# Flexible Antennas: A Review
## 8 Performance of Different Types of Flexible Antennas  
An antenna's performance depends on various parameters like conductivity of the radiation element, dielectric substrates, and different design considerations. A highly conductive radiating element ensures superior gain, efficiency, and bandwidth of the antenna. Choosing a suitable dielectric material is critical for antenna performance. Efficiency and gain are reportedly reduced for a higher value of loss tangent of the dielectric substrate [229]. In addition, dielectric permittivity (\(\varepsilon_{r}\)) affects the bandwidth and the resonant frequency of the antenna. An increased permittivity value enables antenna miniaturization with reduced impedance bandwidth and low radiation losses [229]. Substrate thickness is another factor which can influence efficiency, gain, bandwidth, and directivity. For a flexible antenna, it is always a trade-off in choosing the proper substrate considering thickness, performance, and flexibility at the same time. Apart from the abovementioned factors, antenna patch design, array configurations, and power division transmission lines affect the antenna performance a lot. Patch elements come in various shapes like the rectangular, square, circular, annular ring, triangular, pentagonal, and square or circular with perturbed truncations [230]. These shapes affect polarization patterns, resonant frequencies, return loss, gain, and directivity. A computer-aided design (CAD)  
Figure 16: Antenna for wireless capsule endoscopy (WCE) application. (**a**) Antenna Assembly and (**b**) measurement set up for fabricated antenna [228]. Figure 15: Prototypes of antenna for biomedical applications. (**a**) Proposed antenna, (**b**) integrated capsule system, and (**c**) measurement setup of communication system [227]. software is essential in combination with an electromagnetic wave solver to iteratively evaluate the design and the simulated antenna with different radiating material and substrate combinations. Flexibility and bending due to mechanical stress for on-body measurement needs to be assessed to validate any flexible antenna performance in real world situation. Flexible antennas have to undergo mechanical deformation, such as bending or stretching. It degrades the antenna performance like shifting frequency, changing gain and radiation pattern, and changing antenna polarization for the intended application. For realizing IoT effectively, acceptable performance of flexible antennas are necessary. Research is still going on for trying to find creative ways to enhance antenna performance under stress or bending condition. The effect of mechanical stress on a tunable and compact microstrip antenna on a polyimide substrate was investigated by performing bend and stretch tests [23]. The antenna was bent compressively with a minimum radius of curvature of 86 and 150 mm along the x-axis and y-axis, which resulted in a maximum increase of resonant frequency by 3.1% and 1.3%, respectively. Similarly, tensile bending was performed with a minimum radius of curvature of 79 and 162 mm along the x-axis and y-axis, which resulted in a maximum decrease of the resonant frequency by 4.2% and 0.3%, respectively (Figure 17). An overall 0.9% decrease in the resonant frequency was measured for an applied strain of 0.09% while stretching the antenna along the y-axis. Adhesion tests were also performed for checking the adhesion of the antenna with the substrate. In an earlier report [55], the dipole antenna over the artificial magnetic conductor (AMC) structure maintained the reflection coefficient relatively the same regardless of the bending condition or the presence of a lossy host. The antenna's radiation pattern remained broadside under bending condition and on a filled blood bag with 0.7 dBi gain variation. In another study [56], fabricated antenna for body area networks at 2.4 GHz had overcome the human body proximity detuning effect by varying radiator length.