Environmental High Frequency Characterization of Fabrics Based on a Novel Surrogate Modelling Antenna Technique

Wearable antennas are mostly constructed from fabric or foam, whereas e-textiles are often used as conductive parts. A design obstacle is the lack of knowledge about the electromagnetic properties of these materials. Moreover, most of these fabrics exhibit electromagnetic properties that depend on prevailing atmospheric conditions. In this work, we present a dedicated characterization method to determine the complex permittivity of fabrics or foams, as well as the effective conductivity of e-textiles, and this as a function of relative humidity. The method extracts the constitutive parameters by comparing measured and simulated antenna figures of merit such as input impedance and antenna efficiency. This inverse problem is solved using a surrogate-based optimization technique as implemented in the Surrogate Modeling Toolbox, yielding a fast and accurate characterization. The method is evaluated by characterizing six materials which are exposed to relative humidity levels ranging from 10% to 90%. From the extracted complex permittivities of the six materials, two-phase dielectric mixing models based on the volumetric fractions of the absorbed moisture in the substrates are developed and evaluated in terms of accuracy. For the materials exhibiting a high sensitivity to moisture, the model is observed to be less accurate. However, the worst model accuracy is shown to be comparable with the estimated accuracy of the characterization procedure. For materials with low sensitivity to moisture, the model fits the measured values very well.