# Experimental Demonstration of Beam Scanning of Dual-Metasurface Antenna
## 3 Results  
Full-wave simulations of the designed antenna were performed with the commercial software CST Studio Suite [31]. The varactor diodes were used in a 0-8.2 V biasing range and their capacitance went from 0.233 pF to 0.0548 pF, according to the manufacturer's datasheet [28]. The beam pointing angles obtained from these simulations at the working frequency of 10.65 GHz are listed in Table 1, showing excellent agreement with the main beam directions computed using the approximate model described in Section 2.2.  
A prototype was fabricated and tested (Figure 7). The modulated metasurface and the reconfigurable plane are pictured in Figure 7a,b, while Figure 7c,d show the tapered feeding section on the upper layer and the varactor diodes soldered in the lower metasurface, respectively. Radiation measurements were performed in an NF-FF spherical range; the full radiation pattern was obtained, allowing evaluation of directivity and radiation efficiency.  
Measured far-field patterns at 10.65 GHz for different bias voltages are shown in Figure 8, together with the simulation results, which are in excellent agreement.  
The antenna gain bandwidth was 200 MHz around the working frequency of 10.65 GHz; the impedance bandwidth was larger than the gain one. The scattering parameters S\({}_{11}\) and S\({}_{21}\) of the fabricated prototype are shown in Figure 9: S\({}_{11}\) was always below \(-10\) dB in the working band for all varactors' bias voltages, while S\({}_{21}\) remained below \(-20\) dB, which is very important for the efficiency of a traveling-wave antenna.  
In the considered frequency range, the gain spanned from 11.6 dB to 13.4 dB with varying bias voltage, while the radiation efficiency was between 67% and 76%. The measured aperture efficiency went from 20% to 32%.  
In Figure 8, a side-lobe is noticeable in the backward radiation direction; this lobe can be ascribed to radiation of the \(n=-2\) harmonic, which falls within the fast wave region at the working frequency [23]. These results are consistent with what can be predicted using (2), i.e., the \(n=-2\) harmonic radiating at about \(-41^{\circ}\) and \(-57^{\circ}\) for a bias voltage equal to 0 V and 8.2 V, respectively. The radiation pattern also presents an irregular shape and high side-lobes; this can be ascribed to the anisotropy of the impedance planes, in particular of the lower reconfigurable metasurface. In fact, its unit cell geometry and the placement of the varactor diodes inside it are such that a change in the bias voltage alters not only the component of the sheet impedance in the direction of wave propagation, but also the transverse one. This may cause disturbances in the wave propagation.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Voltage [V]** & **Calculated** & **Simulated** & **Measured** \\ \hline
0 & 22.7\({}^{\circ}\) & 20\({}^{\circ}\) & 19.5\({}^{\circ}\) \\
8.2 & 11.9\({}^{\circ}\) & 13\({}^{\circ}\) & 12.5\({}^{\circ}\) \\ \hline \end{tabular}
\end{table}
Table 1: Calculated, simulated, and measured radiation angles at 10.65 GHz.  
Figure 6: Unit cell of the fully-stacked structure.  
Figure 8: Measured and simulated radiation pattern in the E-plane at 10.65 GHz for different values of bias voltage.  
Figure 7: Fabricated antenna prototype. (**a**) Top modulated metasurface. (**b**) Reconfigurable impedance plane. (**c**) Coaxial cable and matched tapered input section at the left end of the upper metasurface (same tapered section is present at the opposite end). (**d**) Zoom showing the soldered varactors in the lower reconfigurable metasurface.