## 3 Measurement Results
### Reflection Coefficient (S\({}_{11}\)) and Bandwidth  
Figure 6 shows the measured S\({}_{11}\) for the antenna in three cases: the free-space, placed on the chest, and on the arm. Measured and simulated results agree well. The antenna showed good matching at both bands, and S\({}_{11}\) in both bands was lower than \(-\)10 dB. The antenna was measured while it was placed on two subjects with different weights (65 kg and 110 kg) and showed performance similar to the free-space at both bands. Initially, the antenna was relatively flat on the body. The flatness and air and textile layers between the body and antenna provided similar conditions to a free-space scenario. Therefore, the body size had a modest effect on the antenna's performance. The additional denim layer under the ground created better isolation from the body; as a result, there was less effect of back radiation, which is radiation toward the body tissues. The air-gap was chosen to be 10 mm. Similarly, the antenna was placed around the arm (with a radius of 50 mm and 100 mm) maintaining the same air-gap; the impedance bandwidth was not much affected due to the reduced coupling effects between them. Two other conditions that might occur are the bending of the antenna and body humidity (sweating) causing moisture to penetrate the antenna. In the cases, when the antenna was placed on the chest or arm, the antenna still covered the WLAN band without any considerable changes in the levels of S\({}_{11}\). The performance of the antenna under bending conditions was experimentally studied in different locations. The antenna was measured in free-space and on polystyrene cylinders with a dielectric constant of \(\varepsilon_{\mathrm{r}}\approx 1\), for cylinders with various diameters (D in mm) of 100, 80, 70, 60, and 50. Different diameters gave unique antenna curvatures to test whether the desired operating frequency range was maintained under bending conditions. For all cases, there was a minor shift in the center frequency. Overall, the measurement results show that the antenna works within the required band without any considerable frequency detuning. The measured S\({}_{11}\) values at different bends are shown in Figure 7.  
Wearable devices are often affected by the humidity of the body. We analyzed the case where the proposed antenna was dampened by moisture by applying water to the textile substrate until it was saturated, as shown in Figure 8. The resulting S\({}_{11}\) was severely affected by the wet condition. After one-hour, further changes in the results occurred due to the evaporation of water from the whole substrate. After 2 h, the measured S\({}_{11}\) results were similar to those measured after one hour, which showed that the evaporation was completed after one hour. As more time passed and the antenna slowly dried and cooled back to the room temperature of 26 \({}^{\circ}\)C along with an air humidity of 30%, the measured S\({}_{11}\) values were similar to those measured before adding the moisture.  
Figure 8: Measured S\({}_{11}\) of the MMAs in wet conditions (Ref (free-space), initial condition, after 1 h, 2 h, and 3 h).  
Figure 6: Measured S\({}_{11}\) of the MMAs in free-space and on different locations of the human body (chest, 50 mm; small size arm, 50 mm diameter; and large size arm, 100 mm diameter).  
Figure 7: Measured S\({}_{11}\) of the MMAs deformed on various cylindrical curvatures (d = 50 mm, 60 mm, 70 mm, 80 mm, and 100 mm).