# A Tunable Microstrip-to-Waveguide Transition for Emergency Satellite Communication Systems
## 2 Transition Design  
In the ground station, the operating frequency band of the transmitting terminal is from 14 to 14.5 GHz, and that of the receiving terminal is from 12.25 to 12.75 GHz. When the transmitter works, it generates noise signals within the receiving frequency band. The noise signals inevitably transport to the receiving terminal by the duplexer because the duplexer cannot completely attenuate them. In order to enhance the suppression, in the conventional solution, an extra waveguide filter is introduced between the transition and the duplexer, as shown in Figure 1. In our design, the microstrip-to-waveguide transition located between the power amplifier and the duplexer integrates the out-of-band suppression function, and thus, the waveguide filter becomes unnecessary. According to the microwave theory [35], a \(\lambda/2\) open-ended transmission line can be regarded as a resonator. When the resonator is located along the E-plane of the waveguide, it obstructs the propagation of electromagnetic waves. Hence, to enhance suppression within the receiving frequency band, we use open-ended metal probes to construct the resonator, as shown in Figure 2. As \(\lambda/2\) is much larger than the \(b3\) of WG3, we utilize three metal probes instead of one metal probe, and its total length is determined by:  
\[m1+m2+m3=\lambda/2 \tag{1}\]  
\[\lambda=c/f \tag{2}\]  
where \(c\) is the speed of light in a vacuum and \(\lambda\) is wavelength at the receiving frequency band. Figure 1: The location of the transition in the ground station. As shown in Figure 2, the proposed transition comprises a microstrip patch, three matched patches, three waveguides (WG1, WG2, WG3), and three metal probes. As we can see, three metal probes are introduced into WG3 to achieve out-of-band suppression within the receiving frequency band, thereby eliminating the extra waveguide filter and reducing the size of the device. Importantly, one can change the suppression frequency by adjusting the total length of the metal probes according to Equation (1). Furthermore, three matched patches and WG2 are used to compensate the match within the transmitting frequency band. Here, the microstrip utilizes Rogers Ro 4003(tm) dielectric, which is characterized by a relative permittivity of 3.55 and a loss tangent of 0.0027. The cavity shell is aluminum. To achieve the desired performance, we employ the High Frequency Structure Simulator (HFSS) of ANSYS Electronics Desktop 2018.2 [36] to analyze the microstrip-to-waveguide transition. In the simulation, the solution type in the HFSS is set as \"Modal\". The parameters are listed in Table 1.  
Figure 3a,b shows the S parameters of the transition without the three matched patches and WG2, respectively. It is clear that the patches and WG2 can enhance the match in the transmitting frequency band and hardly generate influence on suppression within the receiving frequency band. Figure 3c shows the S parameters of the transition without \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value (mm)** & **Parameter** & **Value (mm)** \\ \hline \(a1\) & 11.8 & \(g2\) & 3.2 \\ \(b1\) & 4 & \(a3\) & 15.8 \\ \(h1\) & 7.5 & \(h3\) & 7.9 \\ \(m1\) & 3.56 & \(h3\) & 7.8 \\ \(g1\) & 3.8 & \(g3\) & 0.67 \\ \(g4\) & 1.14 & \(l0\) & 1.5 \\ \(w1\) & 0.3 & \(l1b\) & 0.84 \\ \(w2\) & 0.34 & \(w4\) & 0.4 \\ \(l2\) & 1.24 & \(l4\) & 1.8 \\ \(a2\) & 13.4 & \(w0\) & 0.42 \\ \(b2\) & 7.2 & \(l1a\) & 1.8 \\ \(h2\) & 4.5 & \(w3\) & 1.4 \\ \(m2\) & 3.56 & \(l3\) & 0.85 \\ \(m3\) & 5.18 & \(r\) & 0.75 \\ \(w\) & 2.3 & \(re\) & 1.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The parameters for the MTM BPF. Figure 2: Configuration of the microstrip-to-waveguide transition (green color: Rogers Ro 4003(tm) dielectric, silver gray color: cavity shell). the three metal probes. The results clearly indicate that the three metal probes in WG3 can significantly improve out-of-band suppression within the receiving frequency band, especially with \(\,\)1S\({}_{21}\)\(\,\)\(-\)\(-\)55 dB at 12.75 GHz.