# Flexible Antennas: A Review
## 4 Applications of Flexible Antennas under Different Frequency Bands
### Below 12 GHz  
The development of flexible materials has paved the way for innovation in antenna designs and new applications that were not possible with rigid substrates. For flexible antenna applications below 12 GHz, RFID tag or smart card systems are typically designed using the flexible antennas at ultra-high frequency (UHF) band. The ultra-wideband applications of the flexible antenna cover WiMAX, WiFi, lower band of 5G [131], and one of the ISM radio bands. For flexible display devices working in the UHF band, a dipole antenna was reported on the Kapton polyimide substrate [132]. Kapton substrate ensures mechanical robustness and low dielectric loss for this antenna. In the UHF spectrum, antennas for smart cards and RFID tags dominate. Flexible RFID tags for non-invasive sensor applications like patient tracking in the medical system, internet of things (IoT) devices, childcare centers, humidity, and temperature sensing have been reported [133; 134; 135; 136]. In an earlier article [137], a photo paper-based flexible inkjet-printed RFID tag antenna was reported for UHF applications. The antenna had an omnidirectional radiation pattern with 4.57 m coverage with a universal UHF band, 865-960 MHz. A temporary tissue type flexible RFID tag antenna  
Figure 10: Embroidery and stitching based flexible antennas. (**a**) Novel mixed embroidered-woven textile integrated waveguide (TIW) antenna: top and bottom views [128], (**b**) embroidery metamaterial antenna manufacturing process with stitch pattern embroidery layout and embroidered antennas [129], and (**c**) fabricated e-textile based on a slotted patch antenna created with a sewing machine and copper tape [127]. was proposed in a prior study [133], which had a maximum range of 1.2 m. In another study [32], graphene nanoflakes printed flexible antenna was reported that was mainly a meandered line dipole antenna. It covered frequency from 984 to 1052 MHz with a radiation efficiency of 32% and a gain of 4 dBi. The new 3-D printing technology was used to design an RFID tag antenna in [138]. And in [111], a button-shaped RFID tag antenna was proposed combining 3-D and inkjet printing technology. This antenna showed a maximum reading range of 2.1 m. A flexible 3-D printed RFID tag antenna [139] was shown that achieved a maximum 10.6 m read range, and even in several stretching conditions, it covers more than 7.4 m.  
Flexible antennas offer a promising solution for body-centric medical, consumer electronics, and military applications. For wearable applications, besides UHF bands, a 2.45 GHz frequency is used extensively for ISM applications. A low profile, lightweight, and robust antenna is preferred for this type of application. For intrabody telemedicine systems, a flexible photo paper based antenna was proposed that operates at 2.33-2.53 GHz [80]. In another study [140], a wearable textile logo antenna was proposed that was designed for military applications operating at the ISM band. In the literature, many other flexible antennas were reported for on-body applications that fall in the ISM frequency band [141; 142; 143; 144; 145]. Another frequently used antenna type is the ultra-wideband (UWB) antenna. In 2002, Federal Communication Commissions (FCC) defined the UWB spectrum ranging from 3.1 to 10.6 GHz to comply with the demand for higher data rate. The proliferation of body-centric communications, a subcategory of wireless body area networks (WBAN), has encouraged researchers to focus on flexible wideband and ultra-wideband antennas. UWB antennas have significant features such as small electrical size, low cost, low power spectral density, and high data rate [146], and have found extensive use. Because of the lower spectral density, the antenna is less prone to interference with other signals [147]. The UWB antenna made with the textile substrate can be used for on-body applications because it has minimal effect on the human body [148; 149; 150; 151; 152]. Paper-based inkjet-printed UWB antenna was introduced first in earlier studies [153; 154].