# Conceptual Design of a Semi-Dual Polarized Monopulse Antenna by Computer Simulation
## 2 Feed Design
The performance characteristics of the designed feed are simulated using the CST Studio Suite. Figure 6 shows the reflection coefficient (S1,1 = S2,2) at the sum port coaxial input. It is less than \(-15\) dB at 14.5-16.0 GHz. The reflection coefficient (S3,3 = S4,4 = S5,5 = S6,6) at the difference channel coaxial input is less than \(-11\) dB at 14.5-16.0 GHz. A scattering parameters value of \(-40\) dB is as good as values smaller than \(-40\) dB, and, thus, we have truncated values of less than \(-40\) dB. This also applies to directivity values of less than \(-30\) dB. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & \(\mathbf{d}\) & \(\mathbf{a}\) & \(\mathbf{b}\) & \(\mathbf{g}\) \\ \hline Value & 46.00 & 11.61 & 5.81 & 1.00 \\ \hline Parameter & \(T_{1}\) & \(T_{2}\) & Choke slot depth & \(C\) \\ \hline Value & 2.00 & 2.00 & 6.50 & 3.90 \\ \hline Parameter & \(L\) & \(K\) & \(E/F\) & \\ \hline Value & 2.90 & 10.50 & 1.27/4.11 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Dimensions (mm) of the designed feed. Figure 5: Dimensional parameters of the designed feed and the coaxial probe for the difference channel waveguide. The isolation (negative of S1,2) between the sum channel Port 1 and Port 2 is greater than 13.5 dB at 14.5-16.0 GHz. It is increased by 6.5 dB from the value of 20.0 dB (negative of S2(1),1(1)) in Figure 3b where the sum channel square waveguide is perfectly matched. The increase in the sum channel isolation is due to reflection at the square waveguide aperture and external coupling by currents flowing on the feed's flange surface. The RHCP directivity patterns of the designed feed are shown in Figure 7 in \(\varphi=0^{\circ}\) and \(45^{\circ}\) cuts where the feed aperture lies in the \(xy\) plane as shown in Figure 5. Due to structural symmetry, the LHCP patterns are the same as the RHCP patterns. The pattern in the \(\varphi=90^{\circ}\) cut is the same as that in the \(\varphi=0^{\circ}\) cut. Similarly, the pattern in the \(\varphi=-45^{\circ}\) cut is the same as that in the \(\varphi=45^{\circ}\) cut. In Figure 7, we can observe that the directivity pattern in the \(\varphi=0^{\circ}\) cut is quite similar to that in the \(\varphi=45^{\circ}\) cut, showing a good angular symmetry in the directivity. The maximum directivity ranges from 6.2 dBc to 6.6 dBc at 14.5-16.0 GHz. The 10 dB beamwidth ranges from 152.2\({}^{\circ}\) to 150.3\({}^{\circ}\) when the frequency changes from 14.5 GHz to 16.0 GHz. The use of dual chokes reduces the boresight gain by 2-3 dB (not shown in this paper), which is a fortuitous result since the wave radiated at small angles around the feed boresight is reflected back from the reflector surface toward the feed itself. Part of the reflected wave enters the feed aperture, increasing the reflection coefficient, while some of the reflected waves are scattered by the feed, degrading the performance of the reflector antenna. The use of the dual chokes reduces the back radiation by 4-6 dB. Figure 6: Reflection and coupling coefficients of the designed feed. Figure 7: RHCP directivity patterns of the sum channel. Figure 8 shows the RHCP phase patterns of the sum channel with a far-field origin at the phase center, which is found to be 0.2 mm into the square waveguide from the aperture plane. Over the 10 dB beamwidth (approximately 150\({}^{\circ}\)), the sum pattern phase variation is about \(\pm\)10\({}^{\circ}\) at 14.5-16.0 GHz. Figure 9 shows the axial ratio (AR) patterns of the sum channel. In the \(\varphi\) = 0\({}^{\circ}\) cut, the axial ratio is less than 3 dB at 14.5-16.0 GHz over the 10 dB beamwidth (\(-75^{\circ}\leq\theta\leq 75^{\circ}\)). In the \(\varphi\) = 45\({}^{\circ}\) cut, it is less than 10 dB. The square waveguide radiating the TE\({}_{10}\) mode does not yield a circular symmetric axial ratio pattern. Figure 10 shows the directivity patterns of the azimuth difference channel.