# Flexible Antennas: A Review
## 8 Performance of Different Types of Flexible Antennas
The multilayer inkjet-printed microstrip fractal patch antenna showed excellent stability and tolerance under different bending radii of curvature [60]. The inkjet-printed antenna  
Figure 17: Experiment process of compressive and tensile bending and stretching the antenna along the x-axis and y-axis [23]. reconfigurable antenna for WLAN/WiMAX wireless devices was fabricated and tested for both flat and curved geometries of different radii with well-maintained radiation characteristics [63]. The effect of antenna bending in x and y direction had been reported in [64] for reconfigurable dual-band dual polarized monopole antenna. A numerical and experimental study of the impact of bending on the characteristic parameters of a flexible ultra-wideband (UWB) made of thin PEDOT polymer sheet on sticky tape substrate antenna was reported earlier [42]. Antenna characteristics, including S-parameters, polarization, and radiation patterns, were examined with regards to the bending angles from 0\({}^{\circ}\) to 180\({}^{\circ}\) (Figure 18). Though the antenna performance was found satisfactory even with acute bending angles, the polarization direction was changed. The array configuration [81] exhibited a return loss of more than 15 dB in both frequency bands. Higher cross-polarization can be improved using different feeding networks at both layers. To demonstrate the flexibility and mechanical stability of the antenna arrays, they were flexed several times and recharacterized. The return loss and radiation patterns were unchanged within the repeatability of the measurement equipment. The simulated and measured data of an mm-wave antenna and its antenna arrays performance regarding impedance bandwidth, radiation pattern, and realized gain for laser printed and inkjet-printed processes over Ka-band (26-40 GHz) was reported earlier [82]. The conformity analysis of the antenna was done by bending it along the cylindrical surface of different radii only for the reflection coefficient parameter (Figure 19). Though inkjet-printed high-gain, multidirector Yagi-Uda antennas for use within the 24.5-GHz ISM band was fabricated on flexible LCP substrate, the antenna performance for the conformal  
Figure 19: Comparative return loss of planar vs. conformal antenna configuration along the cylindrical surface of radii, \(r_{1}=6\) mm, \(r_{2}=8\) mm, and \(r_{3}=10\) mm [82]. Figure 18: Impact of bending on antenna performance. (**a**) 3-D bending setup and definition of the bending angle in CST microwave studio, and (**b**) simulated \(|S_{11}|\) for different bending angles [42]. application was not studied [95]. It is expected that antenna characteristics parameters change due to the interaction with the lossy human body tissues. Hence, the permittivity of human tissues and different conductivity values influence the reflection coefficients, affect the power absorbed by the body, and decrease the radiation efficiency of the antenna [231]. Moreover, the specific absorption rate (SAR) limit has to be considered while designing a wearable/flexible antenna. The SAR limit is regulated for wearable devices, including antennas, which quantifies the amount of EM radiation a human body can safely withstand without any health hazards and is defined as the power absorbed per unit mass of tissue [232]. The new miniaturized cavity-backed substrate integrated waveguide (SIW) textile antenna operating at 2.45 GHz was reported earlier [122]. In this work, the effects of bending the antenna were considered when calculating the reflection coefficient, gains of 5.28 and 5.35 dBi, and efficiencies of 73% and 74.3% in free space and on-body measurement which is appropriate for flexible wearable applications. SAR values for thigh, upper arm, and chest were found to be 0.297, 0.358, and 0.380 W/kg, respectively, which were less than 2 W/kg, which is the European limit. A low-cost wideband textile antenna based on the SIW technology on a woolen felt was presented in a previous article [120].