## 3 Results and Discussion
### Evaluation of Button Sensor Antenna: Simulated Results
#### 3.1.1 Button Sensor Antenna Feed Locations  
Four feeding positions on the chassis were examined: the center, the long edge, the short edge, and the corner. A ground plane (45 mm \(\times\) 45 mm) is used. The lower frequency range is served by the imaginary part of the input impedance (Xin) in a button sensor antenna. When the resistance (Rin) is low, it signifies that the design is difficult to match to a 50 \(\Omega\) input impedance and, as a result, the antenna cannot efficiently radiate. When the button sensor antenna's feed point is placed at the center, short edge, or corner of the ground plane, its lower (upper) resonance frequency moves to 2.61 (6.20) GHz, 2.42 (6.00) GHz, 2.31 (7.00) GHz, respectively. In all cases, the BSA is not matched at 5.6 GHz. The resistive part of the input impedance is 26.92 (5.00) \(\Omega\), 42.32 (13.00) \(\Omega\), and 11.00 (29.00) \(\Omega\), respectively. The button sensor antenna's resonance is moved to 2.45 GHz when the feed point is located at the long edge. In this location, the input resistance is 51.12 (47.42) \(\Omega\), at the 2.45 GHz lower and 5.6 GHz higher bands, respectively. The long-edge location provided S11 bandwidth (BW) lower than \(-\)10 dB of 2.42 GHz to 2.47 GHz and 5.37 GHz; these were called the lower and upper bands. The coupling between the top button and the textile ground causes changes in input impedance for different positions. The long-edge location produces a strong ground coupling. In this case, the ground plane works like a radiator, increasing the resistive part of the input impedance. Furthermore, the simulation shows that the x- (horizontal) and y- (vertical) components of currents are out of phase at the center, short edge, and corner. These components cancel each other and do not contribute much to the improvement of the antenna's impedance bandwidth. On the other hand, the x- (horizontal) and y- (vertical) components of the surface current are in phase in the long-edge corner, creating a strong radiated field.  
Figure 7a depicts the surface current at the lower band that follows the A-B-C path (\(l_{1}\)). It shows that the current path is half a wavelength at 2.4 GHz, resulting in an omni-directional radiation pattern. Figure 7b depicts the current route at the higher band that follows the A-B-D pattern (\(l_{2}\)). This current takes a 1/4 wavelength path at 5.6 GHz. It also shows a significant amount of current on the ground plane, which will perform as a radiator due to shorting pin. Therefore, the modes depend on the top patch, bottom patch, and shorting pin. Figure 8 depicts the simulated reflection coefficients (S11) of the button sensor antenna (BSA), as well as the impact of feeding locations on the S11. It can be seen how changing the button sensor antenna's feeding point has a considerable impact on the impedance matching level and resonant frequency bands. Due to the presence of the lossytissues, the radiation pattern shows a wider beam with a wider bandwidth for the on-body case. As illustrated in Figure 9, omnidirectional radiation patterns are obtained.  
Figure 8: The reflection coefficient (S\({}_{11}\)) of the BSA for different feed locations (lower band markers are at 2.22 to 2.8 GHz; upper band markers are at 5.0 to 5.9 GHz).  
Figure 7: (**a**) Surface current distribution (left side) and magnitude (right side) at lower band 2.45; (**b**) Surface current distribution (left side), and magnitude (right side) at the upper bands at 5.6 GHz.