# Knitted Microwave Transmission Line for Wearable Electronics
## 3 Results  
The technology described above enabled the fabrication of a prototype textile microstrip line. A visual representation of the prototype line is provided in Figure 12. This prototype was subsequently employed in the next phase of research to conduct measurements of the transmission line's characteristic parameters. For this purpose, a measurement setup, illustrated in Figure 13, was utilized. This comprised a vector network analyzer (Anritsu MS4647B, Atsugi-shi, Japan) and measurement probes in the form of open-ended coaxial cables (Huber+Suhner Multiflex 86, Herisau, Switzerland), which were connected to the transmission line for testing. Prior to the measurements, the analyzer was calibrated, and the calibration plane was shifted to the connection point between the coaxial probe and the tested line.  
Using the measurement setup described above, the parameters characterizing the textile transmission line were measured. Figure 14 presents the measurement results of the \(S_{11}\) parameter module (in red). The corresponding simulation result (in blue), which exhibited the closest match, is superimposed on the measurement data. The best agreement was obtained for simulations with a substrate relative permittivity of \(\varepsilon_{r}=1.5\) and a dielectric loss tangent of tan (\(\delta\)) = 0.15. Figure 15 shows the measurement results for the \(S_{21}\) parameter of the prototype line. In this case, as well, the closest simulation result, obtained for the same material parameters as for the \(S_{11}\) parameter, was superimposed on the measurement data.  
Figure 11: The knitted fabric: (**a**) top view showing the transmission path weave (top of the sample); (**b**) cross-section; (**c**) view of the bottom layer (ground plane).  
The transmission line presented in our paper is intended for wearable applications, where it may be subjected to bending due to the curvature of the human body. While twisting is less likely, bending along the longitudinal axis is a significant concern. To evaluate this, we conducted bending tests on a cylindrical surface with a 100 mm radius. The experimental setups for convex and concave bending are depicted in Figures 16 and 17. A Styrofoam form was utilized to introduce deformation into the prototype textile line. This choice of material was based on the need to minimize measurement artifacts. Styrofoam's electrical properties (at 3 GHz, \(\varepsilon_{r}\) = 1.03, tan (\(\delta\)) = 0.0001 [40]), being comparable to those of air, ensured that the fixture would have a negligible impact on the measured parameters. The results of these measurements, specifically the impact on the \(S_{11}\) and \(S_{21}\) parameters, are presented in Figures 18 and 19.  
Figure 12: Prototype of the knitted transmission line: (a) top side; (b) bottom side (ground plane).  
Figure 13: Measurement setup for transmission line characterization.  
Air permeability is a crucial characteristic of wearable devices as it ensures user comfort. The developed prototype line underwent air permeability testing. The air permeability of the knitted structure shown in this work gave an average result of 861 dm\({}^{3}\)/(m\({}^{2}\).s). This is similar to other knitted fabrics of this kind. Several knitted fabrics with similar structural parameters (tightness, surface mass, thickness) made of polyester and cotton yarns were analyzed, and average results were obtained for them in the range of 814-901 dm\({}^{3}\)/(m\({}^{2}\).s).  
Figure 14: \(S_{11}\) obtained from measurements of prototype line compared with results of simulations for \(\varepsilon_{r}=1.5\) and \(\mathrm{tg}(\delta)=1.5\).  
Figure 15: \(S_{21}\) obtained from measurements of prototype line compared with results of simulations for \(\varepsilon_{r}=1.5\) and \(\mathrm{tg}(\delta)=1.5\).  
Figure 16: The experimental setup for convex bending.  
Figure 17: The experimental setup for concave bending.  
Figure 18: Measurement results of \(S_{11}\) parameter for bent transmission lines compared with a straight line.