# A Dual-Polarized Omnidirectional Rectenna Array for RF Energy Harvesting Harvesting
## 2 Rectenna Array Design  
### Antenna Design  
A dual-polarized omnidirectional antenna array operating at 2.45 GHz is proposed, which comprises a horizontally polarized array and a vertically polarized array. The structure of the dual-polarized omnidirectional antenna array is shown in Figure 1. The horizontally polarized antenna subarray is designed with a double-layer structure to improve the gain, which contains two identical horizontally polarized omnidirectional elements, 1 and 2. The green and orange parts in Figure 1c,d represent the metal layers printed on the top and bottom of the horizontally polarized element, respectively. Each of the metal layers is printed on a 30-mil-thick Rogers RO4350B substrate with a dielectric constant of 3.66 and a loss tangent of 0.0037. The distance between the two horizontally polarized elements 1 and 2 is approximately one wavelength. From Figure 1c,d, each curved radiating branch of the horizontally polarized element is connected to the tapered balun by a tapering microstrip line and converges at the feed point. The location of the feed point is at the center of the circular dielectric substrate. As can be seen from Figure 1a,b, the vertically polarized antenna subarray consists of four identical dipole elements, and the design introduces a gradient balun structure to improve the impedance matching of the antenna elements. This subarray contains two inner and outer layers of C-UV 9400E cylindrical dielectric substrates of different heights. The dielectric constant and loss tangent of the C-UV 9400E substrate are 3 and 0.05, respectively. The thicknesses of both cylindrical dielectric substrates are 0.8 mm. The yellow part in Figure 1a,b is a metal reflector printed on the inner cylindrical dielectric substrate. The dipole elements are co-formed on both sides of the outer cylindrical dielectric substrate. The inner and outer cylindrical dielectric substrates are connected and fixed by eight nylon columns to improve the structural stability of the antenna array. The two circular dielectric substrates and the inner and outer cylindrical dielectric substrates are placed concentrically in the layout. The inner cylinder dielectric substrate is placed between two circular substrates and fits closely to the bottom of the upper dielectric substrate and the top of the lower dielectric substrate, respectively. Each part of the antenna array was simulated and optimized using the software Ansys HFSS. The optimized dimensions of the proposed antenna array are given in Table 1. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Parameter** & **Value (mm)** & **Parameter** & **Value (mm)** & **Parameter** & **Value (mm)** & **Parameter** & **Value (deg)** \\ \hline \(R_{0}\) & 18.1 & \(d_{1}\) & 0.9 & \(H_{2}\) & 122 & \(\theta_{1}\) & 42.5 \\ \(R_{1}\) & 37 & \(d_{2}\) & 1.2 & \(H_{3}\) & 0.8 & \(\theta_{2}\) & 3 \\ \(R_{2}\) & 22.6 & \(d_{3}\) & 0.3 & \(H_{4}\) & 0.8 & & \\ \(R_{3}\) & 29.9 & \(d_{4}\) & 0.7 & \(w_{\text{a}}\) & 6 & & \\ \(R_{4}\) & 35 & \(d_{5}\) & 5.6 & \(w_{\text{b}}\) & 1.7 & & \\ \(R_{5}\) & 2 & \(d_{6}\) & 0.9 & \(l_{\text{a}}\) & 28.2 & & \\ \(R_{6}\) & 3 & \(H_{0}\) & 0.762 & \(l_{\text{b}}\) & 23.2 & & \\ \(d_{0}\) & 0.3 & \(H_{1}\) & 60 & \(l_{\text{c}}\) & 2.5 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Optimized dimensions of the proposed antenna array. Figure 1: Structure of the antenna array: (**a**) 3D view; (**b**) Overall layout; (**c**) Top view of the horizontally polarized element; (**d**) Bottom view of the horizontally polarized element; (**e**) Zoomed-in view of the vertically polarized element. To verify the performance of the design, the antenna array was fabricated and measured. Figure 2 provides photographs of the fabricated array. Due to the limited internal space of the inner cylindrical dielectric substrate, it is difficult to use conventional SMA connectors for soldering. Thus, 50-\(\Omega\) SMA connectors with RF cable wires are used for power feeding to each element of the antenna array.