## 8 Conclusions  
Beam steering is essential in the modern communication landscape dominated by technologies like SATCOM and COTM. Mass-produced, cost-effective solutions are necessary to meet the expanding consumer base. Beam steering antennas based on passive planar metasurfaces offer compact, low-profile, and cost-effective solutions with steering capabilities similar to phased arrays but without the need for active RF components that require heat sinks and complex feed networks. These planar metasurfaces, specifically known as phase-gradient metasurfaces, enable precise control over the direction of electromagnetic waves by incorporating passive phase-shifting/phase-transforming cells. This article presents a comprehensive analysis of various design techniques found in the literature for dual-band phase-shifting cells in PGMs. These designs aim to enable full duplex communication using a single shared aperture. Since the fundamental element in a PGM is a phase-transforming cell, selecting a suitable dual-band unit element is crucial to ensure the desired performance of the overall steering antenna system. The dual-band cells described in the literature have been comprehensively classified based on their structure in this article. Furthermore, to achieve dual-band resonance, five main techniques have been utilized in the reported designs for the selection and tuning of resonant shapes within the cells. * Optimized phase pair [32; 59; 84]: This technique may result in the design of concentric cells and smaller unit cells but requires more optimization and complex computations with larger datasets. * Interleaved resonant elements [31; 60; 63; 64; 65; 66; 68; 69; 83; 100; 63; 60; 66]: This technique enables independent tuning but may result in larger cell sizes that may lead to phase quantization error or grating lobes particularly at a higher-frequency band. The placement of cells to form the PGM needs careful investigation to avoid corner element \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline
**Ref.** & **Steering Mechanism** & \begin{tabular}{l} **Operating Frequency** \\ **(GHz)/Bandwidth** \\ **(\%)** \\ \end{tabular} & \begin{tabular}{l} **Steering** \\ **Range (\({}^{\circ}\))** \\ \end{tabular} & \begin{tabular}{l} **Total** \\ **Antenna** \\ **Height** \\ \end{tabular} & \begin{tabular}{l} **Peak** \\ **Gain** \\ **(dBi)** \\ \end{tabular} & \
\begin{tabular}{l} **Gain** \\ **Variation/Scan** \\ **Loss (dB)** \\ \end{tabular} &
\begin{tabular}{l} **Technique** \\ **Varia-** \\ **tion/Scan** \\ **Loss (dB)** \\ \end{tabular} \\ \hline
[35] & Feed Translation & 19.8, 28.2/19.8, 12.0 & \(\pm 20,\pm 30\) & \(\approx 11\lambda_{H}\) & 27.1, 1.5, 3.4 & TA \\  & & & & 29.9 & \\
[36] & Feed Translation & 3.5, 28/N-A & \(112,56\) & & 6.4, & Multiple \\  & & & & 19.6 & Antenna \\
[66] & Feed Translation and rotation & 19.5, 29/11.3, 11.4 & \(\pm 20,\pm 18\) & 10.7\(\lambda_{H}\) & 25.9, 29 & 3.0, 3.2 & TA \\  & & & 20.4, 29.6/4, 8 & \(\pm 50,\pm 50\) & 12.4\(\lambda_{H}\) & 23.4, & 1.8, 2.5 & TA \\  & & & 25.3 & & & \\ \hline \hline \end{tabular}  
N-A: Not Available, TA: Transmitarray, NFMS: Near Field Meta-steering.  
\end{table} 
Table 4: _Cont._ \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Parameter** & **NFMS** & **TA** & **RA** \\ \hline Feed Blockage & No & No & Yes \\ \hline Antenna Overall Height & Least & Higher & Higher \\ \hline Aperture Efficiency & Medium & Higher & Higher \\ \hline Dual-band beam steering & Easier & Easier & Complex \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of analysis for dual-band beam steering antennas incorporating metasurfaces. shape discontinuities. Smaller unit cell topologies need to be explored to design a PGM that can allow better resolution in phase correction/tunneling and enhance the overall steering range of the antenna system. * Orthogonal polarised resonant elements [61; 62; 63; 64; 100]: Such cells allow independent phase tuning at the dual frequency bands, minimising the need for time-consuming computations.