## 2 Very Compact Waveguide OMT
### Experimental Results  
A prototype was manufactured using high-precision milling. The waveguide OMT is composed of two parts, as shown in Figure 2, and assembled with alignment pins for higher precision. The conical horn used for test purposes is also shown in Figure 2. Note that the simple mechanical design of the OMT is compatible with a mill diameter of 2 mm. A much smaller mill diameter is required for the designs in [21; 22], leading to higher CNC machining time and thus higher costs, while also resulting in higher sensitivity to manufacturing tolerances. The complete assembly was tested in the compact antenna test range (CATR) at ESA-ESTEC, The Netherlands. The measured scattering parameters (reflection and coupling coefficients) are reported in Figure 3 in _solid lines_, where they are also compared to the simulation's results in _dashed lines_. A reasonably good agreement was obtained with some small discrepancies, particularly with respect to the reflection coefficients; this may be attributed to manufacturing and assembly (e.g., alignment) errors, which were evaluated to be within \(\pm 30\) um. The measured reflection coefficients are better than \(-19.5\) dB, while the measured port-to-port coupling coefficient is better than \(-22.9\) dB over the nominal \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & **Value** & \multirow{2}{*}{**Parameter**} & **Value** & \multirow{2}{*}{**Parameter**} & **Value** \\  & **(mm)** & & & **(mm)** & & **(mm)** \\ \hline \(a_{\text{WR42}}\) & 10.668 & \(d_{1}\) & 5.86 & \(z_{1}\) & 2.54 \\ \(b_{\text{WR42}}\) & 4.318 & \(d_{2}\) & 2.50 & \(z_{2}\) & 2.60 \\ \(a\) & 9.50 & \(d_{3}\) & 3.18 & \(z_{3}\) & 3.45 \\ \(z\) & 4.40 & \(c\) & 1.25 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Design parameters of the very compact two-probe waveguide OMT in the K-band with WR42 flanges. Figure 1: Isometric CAD view of the proposed very compact two-probe waveguide OMT with port numbering (**left**) and associated design parameters in top CAD (**top right**) and side CAD views (**bottom right**). operating band, corresponding to a fractional bandwidth of 15%. Note that the design is constrained here by the horn cross-section and the standard waveguide width. These parameters may be used as additional optimization variables with the expectation of improving the scattering parameters further, possibly extending the fractional bandwidth up to about 20%. As a reference, the simulated results of the standalone OMT (i.e., without the horn) are reported in Figure 3 in _dotted lines_. As anticipated from the dual-linear operation of the OMT, the horn has very limited impact on port-to-port coupling. The impact on the reflection coefficient is more visible but remains within the target value over the nominal operating band. The experimental evaluation of the XPD of the OMT was performed by measuring the on-axis co- and cross-polarized electric field components in a far-field setup, taking advantage of the symmetry of the horn's design. However, the measurement results proved to be sensitive to room scattering and probe imbalance, specifically the lower levels of cross-polarized field. A dual-polarized probe is used in the CATR with some residual error between the horizontal and vertical components. These errors are filtered out by averaging two probe orientations with angular values \(\phi=0^{\circ}\) and \(\phi=90^{\circ}\). The corresponding results are reported for both ports in Figure 4 in _solid lines_ and compared to the full-wave simulation's results in _dashed lines_. The measured XPD of both ports is better than  
Figure 3: Scattering parameters of the compact dual-mode OMT assembled with a conical horn, comparing measured (_solid lines_) and simulated (_dashed lines_) results. The simulated standalone OMT results are overlaid in _dotted lines_. The specifications are marked with a _black thick line_.