## 5 Passive Dual-Band Phase-Gradient Metasurfaces
### Composite Metal-Dielectric Unit Cells
Arranging the elements in a gradient metasurface while avoiding corner discontinuities can also be challenging. A linearly polarized unit cell with vertical and horizontal dipoles corresponding to the dual frequency bands is also utilized to design a dual-band TA for wide beam scanning in [33]. The proposed design can be used for the uplink and downlink Ka-bands. Beam scanning of \(\pm 40^{\circ}\) and \(\pm 30^{\circ}\) is achieved independently at the two frequency bands. The results suggested that the transmitted beam can be scanned over \(80^{\circ}\) for 19.5 GHz and \(65^{\circ}\) for 29 GHz in elevation with only 2 dB scan loss in the peak gain. Displacement of the feed horn antenna at the focal distance from the surface is proposed for tilting the beam. Another similar dual-band unit cell with double horizontal and vertical dipoles was reported in [64]. Another dual-band design allowing independent phase tuning through the cell at dual frequency bands is reported in [67]. The reported cell is a single-layered substrate meta-atom. The two metallic layers at the top and bottom of the substrate layer are composed of a modified Jerusalem cross-resonator seated in a circular hole, a modified complementary split-ring resonator, and a ring connector. The modified Jerusalem cross resonator is used to tailor the phase of the cell at the lower-frequency band \(f_{L}\). In contrast, the modified complementary split-ring resonator is utilized to tailor the phase at the higher frequency band \(f_{H}\). In this case, the PR technique independently tunes the phase response at the dual frequency bands. In [59], a dual-band lens is implemented, operating in orthogonal polarization for the X-band and Ku-band. A rectangular dual-band cell with two orthogonal rectangular patches printed on four metal layers separated by three dielectric layers is used. At \(f_{L}\) = 8 GHz, a y-polarized feed is used, with the resonant elements' dimensions parallel to the y-direction tuned for phase control. At \(f_{H}\) = 14 GHz, an x-polarized feed was used, with the resonant elements' dimensions parallel to the x-axis being tuned for phase adjustment. Recently, another dual-band cell with independent frequency tuning has been reported in [100]. The unit cell consisted of four metal layers and four dielectric layers of a thin substrate. It is composed of interleaved square slot and cross slot resonant elements, following the orthogonal principle of resonance to allow independent phase tuning at the dual frequency bands. The composite unit cells reported in this section provide more design flexibility and wider bandwidth at the dual frequency bands of operation. Moreover, such PGMs can be precisely fabricated at higher-frequency bands using the standard printed circuit board fabrication techniques. However, high-performance laminates are costly. The dielectric electric field breakdown threshold is also lower than that of metals, making it less feasible for space applications or where environmental conditions are harsh [31; 100]. Based on the structural topology of a unit cell, the performance of different cell can be summarized as in Table 2.