# Knitted Microwave Transmission Line for Wearable Electronics
## 4 Discussion  
Based on the analysis of the measurement results for the prototype line, it can be concluded that the textile technology that was used in this research enabled the fabrication of a transmission line whose parameters allow its application in the transmission of high-frequency signals in the 2.4 GHz band. In this frequency range, the line exhibits good impedance, matching a reference impedance of \(Z_{0}=50\ \Omega\). For the center frequency of this band, the value of the \(S_{11}\) parameter does not exceed \(-10\) dB. The developed transmission line section, 115 mm in length, exhibits an acceptable level of signal attenuation in the considered band (i.e., frequencies below 2.5 GHz). The value of the \(S_{21}\) parameter is not less than \(-5\) dB. This may result both from dielectric losses in the textile insulating material and from the limited conductance of the conductive fiber layers. Dielectric loss is a crucial factor affecting the performance of high-frequency transmission lines. Excessive dielectric loss can result in significant signal attenuation and distortion, particularly at higher frequencies. To mitigate these effects, several strategies can be employed, including the careful selection of low-loss textile materials, the optimization of transmission line design, and the utilization of precise fabrication techniques. Previous research has explored various approaches to address dielectric losses in textile-based transmission lines. These include the use of low-loss dielectric materials, the optimization of line geometries, and the development of advanced fabrication techniques. However, at this stage of the research, the transmission line is not optimized in terms of selecting materials with the lowest possible loss. In further research, it will be verified to what extent the selection of fiber material will affect the line attenuation and what effect the parameters of the technological process (such as the density of spacer fibers) have on this phenomenon. In a simplified numerical model of the line, as shown in Figure 7, the attenuation of the line (\(S_{21}\) parameter) depends on the dielectric losses of the insulating material. Assuming a tan (\(\delta\)) value of 0.15, the simulation results were found to be in agreement with the measurement results. This value can be adopted for the design of lines of this type in order to account for energy losses in both the dielectric and in the conductive layers. As shown in Figures 14 and 15, a fairly good agreement between the simulation and measurement results was obtained. The slight discrepancies between them may be  
Figure 19: Measurement results of \(S_{21}\) parameter for bent transmission lines compared with a straight line. attributed to the influence of the open-ended coaxial probe on the measurement result. This point introduces a discontinuity in the measurement setup structure and can be a source of additional errors. Nevertheless, the overall good agreement between the measurements and simulations indicates that the proposed method allows for the effective determination of material parameters of a complex structure such as spacer fabric. The simplification of the composition of fibers and air to a homogeneous material characterized by permittivity and dielectric loss appears to be a sufficient approximation for engineering applications. Figures 18 and 19 demonstrate that bending the transmission line with a radius of curvature of 100 mm has a negligible impact on its electrical performance. Interestingly, when using a convex fixture, a minor improvement in impedance matching and a reduction in attenuation were observed at 2.4 GHz. This phenomenon could be attributed to a slight reduction in the thickness of the dielectric substrate. These results indicate that further optimization of the transmission line design by varying the dielectric substrate thickness may yield beneficial outcomes.