## 2 Substrate Materials for Wearable Antennas  
The substrate material selection for a wearable antenna design is of crucial importance. The substrate selection requires a low loss material so as to have better chances of increased antenna efficiency when placed on the body. The work in [2] where a Planar Inverted F-Antenna (PIFA) was fabricated out of a foam sheet spacer as the substrate and copper plated nylon for conductive sections states that a low loss conductive fabric is critical to the performance of a textile antenna. For wearable textile antennas the substrate material is required to be flexible. Textile substrate dielectric properties vary with the choice of material and the frequency. The accurate characterization of the properties of a textile substrate is fundamental before designing a textile antenna. A summary of the properties that various researchers use for antenna design are outlined in Table 1. \begin{table}
\begin{tabular}{c c c c c} \hline
**Material** & **Permittivity (\(\varepsilon_{x}\))** & **Loss Tangent (tan\(\delta\))** & **Frequency (GHz)** & **Reference** \\ \hline Denim & 1.40 & & 2.4 \& 5.2 & [3] \\ Denim & 1.40 & & 0.9 \& 1.8 & [4] \\ Leather & 2.95 & 0.16 & 0.9 \& 1.8 & \\ Denim & 1.70 & & 3–12 & [5] \\ Denim & 1.80 & 0.07 & 2.4 \& 5 & [6] \\ Velcro & 1.34 & 0.006 & 2.4 \& 5 & \\ Denim & 1.8–2.0 & 0.014 & 14–40 & [7] \\ Denim (black) & 1.8 & 0.07 & 3.3 \& 5 & [8] \\ Velcro & 1.37 & & 2.4 \& 5 & [9] \\ \hline Felt & 1.38 & 0.023 & & & \\ Fleece & 1.17 & 0.0035 & & & \\ Moleskin & 1.45 & 0.05 & & & \\ Panama & 2.12 & 0.018 & & 2.6–3.95 & [10] \\ Silk & 1.75 & 0.012 & & & \\ Tween & 1.69 & 0.0084 & & & \\ Perspex & 2.57 & 0.008 & & & \\ PTFE & 2.05 & 0.0017 & & & \\ \hline \end{tabular}
\end{table} 
Table 1: Substrates’ dielectric properties. From the above table it is clear that textiles typically have a low relative permittivity. The work in [12] studied the effect of different textile materials on wearable antenna performance. A patch antenna for Global Positioning System (GPS) operation was designed and five antennas were fabricated [12]; all of them used copper tape for the conductive sections. The different textile materials used for the substrate were: fleece (thickness, \(t\) = 4 mm), upholstery fabric (\(t\) = 1.1 mm), vellux (\(t\) = 5 mm), synthetic felt (\(t\) = 4 mm) and cordura (\(t\) = 0.5 mm). Non-conductive textile materials have low relative permittivity which reduces the surface waves (for patch antennas) and improves antenna impedance bandwidth. The measured dielectric constant of these textile materials at 1.575 GHz was between 1.1 and 1.7. Therefore, fabric substrate antennas are somewhat larger in physical dimensions than rigid antennas due to the low value of permittivity. Cordura is made by Delcotex, Germany. The name of the fabric is Delinova 200, which is woven from Cordura fibers. Cordura is a polyamide fiber with a nylon and Gore-Tex membrane. Delinova 200 is a textile impregnated with fluorocarbon and coated with polyurethane. Delinova 200--Cordura fabric proved to be an interesting fabric for textile antennas because of its strength, constant thickness and high water resistance properties [1,12]. The effect of using different textile materials as the substrate of a patch antenna was studied in [13]. Four antennas were designed and fabricated. The different textiles that were used for antenna fabrication are: wash cotton, curtain cotton, polyester and polycot. The polyester patch antenna yielded the best antenna performance in terms of gain and efficiency (polyester patch antenna yielded measured gain equal to 9.6 dBi compared to 7.2-7.5 dBi with the other antennas) [13]. The polyester had the lowest loss tangent compared to the other three textiles that were used for antenna fabrication. Additionally, as shown in [14,15] fleece fabric provides sufficient thickness for an adequate bandwidth.