# A Tunable Microstrip-to-Waveguide Transition for Emergency Satellite Communication Systems
## 3 Experiment Results and Discussion  
The ground station with the fabricated microstrip-to-waveguide transition is shown in Figure 4a. Figure 4b shows three waveguides with three screws, and Figure 4c shows the microstrip with three matched patches. Here, three tuning screws are used as the three metal probes for the following two reasons. Firstly, it can sharply reduce the fabrication difficulty and lower the cost. Secondly, it can achieve the tunability of suppression frequency within the receiving frequency band by easily adjusting the screws' lengths.  
Figure 3: S parameters for (**a**) the transition without three patches, (**b**) the transition without WG2, (**c**) the transition without three metal probes, and (**d**) a different \(m1\) (=\(m2\)) value, and (**e**) a different \(m3\) value.  
We utilize the vector network analyzer (Rohde & Schwarz ZNB 40, from 10 MHz to 40 GHz) to measure the S parameters. Here, the microstrip of the transition is connected to a 2.92 coaxial connector (SMK) with 50 \(\Omega\) impedance, while WG3 of the transition is connected to a waveguide-to-coaxial adapter. In the actual product, the microstrip is directly connected to a power amplifier without the coaxial connector, and thus, the type of connector does not impact its performance.  
The measured and simulated \(\left|S_{21}\right|\) and \(\left|S_{11}\right|\) are shown in Figure 5. As we can see, the measured \(\left|S_{11}\right|\) is less than \(-10\) dB from 14 GHz to 14.5 GHz, typically \(---40\) dB at 14.15 GHz. While the measured \(\left|S_{21}\right|\) is less than \(-20\) dB within the receiving frequency band of 12.25-12.75 GHz, with a typical value of \(-48\) dB around 12.55 GHz. As we can see, there is a little deviation between the measured and simulated results. On the one hand, it can be caused by inevitable machining and measured errors. On the other hand, manual adjustment of deviation in \(m1\), \(m2\), and \(m3\) can directly lead to discrepancies in the measurement. As shown in Figure 3d,e, when \(m1\), (\(m2\)), or \(m3\) increases, the transmission zero in the receiving frequency band decreases, which agrees well with the theoretical prediction from Equation (1). Figure 5 shows that the measured transmission zero is lower than simulated one, indicating that we should increase \(m1\), (\(m2\)), or \(m3\). Furthermore, reflection zero in the operating frequency band increases with the increasing of \(m3\). Meanwhile, a large \(m1\) (\(m2\)) would lead to mismatches in the operating frequency band, as the screws with \(m1\) (m2) are located at the center of the E-plane, which has the strongest electric field. Hence, it is better to fine-tune \(m1\) (\(m2\)) and coarsely adjust m3 to make the simulated results closer to the measured ones.  
According to the above analysis, we further perform the simulation and thus obtain the amended results with \(m1=m2=3.62\) mm and \(m3=5.3\) mm, as shown in Figure 5. The results indicate that the primary source of the discrepancy between the measured results and the simulated ones is derived from the difference in the screw lengths in measurement relative to the simulated ones.  
Furthermore, we obtain the simulated S parameters for different \(m3\), as shown in Figure 6. As \(m3\) changes from 5.5 mm to 5.2 mm, the suppression frequency ranges from 12.25 GHz to 12.75 GHz in the receiving frequency band, and \(\left|S_{11}\right|\) can keep less than \(-10\) dB within the transmitting frequency band from 14 GHz to 14.5 GHz. Though the measured results exhibit differences with the designed ones, the proposed transition has demonstrated its tunable capability to significantly enhance out-of-band suppression within the whole receiving frequency band.  
Figure 4: Fabricated prototype of the microstrip-to-waveguide transition. (**a**) The ground station, (**b**) three waveguides with three screws, and (**c**) the microstrip with three matched patches.