# A Miniaturized RHCP Slot Antenna for Wideband Applications Including Sub-6 GHz 5G
## 3 Design Steps  
The design procedure for the proposed antenna is illustrated in Figure 2, which follows (Ant. 1-Ant. 6) to reach the final structure. The simulation results of the reflection coefficient (\(S_{11}\)) and the broadside axial ratio (AR) of each antenna structure are demonstrated in Figures 3a and 3b, respectively. Ant. 1 is comprised of a conventional wide-slot antenna with a rectangular tuning stub fed by a microstrip line. The length of the slot (L) is approximately equal to half the guided wavelength corresponding to the resonant frequency (2.2 GHz) L \(\cong\lambda_{g}/2\), where the guided wavelength \(\lambda_{g}\) is given by \(\lambda_{g}=\lambda/\sqrt{\epsilon_{r}}\), \(\lambda\) is the free-space wavelength corresponding to the resonant frequency (2.2 GHz), and \(\epsilon_{r}\) is the dielectric constant of the substrate. The impedance BW in this step is equal to 1.39 GHz and resonates at 2.3 GHz, while the AR value is much greater than 3 dB, which does not appear in the figure. In other words, there is no circular polarization in this case. A horizontal rectangular stub with a width of \(W_{s1}\) is protruded from the left side of the radiating slot in Ant. 2, which shifts the resonant frequency to 2.15 GHz with a narrow impedance BW of 510 MHz due to the increase in the surface current path around the radiating slot, while the AR appears less than 3 dB because two orthogonal fields are generated. The specific shape and dimensions of the rectangular stub in Ant. 2 were determined through parametric studies aimed at optimizing the axial ratio performance. Although the geometry of the parasitic element is novel, the approach is based on previous studies working on CP antennas. For instance, studies have demonstrated that incorporating parasitic elements can significantly enhance the circular polarization characteristics by introducing additional orthogonal resonances that lead to attaining low AR values [26]. Thereby, this antenna provides CP with a narrow ARBW of 960 MHz. In Ant. 3, a rectangular strip is integrated into the edge of the tuning stub, which enhances the horizontal path of the current, and the AR has been improved significantly to extend along the range (1.9-4.4 GHz) with an ARBW of 2.5 GHz. Meanwhile, the impedance matching of the antenna is lost because the coupling between the feedline and the radiating slot has deteriorated. The mismatch in the impedance is reduced in Ant. 4. It shows that the tuning stub and feedline are translated to the right to add an additional  
Figure 1: Structure of the proposed antenna: (**a**) front view and (**b**) back view (unit: mm). resonant frequency. In addition to this, a vertical strip with a length of \(L_{p}\) is placed on the tuning stub to obtain an impedance BW of 810 MHz from 1.86 to 2.67 GHz. Ant. 4 just improves the impedance matching that was lost in Ant. 3. By cutting the upper corner of the tuning stub, Ant. 5 provides better performance for impedance matching than Ant. 4. The ARBW extends along a wide range from 1.96 to 4.64 GHz because this tapering tunes both the horizontal and vertical current components in such a way that generates two orthogonal equal magnitude components. Finally, Ant. 6 is presented to tune the coupling between the feedline and radiating slot to obtain a wideband impedance BW and ARBW by adding the rectangular parasitic patch to the slotted ground directly behind the tuning stub and attaching the horizontal strip to the tuning stub. The resulting structure provides an impedance BW of 2.8 GHz (1.90-4.7 GHz) and a 3-dB ARBW of 2.52 GHz along the range (1.90-4.42 GHz). Specifically, the work in [33] stated that a combination of a symmetrical slot length and a rectangular parasitic patch successfully enhanced the 3 dB ARBW. Figure 4 shows the position of the parasitic strip on the slotted ground plane with respect to the tuning stub. To explain the importance of adding this strip in terms of the \(S_{11}\) and AR, Figures 5a,b, respectively, depict the simulation results with and without the parasitic strip.