# SIW Cavity-Backed Antenna Array Based on Double Slots for mmWave Communications
## 3 Experimental Validation  
The design was experimentally validated using a complete prototype as detailed in Figure 6. The prototype was formed by two layers, depicted in Figure 6a. The bottom layer contained the corporate feeding network that included the one-to-four power divider and the coaxial feeding port. Notably, a hole was drilled at the center of the coaxial port in order  
Figure 5: Parametric study of the second slot of the double-slot: (**a**) sweep of parameter \(\mathrm{s}_{\mathrm{\beta}2}\), (**b**) sweep of parameter \(\mathrm{l}_{\mathrm{\beta}2}\), and (**c**) sweep of parameter \(\mathrm{w}_{\mathrm{\beta}2}\). to properly insert the inner conductor of the employed connector. The inner conductor of the commercial connector was precisely cut to obtain a length of 1.05 mm, which provided the expected impedance bandwidth. The top layer contained the four backed cavities and the radiating slots. The two layers were stacked and the assembly was fixed using mechanical pressure. Some details of the assembly are shown in Figure 6b. The locations of the assembly screws were at the furthest part from the center of the plates to increase alignment accuracy. The antenna prototype was characterized inside the anechoic chamber of the SWAT Group at the University of Granada. Some details of the measurement setup and the antenna under test (AUT) position are shown in Figure 7, where the E-plane and H-plane of the antenna are indicated. In the measurements, a transition from 1.85 to 2.92 mm was used since the ports of the vector network analyzer (VNA) R&S-ZVA67 were 1.85 mm and the antenna connector was 2.92 mm [20]. An open-short-match (OSM) calibration was performed to remove the effects of the RF cable of the VNA as well as its 1.85 mm connectors. The 1.85 to 2.92 mm transition could not be included in the calibration setup, although its insertion losses were low (around 0.25 dB) and could therefore be neglected. The antenna circuital matching measurement results are provided in Figure 7b, which shows the measured reflection coefficient value of the complete prototype, as well as the simulated value for the sake of fair comparison. The samples acquired from the VNA were obtained each 32 MHz, representing a suitable sampling for measurement. The antenna provided a \(-10\) dB bandwidth of around 1.6 GHz, with measured results agreeing with the expected ones from the simulation stage. This represents a good indicator of the proposed antenna assembly. If there had been any misalignment, the coefficient would have been affected, especially for misalignment in the \(y\) direction of the antenna. The results show the double resonance that produces the desired frequency band due to the use of double slots in the cavity-backed antenna. Regarding the measurement results of the radiation patterns  
Figure 6: Prototype: (**a**) layer details; (**b**) complete assembly and connector details. of the antenna, Figure 8 shows the simulated and measured H-plane and E-plane radiation patterns of the proposed SIW antenna at 35.4, 36.2, and 37 GHz, normalized to the steering direction. The radiation patterns in both planes match the expected pattern distribution. In the E-plane patterns, some side lobe levels are around 3 dB above the reference values. Figure 9 depicts the measured 3D radiation patterns (spherical acquisition) of the proposed SIW antenna at 35.4, 36.2, and 37 GHz. The complete spherical radiation patterns match the expected performance of the array antenna since pencil-shape beams were produced in the whole operating frequency band. Figure 7: (**a**) Measurement setup details. (**b**) Simulated and measured \(|\mathrm{S}_{11}|\). Finally, directivity and gain measurements are provided in Figure 10. The results were obtained for the main beam direction, that is, for azimuth (\(\phi\)) and elevation (\(\theta\)) angles equal to 0\({}^{\circ}\). As shown, the simulated and measured gain values are in good agreement.