## 2 Optically Transparent Antennas
### Transparent Antennas Using Mesh Grid
These antennas operate between 2.2-2.5 GHz, as shown in Figure 14. They are suitable for integration over solar cells. Some authors also proposed transparent antennas in the mm-wave band. In [46], they achieved high transparency using fabrics of select textiles. To achieve transparency, very fine mesh was used, operating between 27 and 35 GHz. Low-cost acrylic was used as substrate material. This scheme seems to be a strong option for 5G applications that may be used in autonomous vehicles. Figure 15 shows its basic topology. A transparency level of above 70% for veil shield was achieved, but it was less than 50% for radio screens. Good simulation results were achieved, showing a bandwidth of around 8 GHz. Authors in [86] presented the copper honeycomb mesh operating at an 80 GHz frequency band. This topology promises to give better results that are similar to a solid structure. Likewise, in [87], a 60 GHz antenna was designed using silver (Ag) mesh from which 12 dBi gain was achieved along with optical transparency of 75%. The results show that increasing the density of the mesh will decrease the antenna transparency, but on the other hand, the transmission line losses are also decreased. The antenna designs are shown in Figure 16a,b. A novel double-sided micromesh metal layer 4 \(\times\) 2 optical transparent antenna was presented in [88], working at 60 GHz frequency. This scheme provided transparency above 80%. A reasonable gain of 13.6 dBi was achieved at the operating frequency. Later, this antenna was compared with an opaque antenna with copper radiating patch at 59.7 GHz, providing 15.6 dBi gain, as shown in Figure 17.  
An optically transparent water patch wideband antenna producing the pattern of broadside radiations was presented in [89]. The ground plane and the patch were enclosed in a Plexiglass transparent container containing distilled water. High optical transparency was achieved using air as substrate material, providing a low-cost solution. The water patch was excited using an L-shaped metallic probe in the air substrate. The impedance of 34.9% was achieved at operating frequency between 1.61 GHz-2.29 GHz and SWR \(<\) 2. Using this scheme, a maximum efficiency of up to 75% can be achieved with a maximum gain of 6.6 dBi. Figure 14: Meshing using rectangular and circular patch [45]. The paper [90] discussed the design of external transparent patch and dipole antennas working at 470 MHz to 772 MHz for ultrahigh-definition (UHD) television applications. Optical transparency of around 70% was achieved using metal mesh topology having a sheet resistance of 0.04 \(\Omega\)/sq. The average efficiency capacities fed transparent feed was 83% with a peak gain of 6.2 dBi, whereas, in the case of a transparent dipole, the peak gain was 2.4 dBi and efficiency was 72%. Therefore, the results were satisfactory as compared to the nontransparent antennas, providing good scope for their use in UHD TV applications. A summary of the optical transparent antennas made using thin-film materials and mesh-grid technologies is given in Tables 1 and 2 respectively, where optical transparent  
Figure 16: Examples of optical transparent antennas. (**a**) Honeycomb mesh [86]. (**b**) Silver meshing [87]. Figure 17: \(4\times 2\) multilayer optical transparent antenna [88]. Figure 15: Transparent frequency selective surface (FSS) antennas in the mm-wave band [46]. antennas operating at different frequencies, using various technologies and substrate materials, are discussed. Antenna gain, bandwidth, efficiency, transparency, and transmittance are recorded in Tables 1 and 2.