## 4 Conclusions  
Various cases of placing the second element of the two-port MIMO antenna are studied and verified by testing the hardware prototype in this paper. The antenna offers wideband and wide ARBW along with high gain. The two-port MIMO is constructed to get LHCP and RHCP. The second element of the two-port system is placed in various orientations to reference the first element to study the bandwidth, gain, and ARBW of the suggested mm-wave antenna. The suggested antenna MIMO systems offers wideband of 25.5-30 GHz with peak gain of 8.75 dBi. The antenna provides circular polarization with a wide bandwidth of 25.2-28.3 GHz, covering almost all bands of 28 GHz 5G applications. The reference antenna and its four cases of MIMO configuration are designed by using High Frequency Structure Simulator (HFSSv9). Moreover, the results in terms of bandwidth, mutual coupling, gain, and ARBW of all four cases are compared. Alternatively, the results of the suggested design are also compared with published work, which show that the suggested work offers wideband, high gain, wide ARBW, simple geometry compact size, offering both RHCP/LHCP and adoption of a simple design approach. The results, discussion, and comparison table show that the suggested two-port MIMO antenna is the best and most solid applicant for future mm-wave devices for CP applications. Conceptualization, D.H.E. and M.F.A.S. ; methodology, software, validation, F.T., H.A.H. and M.S.A. ; formal analysis, investigation, resources, H.A., D.H.E. and Y.H.E. ; data curation, M.T.H., M.F.A.S. and S.Y.A.F. ; writing--original draft preparation, F.T., H.A.H., M.S.A. and H.A. ; writing--review and editing, D.H.E., Y.H.E., M.T.H., M.F.A.S. and S.Y.A.F. ; project administration, D.H.E. All authors have read and agreed to the published version of the manuscript. This research is funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R238), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. This research is also funded by the College of Technical Innovation, Zayad University, Dubai, United Arab Emirates and the College of Engineering and IT, University of Dubai, United Arab Emirates. **Data Availability Statement**: Not applicable. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Ref.** & **Antenna Size** & **Operating** & **Bandwidth** & **Gain** & **Gain** & **Axial Ratio** & **Antenna Type** & **Bandwidth** & **and Mechanism** \\  & **mm\({}^{3}\)** & **(GHz)** & **(GHz)** & **(dBi)** & **Polarization Type** & **(GHz)** & **and** & **Mechanism** \\ \hline
[11] & \(19\times 17\times 0.8\) & 28 & 27–29.5 & 8 & RHCP & 27.3–29 & & \\  & 38 & 35–38.2 & 8.7 & LHCP & 35–38.2 & & \\
[12] & \(180\times 180\times 1.67\) & 19.7 & 17.2–22.7 & - & RHCP & 17–19 & & \\  & & & & LHCP & & & \\
[13] & \(70\times 63.5\times 2.2\) & 28 & 27–29.5 & - & LHCP & 27.7–28.3 & SIW Antenna Array \\
[14] & \(28.7\times 27\times 2.5\) & 28 & 26.9–30.7 & 11.86 & CP & 27.3–29.65 & Series slot fed MIMO array \\
[15] & \(20.4\times 20.4\times 0.5\) & 27.5 & 24.5–31 & 11 & CP & 25–29.3 & Meta-surface-based antenna \\
[17] & \(10\times 10\times 0.51\) & 32 & 28–39.2 & 12 & CP & 34–38 & CPW fed Antenna with CRLH-MTM \\
[18] & \(20\times 20\times 0.254\) & 30 & 26.5–34.7 & 9.5 & RHCP & 28.5–32 & DRA array \\
[19] & \(10\times 10\times 15\) & 28 & 24.2–30.4 & 8.5 & CP & 24.5–30 & ME dipole Antenna \\
[22] & \(10\times 10\times 2.199\) & 28 & 27.7–29.55 & - & LHCP & 27.7–29.25 & SIW Array Antenna \\ This Work & \(20.5\times 12\times 0.79\) & 28 & 25.5–30 & 8.75 & RHCP & 25.2–28.3 & MIMO Configuration \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of proposed study with state-of-the-art.