# 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
Figure 7a shows the simulated results of S\({}_{11}\) and S\({}_{21}\) with various distances between slot aperture and the director (\(d\)). In this case, the length of the director (\(l\)) is fixed at 2.7 mm (\(\approx\)0.52 \(\lambda_{\rm g}\)). When \(d\) is 1.6 mm (\(\approx\)0.31 \(\lambda_{\rm g}\)), although the S\({}_{11}\) performance is not the best, the S\({}_{21}\) performance is the most stable in the target band. The simulated S\({}_{21}\) is lower than \(-\)19 dB over the target band when \(d\) is 1.6 mm. Also, the simulated radiation pattern with \(d\) of 1.6 mm is stable, as shown in Figure 7b. If the distance is too close or too far, such as \(d\) is 1.2 mm (\(\approx\)0.23 \(\lambda_{\rm g}\)) or 2.2 mm (\(\approx\)0.43 \(\lambda_{\rm g}\)), the radiation pattern deteriorates. Figure 5: Parameter studies of the dimension of Slit 2 (dimension of Slit 1 is fixed): (**a**) antenna structure with Slit 1 and Slit 2, (**b**) simulated results of the reflection coefficient and mutual coupling with various lengths (\(h_{2}\)) of Slit 2 (\(w_{2}\) is fixed at 0.3 mm), and (**c**) simulated results of the reflection coefficient and mutual coupling with various widths (\(w_{2}\)) of Slit 2 (\(h_{2}\) is fixed at 0.6 mm). Figure 6: The \(1\times 2\) array antenna with directors and optimized slits. Figure 7: Parameter studies of the distance between the slot aperture and the director (\(l\) is fixed at 2.7 mm): (**a**) simulated results of the reflection coefficient and mutual coupling with various distances (\(d\)), and (**b**) simulated radiation patterns with various distances (\(d\)) in the \(xz\) plane at 28 GHz. The length (\(l\)) of the director is also an important factor for the quasi-Yagi antenna. Figure 8a shows the simulated results of S\({}_{11}\) and S\({}_{21}\) with various \(l\). In this case, \(d\) is fixed at 1.6 mm. When \(l\) is 2.7 mm (\(\approx\)0.52 \(\lambda_{\text{g}}\)), the S\({}_{21}\) performance is the most stable in the target band. Also, the simulated radiation pattern with \(l\) of 2.7 mm is stable, as shown in Figure 8b. If \(l\) is longer than 2.7 mm, the radiation pattern deteriorates. Therefore, we considered that the optimized length and distance of director are 2.7 and 1.6 mm, respectively. The effects of the slits on the mutual coupling and radiation performance become more apparent when considered together with the director. The surface current flows from port 1 to port 2 in a \(1\times 2\) array antenna without slits as shown in Figure 9b. The surface current is concentrated at the edge of the ground plane, which is indicated by a dotted red circle in Figure 9b. Figure 8: Parameter studies of the length of the director (\(d\) is fixed at 1.6 mm): (**a**) simulated results of the reflection coefficient and mutual coupling with various lengths of the director (\(l\)), and (**b**) simulated radiation patterns with various lengths (\(l\)) in the \(xz\) plane at 28 GHz. Figure 9: The \(1\times 2\) array antenna without slits: (**a**) top view, (**b**) simulated surface current distribution at 28 GHz, and (**c**) simulated E-field distribution in the E-plane at 28 GHz. To mitigate the effect of the surface current, as shown in Figure 10a, we inserted an optimized slit (Slit 1) at that position. By inserting 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 10b. However, the flow of the surface current on the substrate between adjacent directors still exists, and this current contributes to increase the mutual coupling. Electromagnetic coupling occurs between the directors and the surface current concentrated at the edge of the ground plane, which is indicated by the solid blue square in Figure 10b, thereby generating mutual coupling between electromagnetic waves propagating in the \(+z\) direction. This mutual coupling between the propagating electromagnetic waves is described as the surface current on the substrate, which is indicated as a dotted red circle in Figure 10b.