# A High-Gain and Wideband MIMO Antenna for 5G mm-Wave-Based IoT Communication Networks
## 2 Proposed Antenna Design Procedure  
### Antenna Element and MIMO Configuration  
The geometry of the proposed design is demonstrated in Figure 1. A Rogers family substrate, i.e., RO-4350B with a thickness of 0.168 mm is used in the proposed design. The proposed system was designed and simulated using computer simulation technology (CST) software. A single element is backed by a truncated ground plane with a front side radiating structure modification to attain a wideband operation with optimal gain, efficiency, etc. Table 1 provides the dimensions summary of the proposed antenna element.  
The following expression can be utilized to estimate the feedline width [14].  
\[fw=w_{Z_{o}}=\left(\frac{377}{Z_{o}\sqrt{\varepsilon_{r}}}-2\right)\times h \tag{1}\]  
The Equation (1) is based on a combination of components, i.e., feed line width (\(fw\)), impedance (\(Z_{o}\) = 500 ohm), substrate thickness (\(h\)), and dielectric constant (\(\varepsilon_{r}\)). The reflection coefficient for the proposed antenna element is analyzed in Figure 2. Based on the \(-\)10 dB criterion, the response is well below \(-\)10 dB for the desired operational bandwidth. The  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value (mm)** & **Parameter** & **Value (mm)** \\ \hline Al & 14.0 & Aw & 10.0 \\ fw & 0.5 & cl1 & 3.6 \\ fl & 6.0 & cl & 3.6 \\ bl & 1.95 & Gl & 5.0 \\ bw & 2.0 & – & – \\ \hline \hline \end{tabular}
\end{table}
Table 1: Dimensions of the proposed antenna element.  
Figure 1: Antenna element geometry: (**a**) front, (**b**) back.  
design evolution steps of the proposed antenna are shown in Figure 2. Initially, a single radiating structure (step 1) provides a narrow bandwidth, whereas introducing a similar radiating structure on the top of the first radiating structure in step 2 yields an improvement in the bandwidth. Finally, in step 3, adding a sharp leaf-type radiating shape on the top of the first two structures results in a wider bandwidth. Thus, the structure achieved in step 3 is selected for subsequent processes, as it operates with a wide bandwidth and a meaningful magnitude of reflection coefficient. The basic antenna element (antenna) is a variation of the well-known circular broadband patch antenna tuned to work in the frequency range of 16.7-25.4 GHz. The mathematical modelling described in [26] was also helpful in determining the final design, including these design evolution steps. The single element is then extended to two-port MIMO configuration, as shown in Figure 3, such that it could be helpful to achieve field decorrelation. The overall dimensions of the proposed MIMO configuration are 14 mm \(\times\) 20 mm \(\times\) 0.168 mm.  
The reflection coefficient for the proposed MIMO antenna is shown in Figure 4a, which is well below \(-\)10 dB. The isolation between the antenna elements is shown in Figure 4b. It is important to analyze the isolation between the MIMO antenna elements because it becomes an issue when more than one antenna element is assembled on a single printed circuit board (PCB) [27; 28; 29; 30]. A maximum isolation level of more than \(-\)30 dB is achieved within the operating band. Overall, satisfactory likeness between the measured and simulated results is achieved. In the figures, sim. represents simulated results, whereas mea  
Figure 3: MIMO antenna geometry: (**a**) front, (**b**) back.  
Figure 2: Reflection coefficient comparison of the proposed antenna element design evolution steps.