## 2 Optically Transparent Antennas
### Transparent Antennas Using Thin Films  
Antenna designed using thin films (ITO) provide advantages in terms of ease of availability, high conductivity, and high transmittance when they are integrated on the glass substrate. However, high cost is the major disadvantage. Zinc-based TCO materials and carbon nanotubes are considered appropriate in terms of performance with low cost. Optical antennas designed using IZTO/Ag/LZTO, in which metal mesh is sandwiched between thin-film layers, provide high transparency, low resistivity, and flexible structure; however, their fabrication process is complex as compared to the single-layer fabrication. Transparent conducting oxide (TCO) material with gallium-doped zinc oxide (GZO) may be considered a good option for the design of a transparent antenna due to its high conductivity. Authors in [70] discussed several materials, including conductive polymers, TCOs, and conductive inks for these transparent antennas. Carbon nanotubes were suggested as a transparent radiating film in [71]. Several types of TCOs include fluorine-doped tin oxide (FTO), Al-doped ZnO (AZO), ITO, and Ga-doped ZnO (GZO). Other materials include ATO, FZO, IZO, and AIZO, as mentioned in [67]. The design of a conductive polymer microstrip fed patch antenna operating at 10 GHz was suggested in [7]. Polyanilinee (Pani) was used as film material with electrical conductivity of \(6\times 10^{3}\) S/m with thickness of 100 \(\upmu\)m. A gain of 2.08 dB was achieved. Figure 4 illustrates the design. Figure 4: Conductive polymer microstrip fed patch antenna [7]. Figure 3: Classification of optically transparent antennas. In [9], a transparent conducting film comprising AZO/silver nanowire (AgNW) on substrate of glass material was proposed through RF magnetron sputtering and spin-coating. The resistivity of AZO/AgWNs stacked films was \(2.15\times 10^{4}\)\(\Omega\)cm. Finally, from these stacked films, a wideband transparent antenna was designed at 2.4 GHz for Bluetooth communication. Therefore, TCOs are suitable for OTA fabrication due to their conductive nature within the RF range. An optical antenna using thin aluminum and zinc oxide layers was designed, as mentioned in [10]. These layers were co-sputtered onto Si and polycrystalline photovoltaic cells. Low-resistivity AZO was formed through the annealing process, as illustrated in Figure 5.  
Another optical antenna with TCO and AZO combination was proposed in [11]. This material provides improved RF conductivity with high transmittance and provides a cost-efficient solution: an antenna array comprising two rectangular inset feed patch elements with dimensions 19 cm \(\times\) 13 cm operating at 2.4 GHz. Realized gain of 5 dB was obtained. RF properties of these materials allow the transparent antennas to integrate with the solar cells. Authors in [72] proposed a transparent antenna working at 20 to 44 GHz frequency. Wide bandwidth was achieved through the rectangular-shaped branches that are attached to the infinite ground plane. Antenna size was taken as 10 mm \(\times\) 12 mm \(\times\) 1.48 mm. A return loss of 10 dB was achieved with a directional radiation pattern along with electrical tilt according to the required frequency. The authors in [12] discussed the transparent cone-top tapered slot antenna for UWB communication, designed to operate between 2.2 GHz to 12.1 GHz integrated onto solar panels along with harvesting EM waves that are converted to electrical energy. A graphene nanoribbon-based terahertz patch antenna operating at 725-775 GHz band using polyimide substrate was designed in [73]. These types of antennas provide low transmit power with higher data rates while enhancing the security of wireless communication. This novel design resulted in 5.71 dBi gain and impedance bandwidth greater than 5%. Authors in [13] used different TCOs, which included ITO, AgHT-4/8, and FTO, as conducting patch material. Figure 6 shows the design, comprising a silicon layer sandwiched between the anode and cathode layer.