# Modeling and Analysis of Wearable Antennas
## 3 Flexible Transmission Line  
Although the sheet-like transmission line can be produced by a dielectric substrate, it can be attached to clothes by using a material with a flexible structure. Figure 7 shows the transmission characteristics when a sheet-like transmission line is constructed using a conductive cloth composed of copper and nickel. It can be seen that the characteristics of the two types of conductive cloths having different compositions are almost equal to each other, as shown in Table 1. The conductive cloth has been fabricated using wrapping machines where the wearable patches were assembled on a flexible sheet which can easily bend with the fabric. The resultant fabric exhibited full metal coverage with good adhesion to the fibers, as well as being conductive and extremely flexible with little effect on its handle and drape properties. In order to make a conductive fabric, cotton was first mercerized followed by immersion in poly (diallyl dimethyl ammonium chloride) solution. Silver nitrate was then reduced on the surface of the fabric which allowed the formation of silver nanoparticles. However, there is a difference between the frequency and the transmission line which is formed in foam form using the copper foil. Since the transmission line composed of the conductive cloth has a flexible structure, it is possible to bend or fold the transmission line. Figure 6: Power transmission by one-dimensional transmission line. Figure 8 shows the transmission characteristics when the flexible transmission line is bent. Degradation of transmission characteristics due to bending hardly occurs. Next, a vast-like waveguide that can be worn on a human body is created using this flexible transmission line. As shown in Figure 9, the ground plate and upper- and lower-layer resonators are formed by conductive cloth, and a vast is created with a structure sandwiched between foams. By using this waist cover, it is possible to efficiently perform inter-device communication such as sensors installed near the human body. In particular, when a wireless device is placed in front of and behind the human body, it is difficult to maintain a communication line because the human body exists as a medium with a large loss, but by using this vast (called a smart suit). This situation can be improved. Figure 10 shows the transmission characteristics between the half-wave dipoles placed in front of and behind the human body via the smart suit and the transmission characteristics when both of the two dipoles are arranged on the entire human body. As shown in Figure 10a, when a half wavelength dipole is arranged in front of and back of the human body, the S21 characteristic has a transmission characteristic of about \(-70\)dB or less, which is about a noise level. Also, the transmission characteristics when a suit is worn on the human body, and the effectiveness of the smart suit are shown. When the smart suit is worn on the back and front of the human body, then the S21 characteristics have transmission characteristics of about \(-25\)dB up to \(-55\)dB. This means that the smart suit improves the loss by \(45\) dB or more as compared with the case when only the antenna is placed on the human body. Figure 10b shows transmission characteristics when the transmitting/receiving antenna is placed on the entire surface of the human body. However, even in this case, the transmission loss is improved by \(20\) dB or more when a smart suit is used, and its effect is obvious. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Symbol**} & **Thickness** & **Electrical** & **Thermal** & **Mass per Unit** & **Elastic** \\  & **(mm)** & **Conductivity at** & **Conductivity at** & **Surface (g/m2)** & **Modulus** \\  & **25 \({}^{\circ}\)C (m/\(\Omega\)-mm2)** & **25 \({}^{\circ}\)C W/(m\(\cdot\)K)** & **Surface (kN/mm2)** \\ \hline Cu & 0.06 & 6.75 & 49 & 55.87 & 145 \\ ST2050 & 0.06 & 6.73 & 45 & 54.31 & 146 \\ SC8100 & 0.04 & 6.81 & 51 & 52.01 & 142 \\ \hline \hline \end{tabular}
\end{table}