# Design and Development of a Wideband Planar Yagi Antenna Using Tightly Coupled Directive Element
## 4 Parametric Study and Analysis of Geometrical Antenna Parameters  
In a standard travelling-wave printed Yagi antenna, the effect of various geometrical antenna parameters such as (i) driven elements' width and length (ii) reflector's spacing, width and length (iii) different or equal length directive elements (iv) number of directive elements and (v) different or equal width directive elements, on critical performance parameter, has been extensively studied in the literature [34; 35; 36]. In our design, at first step we optimized each geometric parameter of the standard printed Yagi's elements to their values resulting the optimum performance of the antenna. For simplicity, we do not report parametric studies of these well-known geometric variables and their effect on antenna's performance. Instead, in this section, we analyze the effect of geometric parameters of the proposed tightly coupled directive element on the gain and reflection coefficient characteristics of the antenna. By multiple full-wave simulation analysis, we studied the effect of length, width and separations (r\({}_{\rm nf}\), r\({}_{\rm hw}\), d\({}_{4}\) and d\({}_{5}\) in Table 2) of the tightly coupled directive element on the gain and bandwidth of the proposed antenna. The length and width of the elliptic shape directive element are presented by their major and minor axis diameters, r\({}_{\rm md}\) and r\({}_{\rm hw}\), respectively. At first, by varying major and minor axis diameters of the said directive element, the variation inflection coefficient and gain values are studied as shown in Figure 2. The major and the minor axis diameters' values are varied over a range 8.2-32.8 mm and 4.4-8.8 mm, respectively. In Figure 2a, the major axis diameter is changed while keeping the minor axis diameter constant to a value of 4.4 mm and so on to 6.6 mm and 8.8 mm as shown in Figure 2b,c, respectively. It can be seen that the printed Yagi antenna experiences a significant change in bandwidth and gain by varying major axis diameter more than the minor axis diameter values. By analyzing these results, the optimum value of major and minor axis diameters are acquired to be 24.6 mm and 6.6 mm, respectively. In the next simulation setup, the position of the proposed directive element is optimized. In Figure 3, it can be seen that translating the said directive element away from the active dipoles critically effects the reflection coefficient thus the gain values over the proposed spectrum. Conversely, while the directive element is placed closer to the dipole arms, the proposed antenna still exhibited a wide bandwidth performance. Figure 1: Layout diagram of the Yagi antenna (**a**,**b**), with first directive element at \(\lambda_{\rm g}\)/4 and geometric parameters of the proposed antenna design (**c**,**d**), with tightly coupled directive element. Figure 2: Parametric study of proposed antenna structural parameters based on reflection coefficient and gain performance analysis with (**a**) \(r_{\rm{MW}}\) = 4.4 mm, (**b**) \(r_{\rm{MW}}\) = 6.6 mm and (**c**) \(r_{\rm{MW}}\) = 8.8 mm. Finally, the performance of the proposed antenna element having a wide tightly coupled directive element, is studied and compared with the antenna without wide directiective element. The major performance parameters such as reflection coefficient, realised gain and input impedance of both antenna elements are calculted and plotted. The simulated results of reflection efficient (S\({}_{11}\)) and realised gain are shown in Figure 4a,b, respectively. The antenna without wide directive element presents lower bandwidth as compared to the proposed design. The former antenna element exhibits reflection coefficient values (parameter S\({}_{11}\)) less than \(-10\) dB in the frequency range 2.5-3.2 GHz, which correspond to a 28% fractional bandwidth value. On the other hand, the proposed antenna presents S\({}_{11}\) less than \(-10\) dB in the frequency range 2.6-4.3 GHz correspond to a 49.3% fractional bandwidth value.