# Optically Transparent Nano-Patterned Antennas: A Review and Future Directions
## 2 Transparent Antennas: A Review
### Metallic Nanostructure  
#### 2.4.1 Nanowire  
Silver nanowires (AgNWs) are flexible and conductive materials with very small nanometer (nm) diameters in their cross-section [25]. Due to this characteristic, silver nanowires are attractive as next-generation transparent electrode materials that can replace the indium tin oxide (ITO) in conventional transparent electrodes. In particular, there is a strong interest in silver nanowire transparent electrodes that can be produced in large areas in flexible touch panels and displays. However, the conventional nanowire transparent electrode has a rough surface and a large contact resistance between the nanowires. There is also a limit to the haze phenomenon, which is reflected in the sunlight and shattered. The turbidity and surface roughness of the AgNW can be affected by the dimensions of the AgNW, which is usually from 40 to 100 nm in mean diameter and 1 to 20 \(\upmu\)m in mean length [25]. It is possible to realize various \(R_{\rm s}\) (10-100 \(\Omega\)/sq) by controlling the density of AgNWs. However, as the \(R_{\rm s}\) decreases, the density of AgNPs increases and the optical transmittance \(T\) decreases and the turbidity increases [26].  
Figure 6: (**a**) Pictures of a radio frequency identification (RFID) half wavelength dipole antenna and (**b**) an RFID meandering dipole antenna and diagram [22]. (With permission from IEEE).  
In Reference [27], patch antennas based on optically transparent silver nanowire (AgNW) coatings operating at 24 GHz and 61 GHz are analyzed, as shown in Figure 7a. This AgNW has a mean diameter of 40 nm and a mean length of 15 \(\upmu\)m and is coated on glass featuring a thickness of 0.22 mm, relative permittivity of 3.81, and loss tangent of 0.0004 at 24 GHz. The electrical conductivity of AgNW is obtained by comparing the simulation results with the measured insertion loss of coplanar waveguide (CPW), assuming a constant coating thickness of t = 40 nm. The electrical conductivity \(\sigma\) = 7 \(\times\) 10\({}^{6}\) S/m is obtained from the comparison. From the result, the sheet resistance of the patch antenna operating at 24 GHz is 0.36 \(\Omega\)/sq. The patch antennas operating at 24 GHz and 61 GHz have radiation efficiencies of 8.9% and 49.4%, respectively, as shown in Figure 7b, and both have an overall optical transmittance of 51% at 550 nm.  
In Reference [28], AgNW patch antennas resonating at 2 GHz for strain sensors are designed. AgNW is embedded in polydimethylsiloxane (PDMS). The final structure features a thickness of 3 mm and an \(R_{\mathrm{s}}\) of below 5 \(\Omega\)/sq. The AgNW patch antenna's resonant frequency changes with the force applied on it. In Reference [29], a AgNW resonating at 3 GHz patch antenna and a 6 GHz two-element patch array are devised. Similarly, AgNW is embedded in PDMS, which features a relative permittivity of 2.67 to 3 and a loss tangent of 0.01 to 0.05. The conductivity of AgNW is approximately \(8.13\times 10^{4}\) S/cm, and it can be considered as 8.13 \(\times\) 10\({}^{4}\) S/cm assuming that the applied strains are relatively small. The radiation efficiency and bandwidth (\(S_{11}<-10\) dB) of the single antenna and antenna array are 41.83%/88 MHz and 48.8%/330 MHz, respectively. The radiation efficiency deterioration is mainly caused by PDMS rather than AgNW.