## 2 Optically Transparent Antennas
### Transparent Antennas Using Mesh Grid
This method is a cost-effective method providing more robust bending than other transparent antennas. The antenna operated at the dual-band of 2.33-2.53 GHz and 4.7-5.6 GHz. The prototype was fabricated and tested. The given frequencies can be used for WLAN along with instrument, scientific, and management (ISM) bands. Two layers of conductors were implemented to improve the antenna performance, the gain, and efficiency of the antenna as a trade-off with the transparency of the antenna. In [84], an optical transparent antenna was designed at 5-6 GHz frequency band using mesh wire of high conductivity on top of Lexan substrate. The arrays showed a peak gain of 6.5 dBi, which is less than the traditional antenna of 7.8 dBi but still acceptable for the transparent antenna. A coplanar waveguide transparent bandpass filter was proposed in [85], comprising an aluminum thin-film structure of the micromesh. The filter was designed through the dc sputtering; thus, high performance was achieved using the thin-film micromesh structure with the resistivity of 10\({}^{-8}\)\(\Omega\)-cm and optical transparency of 78%. The antenna works at 2.4 GHz with a reflection coefficient of 25 dB, along with 5% bandwidth and minimum insertion loss. Authors in [33] (already mentioned above) used 2.45 GHz to design the optical transparent antennas. Several materials were used in their study, noted as metal mesh, multilayer ITO, and conductive grids. Conductive grids provided maximum transparency of 90% as compared to other materials. The conductivity was 1.5 \(\times\)10\({}^{6}\) S/m, 2.8 \(\times\)10\({}^{6}\) S/m, and 1.5 \(\times\)10\({}^{6}\) S/m, respectively. The substrate material in the case of a conductive grid has a minimum dimension of 0.2 mm \(\times\) 0.36 mm \(\times\) 0.003 mm. Radiation efficiency was 56% in the case of metal mesh, along with a higher gain of 4.14 dBi, higher than the other two materials. Similar work to that described above was carried out in [41; 42; 43] at frequencies of 60 GHz, 1.25 GHz, and 28 GHz using materials of gold grid, silver epoxy mesh, and Ag-alloy, respectively. It was observed that the conductivity of Ag-alloy was high, 2.5 \(\times\)10\({}^{7}\) S/m, in contrast to other materials, providing an efficiency 41 dB higher than other materials. In addition, the gain was 9.1 dBi, the highest among the materials used. This scheme provided a scanning angle of \(-\)60\({}^{\circ}\) to 60\({}^{\circ}\). The drawback of this scheme is the higher width of the substrate. Moving towards the concept of an antenna on display, the author in [44] proposed an antenna using photolithography that can be integrated into the display panels of OLED and LCD, thus making a novel optical invisible antenna. This technology was termed \"antenna on display\" (AoD). It gives promising direction toward the millimeter-wave 5G cellular devices. A high optical transparency of 88 percent was achieved with an antenna working on 28 GHz. The authors designed a multilayer schematic to achieve the phased array configuration. A gain of 6.6 dBi was achieved. The dummy grids were introduced to improve the transparency which degrades the gain. Hence, immaturity of the anisotropic conductive film needs to be improved for achieving reasonable antenna gain while maintaining transparency. Conductors with meshed design along with a plane of solid ground were proposed by the authors in [45]. They used circular and rectangular patches in their designs, and the meshing technique provided transparency to the antennas. The authors discussed the trade-off between the optical transparency and antenna effeminacy relation in this paper. They further proposed refined meshing for achieving a good level of optical transparency along with optimized antenna parameters, but the reduction in meshing can help in achieving higher transparency with little issues with antenna performance. The fabrication process also provides the limitations of meshing.