# Conceptual Design of a Semi-Dual Polarized Monopulse Antenna by Computer Simulation
## 2 Feed Design
The position of the probe from the back short and the probe height are adjusted for the lowest reflection at 14.5-16 GHz. Table 1 shows the dimensions of the designed polarizer operating at 14.5-16.0 GHz. The square waveguide has inside wall dimensions of 11.61 \(\times\) 11.61 mm (0.59 \(\lambda\)\(\times\) 0.59 \(\lambda\) at 15.25 GHz), whose cut-off frequency of the fundamental TE\({}_{10}\) or TE\({}_{01}\) mode is 12.92 GHz. The total length of the polarizer is \(L_{1}+L_{2}+L_{3}+L_{4}+L_{5}\) = 42.98 mm. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & \(\mathbf{L_{0}}\) & \(\mathbf{L_{1}}\) & \(\mathbf{L_{2}}\) & \(\mathbf{L_{3}}\) \\ \hline Value & 12.57 & 5.94 & 5.50 & 5.58 \\ \hline Parameter & \(L_{4}\) & \(L_{5}\) & \(H_{1}\) & \(H_{2}\) \\ \hline Value & 1.78 & 11.61 & 1.48 & 3.30 \\ \hline Parameter & \(H_{3}\) & \(H_{4}\) & \(H_{5}\) & \(t\) \\ \hline Value & 5.58 & 9.62 & 11.61 & 1.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dimensions (mm) of the square waveguide septum polarizer. Figure 2: **(a)** Septum polarizer with waveguide modes. **(b)** Dimensional parameters. **(c)** Coaxial probe excitation. **(d)** Dimensions of the coaxial probe. The reflection and transmission coefficients of the designed polarizer are shown in Figure 3a,b. Figure 3a shows the polarizer scattering parameters with waveguide modes (wave ports in the CST Studio Suite), while Figure 3b shows the coaxial probe version. The notation S3(1),1(1) means the transmission coefficient from Mode 1 of Port 1 (denoted 1(1)) to Mode 1 of Port 3 (denoted 3(1)). The curve for S3(2),(1) overlaps that of S3(1),1(1), and, thus, it is not drawn in Figure 3a,b. The isolation between input Ports 1 and 2 (negative of S2(1),1(1)--transmission from Mode 1 of Port 1 to Mode 1 of Port 2) is greater than 20 dB in both cases at 14.5-16.0 GHz. The return loss at input Ports 1 and 2 (negative of S1(1),1(1) and S2(1),2(1)) is greater than 28 dB with waveguide modes and greater than 16 dB with a coaxial probe. Figure 4 shows the transmission phase of the designed polarizer excited with the coaxial probe. With the input Port 1 being energized, the phases of Mode 2 and Mode 1 at Port 3 are compared. The phase of Mode 2 leads that of Mode 1 by 90.2\({}^{\circ}\) to 90.8\({}^{\circ}\) at 14.5 GHz to 16.0 GHz. From the amplitude balance between S3(1),1(1) and S3(2),1(1) and the 90-degree phase difference shown in Figure 4, it is found that the septum polarizer shows a good circular polarization performance as evidenced in the boresight axial ratio of the feed shown later in Figure 9.  
The final step in the design of the proposed feed is an arrangement of the sum and difference channel waveguides, the design of a coaxial probe exciting the difference channel waveguides, and the two choke slots. The dimensional parameters of the proposed feed are shown in Figure 5.  
The standard SMA coaxial probe is used in this work with dimensions \(E\) (1.27 mm) and \(F\) (4.11 mm), shown in Figure 2d. The dielectric material of the coaxial cable is the PTFE with a dielectric constant of 2.08 and a loss tangent of 0.0002. Optimum values of the probe length (\(L_{6}\)) and the probe position (\(C_{2}\)) from the back short are obtained through parameter sweep with the CST Studio Suite and shown in Table 2. The dimensions \(T_{1}\) and \(T_{2}\) are the choke wall thickness and the choke slot width, respectively, where a minimum value is chosen for a small feed frontal area and reliable machining. The depth of the choke slot is 6.50 mm, about 0.33 wavelength at 15.25 GHz. The dimension \(g\) is the thickness of the walls between waveguides, for which a minimum value is chosen for matching the difference pattern with the sum pattern, and also for reliable machining. The distance between the center of the rectangular waveguides of the difference channel is \(a+2g+b=11.61+2.00+5.81=19.42\) mm, which is 0.99 \(\lambda\) at 15.25 GHz. The reflector blockage diameter \(d\) of the feed is 46.00 mm (2.34 \(\lambda\) at 15.25 GHz).