## 2 Optically Transparent Antennas
### Transparent Antennas Using Thin Films
The patch was electromagnetically excited using a coupled feeding technique, while cold soldering was used to feed the connector. The proposed antenna produced a gain of 5.61 dBi operating at 4.9 GHz of frequency. The reflection coefficient of -10 dB was achieved, and optical transmittance was found to be greater than 60%. The authors in [74] used doped ZnO thin films at 2.5 GHz frequency. Hourglass topology was used in the design. Multiarrays were proposed to solve the issues of low gain and antenna efficiency. In [37], an MIMO antenna for WLAN applications was proposed, operating between 2.4 to 2.48 GHz and 5.15 to 5.8 GHz. The antenna used MMMC (micro metal mesh conductive) films. Using SANTE self-assembling nanoparticle technology, these films were constructed. A high transmittance above 75% and low resistance was achieved, along with efficiency greater than 40%. The design of a BLC (branch line coupler) using nanotechnology-based micro metal (MM) conductive film was suggested for a transparent antenna in [21]. The final design, shown in Figure 9, shows high transparency with good conductivity. A micro metal (MM) conductive film-based branch line coupler (BLC) operating at 2.5 GHz was proposed in [21], as illustrated in Figure 9. The proposed design promises high transparency and good conductivity. This proposed transparent antenna design provided a bandwidth of 38%. They can easily be mounted on a building's glass surface, hence providing a basic solution for 5G communication while the major application of the BLC lies in the intelligent transport system for inter-/intrave  
Figure 8: Transparent antenna using borosilicate glass substrate [19]. An H-shaped slot antenna for transparent antennas was fabricated using the AgGL material suitable for urban cellular communication, as suggested by the authors of [22]. Ohmic losses were restricted through transparent and conductive AgGL coating. The results obtained were 5 dBi of gain and bandwidth of around 20% operating at 2 GHz frequency. This scheme is considered suitable for multilayer array designing for cellular networks in urban areas. They proposed a transparent antenna for urban cellular communication for broadband applications with a size of 300 mm \(\times\) 300 mm. Several authors proposed various optically transparent antennas operating at 800 MHz in [32; 75], 1-8.5 GHz as suggested by [23; 24; 76], and 19.5 GHz as recommended by [77]. The antenna size in these studies was taken below 80 mm \(\times\) 80 mm. The conductive coating of single transparent conductive material printed on the substrate on one side was used. The paper [78] discusses the transparent 1D electromagnetic band gap antenna using thin-film alloy working at millimeter-wave frequencies. At 30 GHz frequency, the attenuation of less than 0.3 db/mm was observed. The antenna can be used for AoD application with radiation of 3.4 dBi. Authors in [25] suggested optically transparent antennas working at 750 GHz frequency using multi-walled carbon nanotube (MWCNT)-based transparent conducting oxide materials (TCOMs). Transparent antennas were designed through ITO and tin oxide (TIO) materials. The performance of the transparent antenna was improved using the shorting technique. Using this technique, a return loss of around -40 dB was achieved at the resonance frequency. Furthermore, poor radiation, narrow bandwidth, and the low gain of the conventional transparent antennas can be overcome using MWCNT-based antennas. In [26], AgHT-8 conducting material was used as ground and radiating patch with Plexiglass substrate of size 10 mm \(\times\) 12 mm \(\times\) 1.48 mm, as demonstrated in Figure 3:0 The rectangular-shaped branch helps to provide the wideband feature of the antenna. The antenna operates in a 23.92 to 43.8 GHz range. The proposed scheme provided a bandwidth of 58.71%. This scheme proved to be the most suitable for 5G communication. A reflect optical transparent antenna using ITO was proposed in [79].