# A Tunable Microstrip-to-Waveguide Transition for Emergency Satellite Communication Systems
## 1 Introduction  
With the trend of globalization, the importance of emergency satellite communication systems has become increasingly prominent, especially as regards natural disasters, wars, and other critical events [1; 2; 3; 4; 5]. To achieve outstanding performance for entire systems, we need to introduce innovative technologies to enhance the capabilities of their components including, but not limited to, filters [6; 7; 8], antennas [9; 10; 11], transitions, suitable transmission lines, or others. As the two important transmission lines, waveguides and microstrip lines are usually utilized in combination to enhance the performance of a system. The former can offer low-loss characteristics and the latter has the advantage of integration. Moreover, the transitions between them are especially important and extensively applied to microwave, millimeter wave, and terahertz fields [12; 13; 14; 15; 16; 17].  
Commonly, transitions offer the fundamental function to achieve mode transformation between integrated circuits and waveguides. In addition, they hold immense importance in enhancing the overall system's performance, such as in bandwidth expansion [18; 19; 20; 21; 22; 23; 24; 25], miniaturization [26; 27; 28; 29; 30], dual band operation [31], etc. Typically, the radial shaped probe, the defected ground slot, the radiation probe, or the complementary split ring resonators can be utilized to provide wideband performance [18; 19; 20; 23]. Generally, there are several approaches to achieving miniaturization. One way involves reducing the physical dimensions of the transition itself [26], while another way is to design the transitions such that they are integrated with additional components such as filters and directional couplers [27; 28]. It also is feasible to achieve system miniaturization by designing suitable transitions for miniaturized waveguides such as H-plane waveguides [29] and integrated waveguides [30]. The implementation of dual-band transitions, which can effectively perform the functionsof two separate single-band ones, offers significant advantages in terms of reducing system costs [31]. Furthermore, the tuning screw is one of the common approaches to achieving the tunable functionin microwave devices. [PERSON] et al. proposed dual- and triple-band tunable filters using screws [32]. [PERSON] et al. achieved a mechanically reconfigurable conformal array antenna by introducing tuning screws into the radial waveguide divider [33]. [PERSON] et al. proposed a novel waveguide reflection-type phase shifter with tuning screws to construct a reconfigurable linear phased array antenna [34].  
For the ground stations of emergency satellite communication systems, the transmitting terminal, the receiving terminal, and the antenna are connected by a duplexer. Hence, the amplified noise signals from the transmitting terminal can always result in a worse signal-to-noise ratio in the receiving terminal. For the conventional approach, an extra waveguide filter is used to suppress amplified noise signals but would enlarge the system size. In this paper, we propose a novel microstrip-to-waveguide transition that integrates the function of out-of-band suppression within the receiving frequency band, thus eliminating the need for an extra waveguide filter. Meanwhile, the out-of-band suppression frequency in the receiving frequency band is tunable by adjusting the length of the tuning screws.