## 3 Very Compact Waveguide OMT with Folded Ports
Figure 5: Isometric CAD view of the proposed very compact waveguide OMT with folded single-mode ports (**left**) and associated additional design parameters in side CAD (**top right**) and bottom CAD views (**bottom right**). Figure 6: Simulated scattering parameters of the very compact waveguide OMT with folded single-mode ports assembled with a conical horn (_solid lines_) compared to the reference OMT design with straight single-mode ports (_dashed lines_). The simulated standalone OMT results are overlaid in _dotted lines_. The specifications are marked with a _black thick line_. horn design, indicating that even better performances may be achieved by using these parameters as optimization variables. In particular, the component shows the potential to extend its operation bandwidth up to 21.2 GHz, corresponding to a fractional bandwidth of approximately 20% and covering also the portion of the spectrum in the K-band that is typically allocated for governmental applications. A detailed comparison with alternative compact two-probe waveguide OMT designs is provided in Table 3, highlighting the benefits of the proposed solution. The dimension estimate in the xy-plane takes into account the space required to fold the waveguides along the longitudinal direction [20], i.e., the waveguide's broad wall dimension in the case of side-coupling designs and the waveguide's narrow wall dimension in the case of turnstile designs, as demonstrated in Section 3 for the proposed waveguide OMT design. Note that this dimension estimate is based on the waveguide cavities only and does not account for mechanical constraints, which are assumed to be the same for all designs. The specific waveguide OMT designs reported in this work were also constrained by interfaces required for tests. More compact designs are certainly achievable by implementing well-known techniques to reduce the waveguide cross-sections, such as ridges. The in-plane dimension is important for the design of closely spaced array antennas that are currently considered for broadband communication satellites, specifically in low and medium Earth orbit (LEO/MEO) systems with larger scanning ranges. Furthermore, the mechanical simplicity of the proposed OMT is essential for increasing the manufacturing yield in large arrays and for possibly adapting the design toward additive manufacturing techniques [12]. The waveguide OMT design with folded single-mode ports reported in Section 3 may be easily adapted for such purposes, as it presents very limited overhanging areas. There is also an interest in combining this improved waveguide OMT design with the four-way power divider described in [28] to further enhance its performance. Advantageously, the four-way power divider design does not require folding the single-mode ports, thus enabling further reduced array spacing in the order of 0.7\(\lambda_{0}\), where \(\lambda_{0}\) is the wavelength at the highest operating frequency. Figure 7: Simulated on-axis XPD of the very compact waveguide OMT with folded single-mode ports assembled with a conical horn (_solid lines_) compared to the reference OMT design with straight single-mode ports (_dashed lines_). The simulated standalone OMT results are overlaid in _dotted lines_. The specifications are marked with a _black thick line_.