## 2 Optically Transparent Antennas
### Transparent Antennas Using Mesh Grid  
Antenna design using the mesh-grid technique provides several advantages in terms of ease of fabrication, low cost, and high conductivity; however, reducing grid size may improve the conductivity at the cost of low optical transparency of the antenna. Figure 12 illustrates the advantages and disadvantages of various techniques implemented to design optically transparent antenna. Optically transparent antenna designs using mesh grid for radiating surface are classified to metal mesh, millimeter metal mesh, and grating mesh. Figure 13 shows the pros and cons of these techniques. Metal mesh provides improved conductivity; however, due to increased mesh, optical transparency is compromised. In case of metal mesh, the size of mesh is taken as very small, in micrometers, and it provides normal conductivity but with improved optical transparency. The grating mesh contains nonuniform meshing, providing improved optical transparency. A transparent antenna using metal mesh was presented in [33]. It can be integrated with smart widows, IoT, and mobile devices. The operating frequency was chosen for WLAN (802.11b) at 2.4-2.5 GHz. The conductive material was designed using the square structure of lattice using wired metal mesh between 0.2 to 0.5 nm thickness. The proposed antenna provided 3.4 dBi gain using simulation software, while the measured gain was 5.2 dBi. In this scheme, radiation efficiency greater than 50% was achieved. It was noticed that wired and micro metal meshes were considered to have low resistance and high  
Figure 11: CPW-fed dual-band transparent antenna [29]. conductivity in contrast to transparent antennas. The authors in [34; 81] suggested these metal meshes for the shielding of electromagnetic interfaces. A novel zeroth-order resonant (ZOR)-based micromesh optical antenna structure was discussed in [35]. This antenna was implemented using a mechanical reconfigurable model. ZOR helps in reducing the size while the tortuous micromesh was used as patch material. This structure can undergo deformation by stretching, twisting, and folding without any breakage. This design can operate at 2.94 to 2.46 GHz. The size of the material chosen was 8.32 mm \(\times\) 11.6 mm \(\times\) 0.4 mm. A gain of \(-\)0.02 dB was achieved in this configuration. Similar work was discussed in [36; 82; 83], where metal materials/substrate were taken as tortuous, while the Cu mesh replaced previous works of EGaIn/PDMS, E-textile/fabrics, and AgNPs/PDMS. A \(-\)2.4 dBi gain was achieved at 3.45 GHz, while it was 12.8 dBi at 2.1 GHz. Similarly, at 2.92 GHz, the gain of 0.37 dBi was noted. Figure 12: Techniques to achieve optical transparency. Figure 13: Meshing techniques for optical antenna. A circularly polarized antenna using meshed patch was investigated in [38], where different mesh configurations were proposed. The opto-radio electrical performance was compared. The proximity coupling technique was altered to feed the mesh patch, hence providing improved optical transparency. Different ground plane antennas were fabricated, one with copper and the other with solar cell ground plane. Results gave 2.79 and 3.27 bandwidth at the frequency of 2.43 GHz operational frequency, with a gain of 4.9 and 4.4 dBi, respectively. Authors in [39] proposed a wideband dual-polarized patch antenna with transparency. Conductive films were printed with metal meshes replacing the metal plates. The sheet resistance of 0.5/sq was taken through the material of polyethylene terephthalate. Layers of the planer conductor were separated by poly methyl methacrylate plates used as substrate. Using the stacked round microstrip patches, wide bandwidth was achieved. Results showed 19% antenna bandwidth while radiation efficiency of around 75% was achieved. The antenna transparency of 70% was noticed, and the radiation pattern was also very stable, with a very small back radiation level across the 2.4-2.9 GHz frequency. In [40], transparent wearable antennas using conductive mesh were proposed.