The Characterization of Conductive Textile Materials Intended for Radio Frequency Applications

Antennas constructed in part from conductive textile materials (also known as e-textiles) by means of standard textile manufacturing techniques are currently receiving increasing attention from antenna theorists and antenna manufacturers alike. However, due mostly to the unique fabrication methods employed, these novel materials cannot be treated as simple, equivalent substitutes for the more-conventional metallic antennas. Conductive yarns can have considerably less-than-ideal conductivity, and their inhomogeneous internal structure, with features small with respect to the skin depth, can be difficult to analyze directly in terms of conductive-material bulk resistivity. Furthermore, the undulating and sometimes non-planar nature of stitched or woven conductive textile yarns introduces a significant phase delay that must be properly taken into account. This article describes a method to determine the conductivity, sigma , which accurately represents a lossy inhomogeneous textile conductor for a MoM segment having the same radius as the actual conductive yarn. This method has three steps. First, the resistance per unit length of the textile conductor is determined experimentally, in a transmission-line test cell. Next, this measured resistance per unit length is adjusted to account for the nonuniform current distribution across the multiple yarn conductors. Finally, a surface-impedance formulation is employed to derive an equivalent MoM-segment bulk conductivity that accurately represents the measured conductor's performance. Excess phase delay, inherent in textile conductors, is determined by examination of the phase component of the test cell scattering parameter, S₂₁.