# Design and Development of a Wideband Planar Yagi Antenna Using Tightly Coupled Directive Element
## 3 Proposed Yagi Antenna Design  
The location and geometric parameter (length and width) of the directive elements is very important in designing a Yagi antenna element. The input impedance of the antenna element is mainly affected by the first parasitic directive element. We have exploited this fact to design a wideband Yagi antenna. At first, we designed a Yagi antenna using conventional design approach, where first parasitic directive element is placed at \(\lambda_{\text{g}}\)/4 from the active dipoles, towards end-fire direction as shown in Figure 1. The layout diagram comprising top and bottom planes of the antenna structure are shown in Figure 1a,b, respectively. The design procedure is initiated by the elliptically shaped driven elements of \(\lambda_{\text{g}}\)/4 length printed on both metallization planes of the substrate. The truncated ground plane layer acts as a reflector for the Yagi structure. The antenna is fed by a 50 \(\Omega\) microstrip line printed on the ground plane. The plated-via holes are placed on the two sides (left and right) of the printed circuit board (PCB). These holes are capable of reducing the side-lobe levels (SLL) by concentrating the radiated fields toward the end-fire direction [37]. The antenna is designed on a FR4 substrate with relative permittivity, dissipation factor and thickness of 4.4, 0.025 and 1.6 mm, respectively. By placing directive elements toward the end-fire direction of the antenna, the electromagnetic energy can be concentrated and a broad bandwidth matched to 50 \(\Omega\) input impedance can be achieved. Therefore, to achieve wide bandwidth, we introduced an additional parasitic directive element between the active dipole arms and first directive element called tightly coupled directive element. The first directive element is wider compared to the others, and it is placed in close proximity to the driven elements. The top and bottom plane drawings of the proposed design having additional directive element is shown in Figure 1c and d, respectively. This placement is quite uncommon in conventional printed Yagi-antenna design. It is worth mentioning that the directive elements are placed on the top and bottom planes of the antenna. To achieve efficient electric-field coupling among the directive elements, the bottom plane directive elements are placed with some offset with respect to the top plane directive elements. The purpose is to enhance the effective width toward the direction of propagation. Critical antenna design parameters, such as the first directive-element spacing, length, width, the other directive-elements dimensions (lengths and widths), and the spacing between directive elements, are optimized using the full-wave electromagnetic simulation program CST Microwave Studio 2019 [38]. The final antenna dimensions used in the fabrication are presented in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \(\mathbf{L}\) & \(\mathbf{W}\) & \(\mathbf{L_{f}}\) & \(\mathbf{L_{fg}}\) & \(\mathbf{L_{g}}\) & \(\mathbf{r_{nf}}\) & \(\mathbf{r_{nw}}\) \\ \hline
140 & 60 & 67 & 12.9 & 55.5 & 22.6 & 6.6 \\ \hline \(\mathbf{r_{md}}\) & \(\mathbf{r_{nd}}\) & \(\mathbf{d_{1}}\) & \(\mathbf{d_{2}}\) & \(\mathbf{d_{3}}\) & \(\mathbf{d_{4}}\) & \(\mathbf{d_{5}}\) \\ \hline
24.6 & 4.3 & 15.47 & 15.27 & 13.95 & 7.2 & 6.77 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Optimized parameters of the proposed antenna in mm.