## 2 Optically Transparent Antennas
### Transparent Antennas Using Thin Films
This solar patch transparent antenna was designed at 28 GHz for 5G communication. Their performance was tested with a nontransparent antenna. It was observed that the solar efficiency was reduced due to the shadow effect. This issue was resolved using the thin-film TCO as a radiating patch. The patch thickness of the transparent antenna was taken as four times thicker than the nontransparent one. It was also noticed that FTO gave the highest bandwidth while the return loss of ITO was \(-\)35 dB. Figure 5: CPW antenna fabrication [10]. In [14], an optically transmitted microstrip patched antenna mounted on the solar module surface was proposed, working at 3.4 to 3.8 GHz frequency. Polyester was used as a patch material having a conductive coating. The amorphous silicon solar cells worked for both the antenna ground plane and the photovoltaic generator. This topology resulted in a gain of 4 dBi at the resonant frequency. In [15], silver was fabricated with an Ag monolayer while IZTO was fabricated at a multilayer, shown in Figure 7. These TCO materials were used for laptop computers at a frequency between 5.25 to 5.32 GHz. Results indicate the improved performance of Ag monolayer antennas as compared to the IZTO/Ag/IZTO multilayer. The efficiencies of multilayer antennas were 59-68% and 82-84%, respectively. Authors used ITO (indium tin oxide) for designing an optically transparent antenna in [16]. A thin sheet of the quartz glass substrate was used along with ITO film coatings, and the coaxial probe was used for feeding. This antenna resulted in the measured and simulated efficiency of 56.64% and 59.71%, respectively, with the gain of 4.27 dB and 4.52 dB, respectively. Optical antennas have applications for car network communication as well, which was proposed in [17]. This optical antenna worked at frequencies between 5.75 to 5.85 GHz. ITO material was used for the radiation patch which was placed on top of the glass substrate. FSS (frequency selective surface) acted as a ground for the microstrip antenna. As FSS has good transmittance of light, it helps to achieve directional radiation because it can reflect the electromagnetic waves received at the antenna. For the feeding of the antenna, an SMA connector was used. The results demonstrated in this paper gave good radiation antenna parameters performance (S11 and gain). In [18], a dual-band transparent antenna with two slotted circular rings (SRR) was suggested. AgHT-8 thin film was used as conducting patch for two circular SRRs and partial ground, providing transparency of 80%. Plexiglas material was used as substrate. The proposed antenna has dimensions 35 mm \(\times\) 35 mm \(\times\) 1.84 mm. Impedance bandwidth  
Figure 6: Solar patch transparent antenna [13]. Figure 7: Antenna with Ag monolayer and multilayer of IZTO/Ag/IZTO [15]. of 13.27% at 2.4 GHz and 5.28% at 5.28 GHz was achieved. The resulting band is suitable for smart applications covering IEEE a/b/g and n frequency bands. In [19], antenna efficiency was improved using transparent antenna. The antenna was designed to resonate between 3.1 GHz and 10.6 GHz and the patch was made of ITO film deposited on the borosilicate glass substrate, as shown in Figure 8. A bandwidth of 155% was achieved. The gain was improved by the homogeneous decomposition of the gold nanolayer, which in return reduced the ohmic loss. An optically transparent antenna producing high gain for k-band operating between 17.6 GHz and 30.5 GHz was proposed in [20]. This antenna was intended to work with satellite communication using ITO film and Pyrex glass as substrate material. The proposed antenna provided 12.3 dBi gain with 13.38 dBi directivity and 92% efficiency. An improved VSWR of 1.05, an enhanced bandwidth of 12.898 GHz, and an 11.52 dB realized gain at the center operating frequency of 24.8 GHz were achieved in this design. A low-profile transparent microstrip patch antenna using conductive layers of FTO (fluorine doped in oxide) was proposed in [30].