# A Gap Waveguide-Based Compact Rectangular Waveguide to a Packaged Microstrip Inline Transition
## 6 Experimental Validation  
As mentioned above, a back-to-back transition has been designed for experimental validation. The fabrication process included milling for the metallic component and photolithography for the microstrip line. The aluminum block was realized by CNC machining. The lid and the pins were manufactured in separated metal blocks. For the first transition model, six screws guaranteed the connection between top and bottom parts and four dowel pins the proper alignment. Besides, there were four dowel pins and two screws for substrate alignment and positioning, respectively. The dimensions of the whole block were 13.65 mm in length and 25.05 mm in width. For the second transition, four screws were used for the connection between bottom and top parts. Four dowel pins guaranteed the metallic block alignment and four other pins the substrate alignment. Two screws held the substrate to the block. The dimensions of the whole block were 13.14 mm length and 32 mm width. For both cases the microstrip taper transition was built in 100 mm thick Rogers CuClad 233 substrate (\(e_{r}=2.34\) and \(\tan\delta=0.009\)) with 17 mm copper cladding using a standard photolithography process at the Public University of Navarra's facilities. Figure 9a shows a top view of the microstrip line mounted on the bottom metallic block and of the upper metallic block, which contained the bed of snails. Figure 9b is a perspective view of the metallic blocks for the second model and of the microstrip line printed on the substrate. For both cases the experimental setup was the same. The whole structure was assembled and connected between VDI W-band VNA extenders, and connected to an Agilent PNA-X E3861 vector network analyzer, as can be seen in Figure 10.  
The results of the measurement and its comparison with the predicted response of the back-to-back configuration are shown in Figure 11. The simulation of the back-to-back tra  
Figure 10: Photograph of the assembled structures connected to the VDI W-band VNA extenders for the first (**a**) and second models (**b**). Figure 9: Photographs of the fabricated structures: (**a**) top view of the first transition model aluminum block and microstrip circuit and (**b**) perspective view of the second transition model aluminum block and microstrip circuit. the dimensions of the fabricated components, shown in Table 2. Due to the manufacturing errors, the behavior degraded with respect to that presented in Figure 7. In the lower part of the working bandwidth, both insertion and return losses were slightly higher than in the predicted response, whilst in the upper part of the band the increase of insertion losses was higher, although there was not a significant increase in the return loss. This discrepancy in the measurements was ascribed to the assembly tolerances and to a higher than the nominal value of the loss tangent. Nevertheless, the simulated insertion loss was lower than 1 dB and the mean value was 0.6 dB. However, the mean measured insertion loss increased 1.3 dB with a maximum of 3.8 dB at high frequencies. This effect can be ascribed to larger losses in the substrate. Regarding return losses, these were higher than 14.8 dB with a 19.8 dB mean value, whereas the measured mean value was 17.2 dB and the minimum was 12.15 dB. For the second prototype, the results of the measurement and its comparison with the predicted response of the back-to-back configuration are shown in Figure 12. The experimental results are also compared with the simulation of the back-to-back transition using the dimension of the fabricated component and a misalignment of 150 \(\upmu\)m in the perpendicular to the waveguide axis direction, Table 3.  
Figure 11: Comparison between predicted response (black solid line) and simulated transition taking into account the fabricated value (green dashed-point line) and measured (red dashed line) back-to-back configuration of the first transition model prototype (Figure 1a): (**left**) insertion loss and (**right**) return loss.