## 3 Discussion  
The design of an optical antenna requires careful selection of radiating patch and substrate material. Table 1 exhibits a possible list of substrate and patch materials along with the antenna performance in terms of gain (dBi), bandwidth, efficiency, transparency, and transmittance. Antenna performance parameters aid in the careful choice of appropriate materials for optical antenna, depending on the antenna application. Several substrate materials were tested in the literature; they include PDMS, PET, glass (quartz, Pyrex, soda-line, borosilicate), and silicon, while AgHT-4/8, ITO, FTO, AZO, GZO, and ITO are possible patch materials used. The maximum gain of 11.5 dBi was achieved with Pyrex glass substrate with ITO patch, as mentioned in [20], followed by 9.8 dBi for glass substrate along with AgHT-4 patch material. An improved efficiency of 92% was achieved with an ITO patch on top of Pyrex glass substrate. There is always a trade-off between efficiency and optical transparency. Most of the authors did not mention the transparency of the antenna where good efficiency of the optical antenna was achieved. For achieving higher efficiency, authors suggested Pyrex glass with ITO, and Plexiglass with FTO. Table 1 indicates reasonable efficiency for optical antenna designs achieving 92% (Plexiglass substrate with ITO conducting material), 90% (Plexiglass substrate), and 82% (polyimide substrate and IZTO/Ag/IZTO conducting material). Some applications require ultra-wideband optical antenna. Referring to Table 1, tissue fabric with PDMS substrate provides high bandwidth; similarly, multiband antenna designed using borosilicate glass combined with ITO provides 7 GHz bandwidth, while 10 GHz ultra-wideband is achieved using perspex substrate with AgHT-4. For mesh-grid analysis, referring to Table 2, several mesh materials for the patch are suggested, which include metal mesh, aluminum mesh, micromesh, tortuous, and millimeter mesh. The bandwidths of 90%, 86%, and 81% were achieved using glass substrate with Ag/Ti, FR-4 with MWCNT, and fused silica with tortuous MM, while transparency up to 98% was achieved using thin MM with CoPs, as mentioned in [48]. Similarly, using metal mesh as radiating patch and PET as substrate material, 10.4 dBi gain was achieved. Moreover, diamond-shaped Ag-alloy with glass substrate provided 9 dBi gain, as indicated in [44; 47]. Gallium-doped zinc oxide (GZO) can be considered a good option for the design of a transparent antenna due to its high conductivity. The shadow effect reduces solar efficiency [13]. Silver integrated with Ag monolayer performed better for laptop applications, as compared to IZTO multilayer [15]. AZO offers high transmittance in the visible spectrum and strong RF conductivity at low cost, as mentioned in [11], while providing ease of integration with solar cells. A graphene-nanoribbon-based terahertz patch antenna provides low transmit power with higher data rates while enhancing the security of wireless communication [73]. AZO/AgNWs provide weak shielding against backside electromagnetic fields. However, increasing the stacked films thickness may improve the shielding against backside radiations. TCO materials are capable of providing transparency at optical frequency along with conductivity at RF frequency range. Therefore, these materials are suitable for OTA fabrication. By using AZO/AgNWs stacked films, a wideband transparent antenna was achieved, as mentioned in [9]. Moreover, the advantages and disadvantages of thin film and metal mesh were discussed in the literature. Metal mesh provides low cost and ease of fabrication along with high conductivity, but increasing the meshing network will decrease the optical transparency of antenna; hence, a trade-off between the conductivity and transparency is required. On the other hand, thin films are expensive. ITO provides higher conductivity along with transmittance when integrated with glass, while in terms of ease of availability, ITO materials are a good choice.