its measured insertion loss at 2.45 GHz is 0.26 dB. The inner and outer cylindrical dielectric substrates are fabricated using 3D printing technology. Considering the fabrication cost and complexity, the vertically polarized elements are printed on 45-\(\upmu\)m-thick flexible polyimide films with a dielectric constant of 3.5 and a loss tangent of 0.008. Then, the fabricated films are laminated to the surface of the outer cylindrical dielectric substrate to achieve conformality. During assembly, the corresponding through-holes of the inner and outer cylindrical substrates are aligned and fixed by nylon columns. In addition, the relative positions of the upper and lower horizontally polarized elements should be precisely aligned. Figure 3a shows the reflection coefficients of the horizontally polarized element 1 and the vertically polarized element 3 in the antenna array. It can be seen that the measured results are in good agreement with the simulated ones. The measured \(S\)-parameters of the horizontally polarized element 1 are below \(-10\) dB in the range of 2.16 GHz to 2.77 GHz, corresponding to a percentage bandwidth of 24.7%, while the measured \(S\)-parameters of the vertically polarized element 3 are below \(-10\) dB in the range of 2.17 to 2.58 GHz, corresponding to a percentage bandwidth of 17.3%. At the center frequency of 2.45 GHz, the \(S\)-parameters of both horizontally and vertically polarized elements are below \(-30\) dB. Due to the symmetry, as the performances of those elements with the same polarization are almost identical, the \(S\)-parameters of the rest of the elements are not shown. Figure 3b gives the isolation performances between the antenna elements in the antenna array. Within the operating bandwidth of the antenna array, the measured isolations between the elements with the same polarization are better than 20 dB, while the isolations between those elements with different polarizations are better than 55 dB. Figure 2: Photographs of the fabricated antenna array: (a) Side view; (b) Oblique view; (c) Top view. The radiation patterns of this antenna array were measured in an anechoic chamber. Figure 4 depicts the radiation patterns of the antenna array in the xoy and yoz planes at 2.45 GHz in the horizontal polarization. The measured results show that in the horizontal xoy plane, the antenna array has good omnidirectional radiation performances with a gain fluctuation of 0.85 dB and a peak gain of 5.01 dBi. In both xoy and yoz planes, the cross-polarizations of the antenna array are below \(-\)30 dB. Figure 5 plots the radiation patterns of the antenna array in the xoy and yoz plane at 2.45 GHz in the vertical polarization. The measured results show that in the horizontal xoy plane, the peak gain of the antenna array is 1.83 dBi and the gain fluctuation is 1.36 dB. In the xoy plane, the cross-polarization of the antenna array is below \(-\)30 dB, while the cross-polarization in the yoz plane is below \(-\)40 dB. The measured peak gains are slightly lower than the simulated ones, but within the allowable margin of error, probably due to errors in the fabrication and assembly of the antenna array. From the above results, the antenna array shows good omnidirectional performances in both horizontally and vertically polarized operating states and can be readily applied for RF energy harvesting. Figure 4: Radiation patterns of the antenna array at 2.45 GHz in the horizontal polarization: (**a**) xoy plane; (**b**) yoz plane. Figure 3: \(S\)-parameters of the elements in the proposed antenna array: (**a**) Reflection coefficients; (**b**) Isolation performances.