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

Abstract

Abstract-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.

Wearable Antennas for Off-Body Radio Links at VHF and UHF Bands: Challenges, the state of the art, and future trends below 1 GHz

“…This is due to the fabrication (knitting, weaving) and assembling (additional glues or adhesive sheets) processes applied during construction. An accurate prediction of the wearable antenna performance might require simultaneous characterization of all relevant conductive and non-conductive materials by means of test structures that are fabricated in an identical way as the wearable antenna [17]. An additional complication, resulting from the conventional textile production process, such as knitting, weaving and embroidering, is formed by the typical inhomogeneity, anisotropy and instability (in terms of geometry and environmental conditions) of the fabric's material properties [27] [44].…”

“…An additional complication, resulting from the conventional textile production process, such as knitting, weaving and embroidering, is formed by the typical inhomogeneity, anisotropy and instability (in terms of geometry and environmental conditions) of the fabric's material properties [27] [44]. A textile fabric may be described as a mixture of fibers, air and water molecules [17] [18]. The characterization procedure and subsequent wearable antenna design process will have to cope with these effects.…”

“…Yet, they typically only characterize out-of-plane components, which are the most relevant ones when considering microstrip topologies and when neglecting fringing fields [28]. The wideband material properties found by non-resonant methods may be used as initial values in a second more accurate resonancebased procedure, where narrowband patch antennas are constructed that resonate close to the frequency band of interest [17], [22]. These structures also enable to accurately quantify the effects of relative humidity or moisture [16], [19], [26], [35], of compression or strain [29], and of repeated washing cycles on the material properties [32]- [34], [36].…”

Dual-Band Substrate Integrated Waveguide Textile Antenna With Integrated Solar Harvester

“…The proposed antenna is fabricated using pure copper polyester Taffeta, with a surface resistivity R s = 0.18 Ω/sq [14], as top and bottom layer. A 3.94mm thick closed cell expanded rubber protective foam with ε r =1.495 and tanδ =0.016 at 2.45 GHz serves as substrate [14].…”

“…A 3.94mm thick closed cell expanded rubber protective foam with ε r =1.495 and tanδ =0.016 at 2.45 GHz serves as substrate [14]. After gluing the two conductive fabrics to the substrate with thermally activated adhesive sheets, the backed cavity is constructed by a row of equally spaced flat flange copper tube eyelets along each side of the cavity.…”