## 3 Dual-CP Antenna Operation Principle  
To generate circular polarization, two orthogonal signals with 90\({}^{\circ}\) phase difference between them should feed the antenna. There are many techniques to design dual-circularly polarized antenna with RHCP and LHCP; here, the single hybrid coupler is used as a feeding structure to achieve a 90\({}^{\circ}\) phase difference between its output ports, which are considered the feeding terminals of the antenna. The excitation of port 1 results in RHCP, and the excitation of port 2 results in LHCP. Therefore, when feeding both ports, the antenna radiates both LHCP and RHCP that never interfere with each other. Consequently, polarization diversity is obtained. The coupler is placed between the ground plane and the radiating patch to provide a proximity-coupled feeding for the presented antenna. Four equal slits are printed on the patch to elongate the current path passing through the antenna perimeter, which shifts the resonant frequency of the antenna down to a lower value as a miniaturization technique [1] and to tune the ARBW of the antenna. There are some key parameters that should be studied to help improve the impedance and axial ratio bandwidths to cover the required mid-band 5G applications. Figure 4a,b illustrate the simulated reflection coefficient (\(S_{11}\)) and axial ratio (AR), respectively, of the proposed antenna at different slot length (\(L_{s}\)) values while keeping \(\mathrm{L_{tr}}\) at its optimum value. It can be seen that when \(L_{s}\) is increased, the frequency is shifted to the lower band. This can be interpreted as an increase in the path of the surface current. The optimum value of \(L_{s}\) is equal to 2.2 mm, so the 3 dB axial ratio extends along the range of (3.48-3.81 GHz) with a 330 MHz bandwidth. The second parameter is the length of the output transmission lines of the coupler (\(\mathrm{L_{tr}}\)). The simulated reflection coefficient (\(S_{11}\)) and axial ratio (AR) are exhibited in Figure 5a,b, respectively, at different values of (\(\mathrm{L_{tr}}\)) and \(L_{s}\) = 2.2 mm. This parameter has a significant influence on \(S_{11}\) and AR because it controls the matching between the patch and the hybrid coupler. Thus, it maintains and improves the orthogonal modes to obtain (CP). At \(\mathrm{L_{tr}}\) equal to 12.52 mm, the impedance matching is improved, and circular polarization is achieved. The results are the same when exciting port 2 due to the symmetry of the hybrid coupler design. The parameters that enhance and optimize both the impedance matching and the  
Figure 4: Simulation results of (**a**) reflection coefficient and (**b**) axial ratio of \(L_{s}\) at \(\mathrm{L_{tr}}\) = 12.52 mm. When port 1 is excited, the current distribution of the proposed antenna is demonstrated in Figure 6a-d to show the direction of the current at different time instants (\(\omega t=0^{\circ}\), \(90^{\circ}\), \(180^{\circ}\), and \(270^{\circ}\)), respectively, at 3.7 GHz. At \(\omega t=0^{\circ}\), the current direction is toward the positive \(x\)-axis on the circular patch, while at \(\omega t=90^{\circ}\), the current direction is toward the positive \(y\)-axis. At \(\omega t=180^{\circ}\) and \(\omega t=270^{\circ}\), the current direction is oriented toward the negative \(x\)-axis and negative \(y\)-axis, respectively. It can be seen that the rotation of the current moves in a counter-clockwise direction, which indicates the presence of RHCP in the positive \(z\)-axis direction. On the other hand, when port 2 is excited, LHCP is generated, and the rotation of the current moves in a clockwise direction. Thereby, when both ports are excited, dual CP is achieved. Figure 7 illustrates the 3D axial ratio pattern at different frequencies to verify the coverage of the 3 dB ARBW in the broadside direction. It is evident that from Figure 7a there is no CP at f = 3.3 GHz.