## 2 Structure
### Development of the Junction  
Based on the above-mentioned ideas, four different structures for the transition are initially proposed, which can be seen in Figure 2.  
* Full robust structure: good mechanical fit, but poor soldering due to the difficulty of the soldering paste to flow into the gap, Figure 2a. * Structure with U-shaped aligning elements: easy weldering and sufficient mechanical adjustment. Once soldered, the prototype is very robust thanks to the U-shaped elements and the welding, Figure 2b. * Dowel pins as aligning elements: very easy weldering and sufficient mechanical adjustment with dowel pins. Using external elements, precision does not depend on the resolution of the printer and precision rises. Because of the sintering process [15] used to build the 3D part, deformations are higher than expected due to the circular geometry for the dowel pins. Because of that, results are not that good but still good enough. It is necessary to clarify that other 3D printing technologies do not depend on the sintering process, which is why these results are included, Figure 2c. * Substrate fragments as aligning elements: very easy weldering and sufficient mechanical adjustment with substrate fragments. Using external elements, precision does not depend on the resolution of the printer and precision rises, Figure 2d. With a completely solid structure like the one shown in Figure 2a, it may appear initially that the aforementioned requirements are better met. However, if the transition is manufactured in this way, it will not be possible to directly access the contact zone between  
Figure 1: Electromagnetic propagation in the lines across the transition. the long walls of both waveguides, making it impossible to achieve good electrical contact between them, as there is no way to ensure that the tin will penetrate enough into the structure. Therefore, this structure (Figure 2a) was discarded in this work once fabricated and measured in order to prove the above statement and the results of the measured prototype evidence this. The U-shaped alignment elements, dowel pins, and substrate fragments were selected because they ensure adequate mechanical fit and the connection between the waveguides is fully accessible for performing the necessary soldering. For the soldering to be effective, which is crucial for the operation of the transition, it is important that the edges around the end of the feeding ESIW waveguides that face the 3D waveguide are fully metallised, as shown in Figure 3. Based on the manufactured and measured devices, it has been ascertained that the device exhibits a notably high level of mechanical resistance, largely attributed to the quality of soldering. As such, this particular component does not pose any concerns in any conceivable scenario. The soldering integrity surpasses that of the ESIW circuit, positioning the power circuit as the primary vulnerability. Furthermore, in the specific context of the U-shaped alignment structures, their robustness is even more pronounced. To validate the transition experimentally, a back-to-back prototype of the proposed device was manufactured. The prototype was fed with microstrip lines that will connect it to the Vector Network Analyzer (VNA) for its characterisation. To complete the back-to-back prototype, two microstrip-to-ESIW transitions were added to both the input and output ports. Finally, the prototype was completed with two twin versions of the ESIW-to-3D waveguide transition (see Figure 3), which is what we intend to validate in this work. Figure 4 depicts the structure of the back-to-back prototype used to validate the proposed transition, with the top covers of the feed ESIWs omitted to show all elements in detail. The performance of the back-to-back transition was initially validated through simulations, which were also used to assess the resilience of the proposed solution against possible manufacturing inaccuracies. All the simulations were carried out with the full-wave simulator CST Studio Suite v21.