# Flexible Antennas: A Review
## 4 Applications of Flexible Antennas under Different Frequency Bands
### Above 12 GHz  
According to Radio Society of Great Britain (RSGB), above 12 GHz, the Ku band starts, and these high-frequency bands are primarily used for radar, satellite communications, astronomical observations, radio astronomy, and microwave remote sensing. For remote sensing, radar, and future communication systems, dual-polarization microstrip antenna arrays were reported on LCP substrate operating at 14 and 35 GHz. Dual-polarization and dual-frequency ensures higher capacity data transfer [81]. A flexible, washable, and reusable UWB fully textile-based wearable antenna was designed and analyzed in earlier reports [178]. It maintained excellent efficiency from 3 to 20 GHz conductive for medical monitoring applications and smart garments. For example, flexible graphene antennas in single and array on polyimide substrate (Figure 11a) operating at 15 GHz produced large bandwidth to support higher-speed for 5G applications [179].  
In a prior study [180], a flexible, transparent, and wideband mm-wave slotted monopole antenna was designed, fabricated, and tested by inkjet printing of custom made silver nanowire (Ag NW) ink. The antenna showed ultra-wide bandwidth up to 26 GHz (from 18 to 44 GHz), the high radiation efficiency of 55%, and a maximum gain of 1.45 dBi. A comparison between CPW fed monopole antennas printed on PET and Epson paper operating at 20 GHz was reported earlier [181]. The antennas were printed using CuO based ink using an inkjet printer. A Y-shaped transparent and flexible coplanar waveguide fed (CPW) antennas operating from 23 to 29.5 GHz, covering the necessary frequency bands for 5G wireless communications, was modeled using silver-coated polyester film (AgHT) transparent conductive material over PET substrate in [182]. Another work of high-frequency application using inkjet-printed technology is a proximity-fed patch antenna, designed for the 24-GHz ISM band, which was demonstrated in [183]. This antenna was printed using multilayer printed technology on the LCP substrate. Frequency reconfigurable antennas can operate at different frequency bands. In another study [184], a reconfigurable wearable Millimeter Wave (MMW) antenna has been introduced covering 20.7-36 GHz range in different switch configurations. The antenna was printed on an LCP substrate using inkjet-printed technology. The dielectric characteristics of PDMS have been in V- and W-bands have been tested and experimented in [185].  
A micromachined microstrip patch antenna was designed in the 60 GHz band on the PDMS substrate for assessing and comparing this technology with the alternative one (Figure 11b). A flexible millimeter-wave antenna array exhibits a bandwidth of 26-40 GHz with a peak gain of 11.35 dBi at 35 GHz, and a consistent high gain profile of above 9 dBi in the complete Ka-band [15]. An electromagnetic bandgap (EBG) structured mm-wave MIMO antenna operating at 24 GHz (ISM band), suitable for wearable applications, was proposed in [186] on flexible Rogers substrate (Figure 11c). The antenna parameters were studied in free space as well as on a human phantom under bending. The proposed antenna is suitable for wearable applications at mm-wave range due to its simple geometry and excellent performance in bending and on-body worn scenarios.