# Experimental Demonstration of Beam Scanning of Dual-Metasurface Antenna
## 4 Discussion  
In Table 2, the performances of the antenna described in this paper are compared to those of other solutions found in the literature. In the table, \"efficiency\" stands for radiation efficiency. Radiation efficiency and gain values are comparable to those reported in the other works (above the average). The comparison is conservative because all reported references work at a lower frequency, which inherently entails lower losses. The small form factor, simple biasing and high efficiency make the proposed architecture interesting for several low-profile applications. The angular scanning range achieved with this first prototype is, on the other hand, limited: improvement of this is thus important and the subject of current work. Here, some of the reasons for this limitation are listed and ways to improve it are proposed. A possible degree of improvement is represented by the choice of the dielectric substrate and its thickness, especially for the side lobes. In particular, a higher relative permittivity results in a wider range of surface reactance values, \(X_{\mathrm{s}}\), which allows for a larger modulation factor \(M\) and, consequently, a higher leakage constant [23]. This in turn translates into the reduction of side-lobes. Figure 10 shows the simulated radiation pattern at 10.75 GHz for an alternative design of the proposed antenna, featuring a Rogers RO3006 substrate (\(\epsilon_{r}=6.5\)) of a thickness equal to 2.56 mm: the radiation angle steers from 15\({}^{\circ}\) to 5\({}^{\circ}\) when the bias voltage goes from 0 V to 8.2 V. In this case, no other harmonic beside the \(n=-1\) radiates, and the very small backward-directed side-lobes are due to the spurious radiation of a second, higher-order TM mode. Directivity values are the same as in the previous configuration. A further reduction of side-lobes could be obtained by tapering the modulation index \(M\) along the length of the antenna [32], which was not pursued here. Another margin of improvement involves the geometry of the reconfigurable metasurface's unit cell. The tunable unit cell used in this first prototype was inspired by the literature and had a different original use [27]; a new geometry could be devised to obtain a wider impedance range for the same diodes. Moreover, the design presented in this paper was based on the combination of a capacitive impedance (top) and an inductive one (bottom), and the propagation of the TM mode. However, different combinations of impedances can be employed (e.g., capacitive \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Reference** & **Frequency [GHz]** & **Scanning Range** & **Antenna Length** & **Thickness** & **Efficiency** & **Gain [dB]** \\ \hline
[14] & 5.6 & 21\({}^{\circ}\) & 5\(\lambda_{0}\) & 3\(\lambda_{0}/4\) & 60–75\% & 5–13 \\
[18] Band 2 & 5.75 & 22\({}^{\circ}\) & 7.15\(\lambda_{0}\) & \(\lambda_{0}/17\) & 40–50\% & 9.5–12 \\
[4] & 9.3 & 43\({}^{\circ}\) & 13\(\lambda_{0}\) & \(\lambda_{0}/3\) & 38–46\% & 8–11.8 \\ This work & 10.65 & 7\({}^{\circ}\) & 7.7\(\lambda_{0}\) & \(\lambda_{0}/6\) & 67–76\% & 11.6–13.4 \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison with other LWAs found in the literature. Figure 9: Measured \(S_{11}\) and \(S_{21}\) for two different values of bias voltage. capacitive) that may increase the scanning range if properly designed, especially if a dielectric material with higher electric permittivity is used. In particular, analyses carried out with the approximate analytical model show that use of the TE mode (instead of the TM) yields a much larger steering range. This possibility is currently being studied and will be the subject of future work (excitation of the TE mode is less natural than that of the TM).