# Flexible Antennas: A Review
## 8 Performance of Different Types of Flexible Antennas
From the simulation, the SAR values of the antenna have been found below the American and Canadian limits of 1.6 W/kg on average. It was recommended to use this antenna for wearable applications like jackets without direct contact with the human body. A 2.28-2.64 GHz wearable circular ring slot antenna had a maximum gain of 7.3 dBi in the ISM band, and the efficiency varied between 50% and 60%, and more than 70% with electromagnetic band gap (EBG) technique [233]. The antenna performance was investigated by placing it on different parts of the human body: arm, lap, and stomach. Figure 20 shows the experimental setup of the proposed antenna and S\({}_{11}\) parameter of the proposed antenna under these conditions. The SAR value with EBG structure was found to be 0.554 W/kg, met the SAR requirements for the US standards. The dual-band textile patch antenna offered good coverage for WLAN operating bands fabricated using SIW technology on a felt substrate [234]. The antenna was bent with various bending radius to verify the performances. The total efficiencies of the array in the lower and upper bands were 55% and 60%, respectively. The average efficiencies were caused by the loss of the textile material. However, the upper band of the antenna maintained a good isolation as low as \(-\)35 dB, making it suitable for MIMO applications. The specific absorption ratio (SAR) value on the human body model, which was 0.067 W/kg, was far below the limit value 2 W/kg of European standard. For the all-textile circular ring-slot antenna, a two-third muscle equivalent phantom was used in order to verify the performance on the human body [235]. The external effects were not able to  
Figure 20: Measured S\({}_{11}\) of the proposed antenna placed on human tissue. (**a**) Measurement setup and (**b**) S\({}_{11}\) curves [233]. influence the return loss characteristics of the antenna because of the SIW cavity-backed feed structure, including the clothing-mounted environment and human body. Figure 21 shows the simulation and measurement of the antenna for different bending radius setup. The radiation efficiency and measured peak gain were 37.7% and 3.12 dBi, respectively, which were reasonable for ISM applications. The radiation characteristic of the proposed antenna was insensitive to the phantom effect. In another report [236], an ultra-wideband (UWB) antenna's performance was investigated with changing of substrate's thickness and dielectric constant, as well as bending along a cylindrical structure with 10 and 20 mm radii. It was found that there is not much difference in the bandwidth and the efficiency of the antenna under flat and bent cases which made it suitable in foldable Wireless Wide Area Network (WWAN) terminals, WBAN devices, and medical sensors. The convex and concave bending analysis was performed on an inkjet-printed multiband (covered GSM 900, GPS, UMTS, WLAN, ISM, Bluetooth, LTE 2300/2500, and WiMAX) antenna to characterize the flexibility [237]. For both types of configuration, the frequency shift was not that significant, a slight increase in the gain due to a slight rise in directivity was observed. No significant degradation was found, and the overall performance was satisfactory for various wireless applications for future conformal and flexible electronic devices. Designing an antenna for a wideband operation makes it more immune to frequency shifting due to bending. If the antennas are designed for wideband operation, their resonant frequency could be kept within the required operating region even after bending [17]. Second, symmetrically shaped antennas are less affected by bending in different directions. Bending analysis is essential to test the durability or robustness of the antennas. Repeated bending tests were done to check physical deformations, discontinuities, or cracks on the conductive part of the antenna. Due to physical deformations or cracks on the radiating element, current density changes, which change the antenna polarization in effect.