## 3 Flared SBF Antenna with Iris  
To further improve the reflection coefficient (S11) of the flared SBF antenna, the length of the waveguide feed aperture (a = 40.387 mm) was shortened on both sides by adding thin metal strips as shown in Figure 5. These metal strips are usually referred to as a waveguide iris. The waveguide iris can either model a shunt capacitance or inductance, depending on whether it is located in either the transverse plane of the electric field or the magnetic field [21]. In our case, the iris was placed in the magnetic field, hence it created an inductive element that can provide the necessary matching for the waveguide's characteristic impedance. Improved impedance matching lowers the S11 result, thereby increasing the impedance bandwidth. The flaring angle and the rim height were kept constant at \(\mathrm{\theta}t=20^{\circ}\) and \(\mathrm{Hr}=0.6\)\(\lambda\), as these produced optimized gain, bandwidth, and cross-polarization results in the previous section.  
A parametric study was carried out in this section to investigate the effect of the iris on the reflection coefficient (impedance bandwidth) and the gain of the flared SBF antenna. The length of the iris is the same as the width of the waveguide feed (b = 20.193), while the width of the iris is varied from 1 mm to 5 mm with a step of 1 mm to determine the best value for optimal performance. The results are compared to the design without the iris (0 mm iris width).  
Figure 5: The geometry of the flared SBF antenna with an iris.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**\(\mathrm{\theta}\)f** & **Hr (\(\lambda\))** & **D\({}_{\mathrm{U}}\) (mm)** & **Peak Gain (dBi)** & **Gain @ 5.5 GHz (dBi)** & **Imp. BW (\%)** &
\begin{tabular}{c} **Min. Cross-Pol.** \\ **Ratio (dB)** \\ \end{tabular} \\ \hline \multirow{5}{*}{\(20^{\circ}\)} & 0.4 & 125.9 & 15.41 & 15.27 & 26.4 & 19.73 \\ \cline{2-6}  & 0.5 & 129.9 & 16.58 & 16.34 & 50.6 & 20.4 \\ \cline{1-1} \cline{2-6}  & 0.6 & 133.8 & 17.22 & 17.02 & 55.0 & 21.32 \\ \cline{1-1} \cline{2-6}  & 0.7 & 137.8 & 17.18 & 17.04 & 33.0 & 20.35 \\ \cline{1-1} \cline{2-6}  & 0.8 & 141.8 & 17.18 & 17.15 & 29.1 & 16.9 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Performance comparison of the flared SBF antenna with different rim heights (Hr).  
The effect of the inductive iris on the reflection coefficient (S11), impedance bandwidth, and peak realized gain are provided in Figure 6. Figure 6a shows the reflection coefficient plots for different iris widths (0 to 5 mm) at a 20\({}^{\circ}\) flaring angle. As the width of the iris is increased from 0-3 mm, the frequency bandwidth for S11 \(<-10\) dB increased from 3.3 GHz or 55% (4.3-7.6 GHz) to 3.85 GHz or 66.7% (3.85-7.7 GHz). As the iris width is increased from 3-5 mm, the bandwidth starts to decrease from 3.85-3.7 GHz 62.2% (4.1-7.8). The 3 mm iris width provides the best impedance matching between the waveguide feed and the SBF antenna as it produces the widest bandwidth.  
Figure 6: Flared SBF antenna with different iris widths: (**a**) Reflection coefficient as a function of frequency; (**b**) Peak realized gain as a function of frequency; (**c**) Cross-polarization ratio as a function of frequency.  
Figure 6b,c shows the effect of the iris on the realized gain and the cross-polarization ratio of the flared SBF antenna. As can be seen in the figure, introducing the iris does not affect the gain of the antenna and the cross-polarization ratio of the flared SBF antenna. The peak realized gain and the minimum cross-polarization ratio for all cases were \(\sim\)17 dBi at 5.6 GHz and \(-\)21 dB at 5.6 GHz, respectively. This is expected as the purpose of the iris is to improve the impedance matching.