# Knitted Microwave Transmission Line for Wearable Electronics
## 1 Introduction
Textile transmission lines were fabricated using conductive yarns and ultrasonic welding technology [21]; however, this technology influences the thickness of the dielectric layers, melting the yarns, and may not be suitable for high-frequency applications. The technology of spacer knitted fabrics, also known as layered knitted fabrics or 3D knitted fabrics, has been known for many years and is widely used in many industries. Products manufactured in this way can be found on a daily basis in the furniture industry (as upholstery for office furniture, sofas, and headboards), automotive industry (seats, covers, filters), footwear (specialized insoles for footwear, upper part of shoes), and clothing (protective clothing, sportswear), but new development opportunities are constantly emerging for manufacturers of this type of knitted fabric [22; 23; 24; 25; 26]. Recently, distance knitted fabrics have been used in wound dressings, compression products, channel mats, and  
Figure 1: Schematic view of the wearable wireless system with textile antenna and textile transmission line. composite products [27; 28; 29; 30]. Three-dimensional knitted products can be made of classic knitting yarns made of staple or filament fibers, natural, artificial or synthetic fibers, threads with special electrically conductive properties, technical threads with increased mechanical or thermal strength, or waterproof or luminescent yarns [27; 28; 29; 30; 31]. Spacer knitted fabrics differ from classic knitted fabrics in that they are mostly made of two layers [31; 32; 33; 34], but there are also known works where a three-layer wett product has been obtained [35]. It is worth emphasizing that these layers are not dependent on each other and are often even separated by a certain distance, defined at the production stage. This distance can be achieved in three ways: connections through stitches, through intermediate yarn, or by scooping. The connections between the decks in 3D knitted fabric are extremely important because they determine the subsequent physical properties of the material. Depending on needs, spacing between the layers of 2 to 65 mm can be achieved [36]. Innovative knitted commodities based on wett distance knitted fabrics are created on special machines called crochet machines; flat crochet machines or cylindrical machines can be used for this purpose [37]. The spacer knitted fabric technology allows for the production of textile materials with unique properties that cannot be obtained by any other methods (low specific gravity, high elasticity, appropriate thermoregulatory properties). It is worth noting that it is possible to design a layered structure in which each layer has different properties; these properties can be completely independent of each other or even divergent (e.g., one side of the material can absorb moisture and the other be hydrophobic) [38; 39]. In one preparation stage, a material is made from three or more coils, which makes the manufacturing process itself relatively cheap and simple compared to other methods of creating spatial textiles. A single-process fabrication method for textile transmission lines offers significant advantages for wearable electronics. By seamlessly integrating conductive and insulating elements within a textile structure, this approach enhances mechanical integrity, electrical performance, and scalability. This leads to improved durability, consistent impedance matching, reduced interference, and efficient production. Furthermore, the resulting devices are highly flexible, lightweight, and comfortable to wear. This enables the creation of complex and multifunctional wearable electronic systems, such as smart clothing, health monitoring devices, and interactive garments. By seamlessly integrating transmission lines into clothing, we can create truly functional and stylish wearable devices that blend seamlessly into our everyday lives. In this paper, a microwave transmission line is presented that was fabricated with knitting technology.