# Quasi-Yagi Slotted Array Antenna with Fan-Beam Characteristics for 28 GHz 5G Mobile Terminals
## 2 Antenna Design and Performance
### 1 \(\times\) 2 Array Antenna Design and Performance  
For designing antenna arrays, mutual coupling between radiators should be considered for good antenna performance. As shown in Figure 3a, we designed a 1 \(\times\) 2 array antenna composed of basic slot radiators to investigate mutual coupling (S\({}_{21}\)) between the radiators. The center-to-center spacing is 5.35 mm (0.5 \(\lambda\) at 28 GHz). The surface current on the ground plane between the adjacent radiators flows from port 1 to port 2, which is indicated by a dotted red square in Figure 3b, and this current flow results in high mutual coupling. Figure 2: Single slot radiator: (**a**) without a director, (**b**) with a director, (**c**) side view, (**d**) simulated reflection coefficients, and (**e**) simulated radiation patterns at 28 GHz. To mitigate the effect of the surface current, as shown in Figure 4a, we inserted a slit (Slit 1) in the middle of two slots. Figure 4c shows the simulated results of S\({}_{11}\) and S\({}_{21}\) with various lengths of Slit 1 (\(h_{1}\)). In this case, the width of Slit 1 (\(w_{1}\)) is the same as the width of the slot radiator (\(w_{1}=0.3\) mm). The best performance of S\({}_{11}\) at 28 GHz is achieved when \(h_{1}\) is 2.4 mm. Without Slit 1 (\(h_{1}=0\) mm), the simulated S\({}_{21}\) is lower than \(-11.96\) dB over the target band. However, when \(h_{1}\) becomes longer than 2.4 mm, S\({}_{21}\) converges to below \(-15\) dB over the target band. Figure 4d shows the simulated results of S\({}_{11}\) and S\({}_{21}\) with various \(w_{1}\). In this case, \(h_{1}\) is fixed at 2.4 mm. The best performance of S\({}_{11}\) is achieved when \(w_{1}\) is 0.3 mm, and the variation of \(w_{1}\) has little effect on the performance of S\({}_{21}\). Figure 4: _Cont._  
Figure 3: A 1 \(\times\) 2 array antenna without directors: (**a**) top view, and (**b**) simulated surface current distribution at 28 GHz. Therefore, we considered that the optimized dimension of Slit 1 is \(2.4\times 0.3\) mm. As a result, by inserting the optimized Slit 1, most of the surface current on the ground plane between the radiators is blocked, which is indicated by the solid black square in Figure 4b. As shown in Figure 5a, we inserted another slit (Slit 2), maintaining Slit 1, to enhance the impedance matching by disturbing the surface current concentrated at the edge of the ground plane, which is indicated by the dotted blue rectangles in Figure 4b. In addition, inserting Slit 2 contributes to reduce the mutual coupling when the directors are added. This will be discussed in a later paragraph. Figure 5b shows the simulated results of S\({}_{11}\) and S\({}_{21}\) with various lengths of Slit 2 (\(h_{2}\)). In this case, the width of Slit 2 (\(w_{2}\)) is the same as the width of Slit 1 (\(w_{2}\) = 0.3 mm). The best impedance matching at 28 GHz is achieved when \(h_{2}\) is 0.6 mm. Figure 5c shows the simulated results of S\({}_{11}\) and S\({}_{21}\) with various values of \(w_{2}\). In this case, \(h_{2}\) is fixed at 0.6 mm. The best performance of S\({}_{11}\) is achieved when \(w_{2}\) is wider than 0.2 mm, and the performances of S\({}_{21}\) are almost same regardless of the variation of \(w_{2}\). Therefore, we considered that the optimized dimension of Slit 2 is \(0.6\times 0.3\) mm. Figure 4: Parameter studies of the dimension of Slit 1: (**a**) antenna structure with Slit 1, (**b**) simulated surface current distribution at 28 GHz (when \(h_{1}\) = 2.4 mm and \(w_{1}\) = 0.3 mm), (**c**) simulated results of the reflection coefficient and mutual coupling with various lengths (\(h_{1}\)) of Slit 1 (\(w_{1}\) is fixed at 0.3 mm), and (**d**) simulated results of the reflection coefficient and mutual coupling with various widths (\(w_{1}\)) of Slit 1 (\(h_{1}\) is fixed at 2.4 mm). Figure 5: \(Cont.\)  
As shown in Figure 6, directors are added to the \(1\times 2\) array antenna with optimized slits to reinforce the radiated field toward the director.