## 3 Results and Discussion
### Evaluation of Button Sensor Antenna: Simulated Results
#### 3.1.5 Bending Effects  
The various possible bending conditions that we considered are depicted in Figure 16. When the textile substrate is bent or comes into contact with body tissue, it must maintain its resilience. Depending on the fabric's thickness, the resonance frequency may be moved due to the varying distance from the body tissue layers. Three bending fixtures were employed in this part to study: (a) free space, (b) on the flat phantom model, and (c) on the cylindrical phantom model. The results are compared with those from the flat condition. Figure 17 shows the comparisons of S\({}_{11}\) at various bending radii. The lower band (half wavelength) shifted from 2.45 to 2.345 GHz, whereas the higher band (quarter wavelength) does not show much variation in the frequency band. The frequency band is from 2.350 GHz to 2.45 GHz and 5.0 GHz to 6.0 GHz when the bending diameter (D) is 50 mm, resulting in small changes in the resonance frequency.  
Figure 16: Study cases of the effect of bending of the BSA. (**a**) free space situation, (**b**) flat layered chest phantom (standard size, \(200\times 200\times 50\) mm\({}^{3}\)), and (**c**) arm cylindrical phantom (50 mm radius and 150 mm length).  
Figure 15: Reflection coefficient (S\({}_{11}\)) for the proposed BSA design covering situations using various types of flexible and fabric substrate materials.  
As illustrated in Figure 18, the radiation patterns in both bands are investigated under various bending conditions. The results revealed that the radiation patterns are practically impervious to structural deformation. The two cases of the design bent on the chest and arm phantoms were investigated. The shift in resonance frequencies and bandwidths is smaller in the case of bending in free space than when the antenna is bent on the chest and arm body phantoms with different sizes. Nonetheless, the BSA operates in the appropriate frequency bands. Therefore, we can conclude that the BSA has robustness against a bending radius of \(\mathrm{R}=50\) mm. Please note that we did not include any stretching in the simulations.  
For a substrate with a thickness (h) of approximately 1 mm, the compression of the substrate is around 1% for a bending radius (R) of 35 mm. We consider the bending for a thin substrate; therefore, neglecting the substrate compression is an acceptable approximation [64]. This topology ensures robustness and minimum effects due to bending, stretching, and wet situation. The proposed BSA design was simulated in this research, and performance tests on body bending models were conducted to confirm the design system performance. Therefore, the design performance cannot be affected by the human body, nor can the human body be affected by the design. The antenna's performance is minimally affected by bending, movement, and fabrication tolerances. Therefore, the proposed design shows robustness.  
Figure 17: Simulated \(\mathrm{S}_{11}\) for different bending cases. Chest, normal size \(100\times 100\times 50\) mm\({}^{3}\), and large size \(200\times 200\times 50\) mm\({}^{3}\); arm, small size 50 mm diameter, length (\(l\)), 100 mm, and large size 100 mm diameter, length (\(l\)), 150 mm.