## 5 Discussion  
A back-to-back prototype for Ku-band has been manufactured. The metal piece has been printed using high purity copper (99.9%) and the power lines have been manufactured with Rogers 4003C for the central layer and FR\(-\)4 for the covers. All the involved geometrical parameters are collected in Table 2, and their meaning can be seen in Figure 10. The parameters \(a\) and \(b\) are determined by the ESIW guide, and \(Gap\) depends on the total height of the power circuits. The parameter \(Lms\) does not impact the behaviour of the transition, so this length can have any value. In this case, the length chosen matches with the length of the calibration kit used to discount the effect of the microstrip lines on the measurements. \(Wms\) is determined by the selected input impedance, the operating frequency, and the substrate used to manufacture the microstrip line. The rest of the parameters do not affect the electrical response of the prototype and can be freely chosen in order to optimise manufacturing time, reduce the amount of material used, provide greater robustness, etc. Figure 9: Soldering paste effect (S11): no pour, perfect, and pour. Figures 11 and 12 show four different responses: the ideal back-to-back simulation (_Ideal_) and the three different responses of the fabricated devices with the different aligning elements (_pins_, _substrate_, and _u-shaped_). The prototype has been measured with the Anritsu MS4644A 2-port Vector Network Analyzer (VNA) and calibrated with a custom TRL (thru-reflect-line) microstrip calibration kit. This kit is designed for the Ku-band and the substrate used for both ESIW circuit central layers. This calibration moves the reference measurement plane to the beginning of the microstrip-to-ESIW transition, avoiding the losses introduced by the microstrip lines and the coaxial connector. Therefore, the measurements will show the back-to-back response of the microstrip-to-ESIW transition and the ESIW-to-3D guide transition (which is what is really intended to be evaluated). Consequently, the measured results will always be worse than those of the ESIW-to-3D guide transition alone. The observed degradation both in the simulated and measured responses is mainly due to the non-ideal response of the microstrip-to-ESIW transitions, which had to be included in order to measure with the VNA, and could not be eliminated with the calibration kit. As shown in Figures 11 and 12, in the worst-case scenario, losses do not exceed 1 dB for substrate and U-shaped aligning elements and around 1.5 dB for dowel pins, while in the best-case scenario they approach 0.4 dB. This response is relatively flat, and losses increase as the frequency rises, which is a typical characteristic of most manufactured prototypes. The reflection coefficient is almost always below \(-15\) dB. The discrepancies between the measured and simulated results are considered to be minimal. As a result, the proposed fabrication and soldering method delivers a response that is very close to the ideal simulation case, thereby confirming the validity of the proposed transition and its manufacturing process. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline a & 15.7988 mm & Width\({}_{3D}\) & 36.71 mm \\ b & 0.966 mm & Long\({}_{3D}\) & 46 mm \\ L\({}_{ug}\) & 20 mm & Width & 3.85 mm \\ L\({}_{feal}\) & 25 mm & Gap & 3.07 mm \\ L\({}_{ms}\) & 25 mm & Wall & 2.8 mm \\ W\({}_{ms}\) & 1.82 mm & W\({}_{feal}\) & 30.40 mm \\ W\({}_{add}\) & 7.29 mm & W\({}_{feal2}\) & 20.80 mm \\ L\({}_{ut}\) & 8 mm & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Table of geometrical parameters. Figure 10: Geometrical dimensions of the prototype. The final prototype and its parts can be seen in Figure 13. The device functions correctly and can be utilised to integrate 3D components with planar circuits, such as filters and antennas. The 3D guide has been fabricated in copper, although other alternatives could be considered to reduce costs or minimise losses.