## 6 A Comparison of Different Dual-Band Phase-Transformation Cells  
Figure 9 summarizes the techniques previously used to design a dual-band PGMs incorporated into a TAs or NFMS to achieve beam steering. Based on the literature review presented above, a comparison of different relevant unit cell topologies used for phase correction or beam steering is presented in Table 3. The unit cell topologies investigated in this work extend beyond those specified in Table 3. However, our focus remains on those relevant to dual-band passive PGM design. The survey reveals that the independent performance of the unit cell in each frequency band provides greater flexibility in the design of TAs or NFMS for full-duplex steering applications. When the unit cell's response is independent at the two frequency bands, it allows for arbitrary beam direction. Coincident beam direction at the dual frequency bands is preferred in full-duplex communication systems. In such cases, a cell with an independent phase response can help avoid tedious and time-consuming phase optimization techniques. Dual-polarized cells are \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Cell Topology** & **Dual-Band Design** & **Op. Freq. BW** & **Fabrication, Cost** & **High Temp. App.** \\ \hline All-Dielectric & Complex & High & Cheaper if using PLA or ABS & lose structural integrity \\ \hline All-Metallic & Easy & Low & Laser cutting, suited for flat panels only & Suitable \\ \hline Comp. Metal-Dielectric & Easy & Medium & Traditional printing technique, require expensive laminates & Buffer dielectric breakdown \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of performance of different materials used for cell design. also preferred over linear-only polarization since they can accommodate any polarization, circular, linear, or slant. The summary presented in Table 3 shows that achieving an independent phase response across a broader frequency range is a relatively underexplored area. Many dual-band cell topologies rely on optimizing phase pairs at specific frequency bands, necessitating a lengthy design process. Moreover, when the frequency separation is large, the interleaved placement of resonant elements at the dual frequency bands results in larger cells at the higher frequency band. Grating lobes appear for different wave-incidence angles if the cell size exceeds half the wavelength. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline
**Ref.** & **Resonant Element** & **Topology** & **Number of Layers** & **Cell Size** & **Frequency Capability** & **Polarization** & **Phase Control** \\ \hline
[95] & Double Square Loop & Composite separated by air gap & 4 & 0.6\(\lambda\) & Wideband (29–32 GHz) & CP & NA \\ \hline
[101] & 4-arm structure for one band, 4-leg structure for other-band & Composite and & One metal and one dielectric layer & 0.2\(\lambda_{L}\) and (14.1–16 GHz and 29.2–36.8 GHz) & Dual & Separate tuning of elements by length variation \\ \hline
[102] & Cross dipole for higher frequency, backed by square loops for lower frequency & Composite interleaved structure & Five dielectric and seven metal layers & 0.3\(\lambda_{L}\) and 0.4\(\lambda_{H}\) & Dual-band (20, 30 GHz) & CP & NA \\ \hline
[69] & Split ring center elements for lower frequency, quarter split circle at corners for higher frequency & Composite and four dielectric layers & Five metal and dielectric layers & 0.53\(\lambda_{L}\) and 0.8\(\lambda_{H}\) & Dual-band (20, 30 GHz) & CP & Independent phase tuning by rotation of split rings \\ \hline
[84] & Square Patch Middle and square loop mentions shape against design frequ & Composite and six dielectric layers & seven metal and six dielectric layers & 0.35\(\lambda_{H}\) and 0.35\(\lambda_{H}\) & Dual-band (20, 30 GHz) & CP & Optimal Phase Pair Selection \\ \hline \end{tabular}
\end{table} 
Table 3: Specifications of resonant elements in various references.