## 3 Transition Manufacturing and Test  
The single-iris version was selected for experimental validation because of its higher robustness with respect to manufacturing tolerances. To ease the manufacturing process the RWG was split into two blocks, fabricated with a Form 2 SLA printer by Formlabs and metalized with a two-step process consisting of silver painting followed by electrolytic copper deposition in a copper sulfide solution. Electrical contact between the two manufactured parts was provided by screws. Further details on this SLA additive manufacturing and metallization are discussed in [26]. This process was adopted in order to minimize time and costs and has exhibited good results for other proof-of-concept prototypes [27; 28]. The SIW circuit was manufactured with standard PCB technology and then connected to the RWG, the latter being integrated in a mechanical support structure with mounting screws tightened in six holes drilled in the PCB. A GCPW feeding section for port 2 was preferred over a microstrip line because of a reduced insertion loss due to spurious radiation and a lower influence with the RWG mechanical support. In addition to this, it was easy to integrate a GCPW transmission line with a K-type solder-less connector adopted for the measurements, which is the 2.92-mm jack (female) end-launch connector model 1092-03A-5 by Southwest Microwave. A SIW-to-GCPW transition was designed according to [29] and included in the following simulations. In particular, a properly dimensioned tapered section converts the frequency-dependent SIW impedance to the GCPW 50-\(\Omega\) port impedance with a return loss better than 20 dB in the bandwidth of interest. The SIW-to-GCPW transition geometry, simulated performance, and geometrical parameters are respectively reported in Figure 10 and Table 7.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Reference** & \(f_{\theta}\) **(GHz)** & \(\varepsilon_{r}\) & \(a\) **(mm)** & \(b\) **(mm)** & \(ae\) (mm)** & \(\lambda_{c}\) **(mm)** & \(r=b/\lambda_{c}\) (\%)** & **20-dB** & **Merit Factor** \\  & & & & & & & & & & \\
[12] & 34.90 & 2.20 & 5.25 & 0.508 & 5.05 & 14.98 & 3.39 & 1.15 & 0.34 \\
[13] & 26.00 & 3.40 & 6.05 & 1.905 & 5.78 & 21.32 & 8.93 & 5.83\({}^{*}\) & 0.65 \\
[14] & 29.75 & 3.55 & 4.80 & 0.787 & 4.54 & 17.12 & 4.60 & 3.36 & 0.73 \\
[15] & 92.50 & 3.50 & 1.30 & 0.254 & 1.18 & 4.40 & 5.77 & 11.89 & 2.06 \\
[16] & 65.00 & 2.20 & 2.66 & 0.787 & 2.32 & 6.88 & 11.44 & 23.08 & 2.01 \\
[17] & 33.25 & 2.20 & 5.30 & 0.787 & 4.90 & 14.55 & 5.41 & 38.81 & 7.17 \\
[18] & 36.00 & 2.20 & 5.00 & 0.508 & 4.62 & 13.71 & 3.71 & 16.67 & 4.49 \\
[19] & 34.90 & 2.20 & 4.30 & 1.575 & 3.91 & 11.58 & 13.60 & 25.22 & 1.85 \\
[20] & 18.38 & 2.20 & 8.40 & 0.787 & 8.01 & 23.77 & 6.62 & 27.37 & 4.13 \\A complete prototype is shown in Figure 10(a) after the assembly. In Figure 10(b) the fabricated RWG is open to visualize the internal details. The prototypes were tested with an Agilent N5230A vector network analyzer using the well-known full two-port short-open-load-through calibration technique. Measured performances are reported in Figure 12 and compared to simulated results. Physical properties of copper conductivity and material roughness were taken into account in the simulation. In particular, an estimated metallic roughness on the order of 5 \(\upmu\)m contributed to deteriorate copper conductivity. A de-embedding procedure was also applied to the measured results in order to remove the effects of the coaxial connector as in [30], relying on the datasheet and measured data provided by the manufacturer. The estimated insertion loss for the connector is approximately 0.4 dB at 10 GHz and 0.5 dB at 15 GHz.