# Knitted Microwave Transmission Line for Wearable Electronics
## 2 Materials and Methods
### Microstrip Transmission Line Fabrication Technology  
To fabricate the knitted transmission line, weft spacer knitted fabric technology was selected, as it provides the possibility of making the product using a small amount of raw material, directly from the coil, bypassing the processes of yarn preparation before production, such as rewinding or weaving. A model of 3D werf knitted fabric is made of two outer layers made with a basic weave (usually shaker stitch), bonded with a monofilament thread (separates the outer layers). The monofilament thread does not form loops: it is knitted in the form of take-up, keeping the outer layers at a distance from each other. The construction of the classic spacer knitted fabric is shown in Figure 8.  
The microstrip transmission line was made using spacer weft knitting technology on a Mayer & Cie cylindrical crochet machine (Mayer & Cie, Albstadt, Germany), type OVJA 36. The machine had a needle number of 20 (E20), a cylinder with a diameter of 30 inches, and a needle pitch of 1.27 mm (Figure 9).  
Figure 8: Spacer knitted fabric structure.  
Figure 9: The Mayer & Co knitting machine used in the work to create a transmission line.  
This knitted fabric consists of two outer layers made of basic weaves and an inner layer made of sections of threads running between the outer layers of the knitted fabric.  
The outer layers of the knitted fabric are created with a plain stitch, using two types of yarns: Shieldex Z100 HB yarn (Shieldex, Bremen, Germany) with a linear mass of 117/17 dtex (transmission path and mass plane) and polypropylene monofilament with a diameter of 0.08 mm (the remaining area of the upper layer of the knitted fabric surrounding the electroconductive path). The inner layer, called the spacer, is made of monofilament polypropylene thread. The selection of materials for the textile transmission line was primarily influenced by the capabilities of the crochet machine. This machine, typically used for producing flat structures, was adapted to create 3D structures by adjusting the cylinder position. The needle count of the machine determined the suitable yarn diameter, with 0.08 mm threads proving optimal for smooth operation and precise stitch formation. Extensive experimentation with different yarn types revealed that this particular diameter minimized yarn stress, preventing errors in the knitted fabric. While the type of yarn did not significantly affect production capacity, factors such as machine speed and weave pattern influenced the overall performance.  
All the component weaves forming the spacer knitted fabric are shown in Figure 10, while Figure 11 shows the produced sample taken using an OPTA-TECH SN series stereoscopic microscope (OPTA-TECH, London, UK) in cooperation with the Opta View Version 4.3.0.6001 software. A spatial knitted fabric in the form of the so-called sleeve was obtained. The parameters of the knitted fabric forming the transmission line are presented in Table 2.  
\begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline Fabric Thickness (mm) & 0.47 \\ Surface Mass, Mp (g/m\({}^{2}\)) & 423 \\ Course Density, Pr (course/10 cm) & 202 \\ Wale Density, Pk (wales/10 cm) & 94 \\ Loop Shape Factor, C & 0.46 \\ \hline \end{tabular}
\end{table}
Table 2: Parameters of the knitted fabric forming the transmission line.  
Figure 10: Spacer knitted weaves: (**a,b**) plain stitch; (**c**) monofilament providing distance (not forming the knitted mesh).