# A Novel High Gain Monopole Antenna Array for 60 GHz Millimeter-Wave Communications
## 2 Antenna Design
### The Proposed Single Microstrip Monopole Antenna
#### 2.1.2 The Proposed Ground Antenna Structure  
The DGS played an important role in the broadband characteristics of this monopole antenna as it helped to match the patch antenna with a feed line in the mmWave range of frequencies. The slots in the ground plane created a capacitive load that equalized the inductive nature of the monopole patch antenna to produce closely pure resistive input impedance. As the top plane was optimized by adding several techniques to improve the antenna performance, the ground plane modification would also be introduced and optimized to further enhance the performance obtained from the last design procedure of the radiator. The ground structure was defected by adding a single circular slot at the bottom center, forming ground 2, as shown in Figure 3b. Two rings at the left and right sides of the ground plane were etched to form ground 3, as depicted in Figure 3c. Finally, small rings at a specific distance from each other were embedded in the lower half of the ground plane to form ground 4, as illustrated in Figure 3d. By using the optimum parameters, the impedance bandwidth was improved from 1.5 GHz to 2.38 GHz, as shown in Figure 4, with a maximum gain achieved of 9.9 dBi, as tabulated in Table 2. It could be noticed from Table 2 that the resonance frequency was shifted from 64.25 to 59 GHz by implementing each iteration of the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Steps** & **Frequency (GHz)** & **Return Loss (dB)** & **Bandwidth (GHz)** & **Gain (dBi)** \\ \hline Iteration 1 & 59.5 & \(-\)22.9 & 1.15 & 6.87 \\ Iteration 2 & 63.51 & \(-\)13.7 & 1.45 & 8.1 \\ Iteration 3 & 63.27 & \(-\)24 & 1.43 & 9.3 \\ Iteration 4 & 63.35 & \(-\)22.5 & 1.275 & 8.14 \\ Iteration 5 & 64.25 & \(-\)21.3 & 1.5 & 8.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation results of each patch iteration. Figure 3: Evolution of the ground structure: (**a**) Partial ground, (**b**) Modified ground plane by DGS as circulars slot, (**c**) added DGS as rings slot at the both sides, (**d**) added DGS as small rings slot. ground plane, and the achieved resonance frequency was the desired one with an improved bandwidth percentage of 4%. This would allow the proposed monopole antenna to better utilize the MMW band. The layout of the proposed monopole antenna with all its design parameters is depicted in Figure 5. It was worth noting that the used structures for both the radiator and ground plane were iteration 5 and ground 4, respectively. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Steps** & **fc (GHz)** & **Return Loss (dB)** & **Bandwidth (GHz)** & **Gain (dBi)** \\ \hline Ground 1 & 64.25 & \(-21.3\) & 1.5 & 8.4 \\ Ground 2 & 61.3 & \(-9\) & - & 8.4 \\ Ground 3 & 59 & \(-8\) & - & 8.3 \\ Ground 4 & 59 & \(-24.12\) & 2.38 & 9.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation results of the ground plane iteration. Figure 4: Simulated return loss characteristics of varied ground structures. Figure 5: Configuration and parameters of the proposed monopole antenna (**a**) Front view; (**b**) Back view. A parametric study was performed to determine the optimum radius for the circular slots (Rcp). The tested radii of these slots were 0.92, 1.02, and 1.12 mm, and the return loss (S\({}_{11}\)) results obtained for each case are shown in Figure 6. The lowest return loss at the desired frequency was \(-\)23 dB and was achieved at Rcp = 1.12 mm, while the other values failed to achieve the desired performance. The spacing between the double T-shapes (D2) was also optimized using a parametric study in order to maintain a mutual coupling with the DGS, which was applied later. Three different spaces were tested 1, 3, and 4 mm; the results are shown in Figure 7. The space D2 of 3 mm introduced the lowest return loss and enhancement in bandwidth at the achieved band. Furthermore, a parametric study was carried out on the thickness of the rings etched in the ground plane, and the optimum thickness (R1) was obtained.