## 3 Experimental Verification of the Monolithic Rotman Lens Design
### Experimental Results  
To simplify the overall test setup and test sequence, it was decided to keep unused waveguide ports open. Thanks to the low port-to-port coupling, this is found to have a moderate impact on the performance and still enables the validation of the concept. The measurements were performed in the Microwave Laboratory at the European Space Research and Technology Center (ESTEC), Noordwijk, The Netherlands. The test setup is shown in Figure 9. A WR42 calibration kit was used to perform scattering parameter measurements over the frequency range 16-24 GHz, while custom waveguide transitions and a WR34 calibration kit were used to measure over the range 24-32 GHz. The WR42-to-WR34 transitions were also measured back-to-back to evaluate their impact on insertion losses in the upper frequency range, with a measured mean value around 0.11 dB. First, the power transmission, including spillover, mismatch and insertion losses, are reported in Figure 10. On absolute values, the losses are higher in the lower band than in the higher band. This is mostly because of the higher spillover losses, as the H-plane pattern of the feeding ports has a wider beamwidth at the lower frequencies. It is also noted that the oscillations are larger in the lower band. This is predominantly due to the increased scattering coming from the higher spillover. As anticipated, the insertion losses, which may be evaluated as the difference in power transmission between the ideal model (i.e.,  
Figure 8: Photograph of the manufactured monolithic 10 \(\times\) 10 Rotman lens prototype (**left**) and inner cavity prototype (**right**). perfect electric conductor boundary condition) and the measured prototype, increase with frequency. The difference, on average, was around 0.5 dB in the lower band and increased to 0.8 dB in the upper band. It is noted, however, that a significant portion of the electrical path is due to the transitions for test purposes. The actual insertion losses of the Rotman lens core design are estimated to be about half of the measured figures, as the waveguide transitions have a combined length of about 120 mm, while the average path in the core design is around 150 mm. It is also noted that the measured results show some discontinuity at 24 GHz. This is a consequence of differential calibration and measurement uncertainties between the two calibration kits used for these tests. The results are nevertheless in line with expectations within the operating bands of interest. A representative selection of scattering parameters of the complete 10 \(\times\) 10 Rotman lens prototype is reported in Figures 11 and 12, corresponding to simulated and measured results, respectively. Compared to the core design reported in Section 2, this complete prototype, including transitions, shows slightly degraded reflection and isolation coef  
Figure 10: Power transmission in the double-ridged 10 \(\times\) 10 Rotman lens prototype for port 1 to 5, from lighter to darker color (measurements in _shades of green_; simulations in _shades of purple_). Figure 9: Test setup for the measurement of the scattering parameters of the 10 \(\times\) 10 Rotman lens prototype. ficients, with a worst case of \(-13.6\,\mathrm{dB}\) in the lower band in simulation and \(-11.5\,\mathrm{dB}\) in measurements. Both worst-case results are obtained for port 5, which corresponds to one of the inner ports. This is because the scattering from the inner lens contour tends to reflect back toward the center of the focal arc due to its shape, while this reflection follows a more specular direction for offset input ports. The results remain nevertheless acceptable. The level of the transmission coefficients is also fairly in line with predictions, with slightly higher oscillations due to secondary effects not accounted for in the model and the test configuration (i.e., open-ended waveguides). Similar oscillations are also observed in the phase response as a consequence of the test setup.