Parameters and Factors Affecting Reliability and Accuracy in Measuring Electro-Textile Conductivity Using Transmission Line Method

Designing and analyzing a textile-based antenna requires precise representation of the material in electromagnetic solver. Due to its complex and inhomogeneous thread structure, the exact cross-sectional structure of the conductive textile segment was very difficult to be modeled, and thus the representation was simplified to bulk conductivity characteristic. Incorrect conductivity value will cause significant deterioration in antenna impedance and radiation performance. Therefore, the conductivity for an electro-textile material used for a wearable antenna design shall be accurately measured. Previously, a transmission line technique was proposed, however, the reliability of the method in correlation to operational frequency and other test conditions were not clarified. Furthermore, the practicality of that technique and the proposed equation for an actual electro-textile sample was not demonstrated. In this paper, the validity of the method over wide range of frequencies from 500 MHz to 5GHz was validated through a more practical and simplified equation derived from the two-port transmission line concept. The accuracy of the proposed technique and equation was demonstrated through comprehensive electromagnetic simulation, measurement and analysis. For validation, strip line samples that consist of an off-the shelf conductive fabric called SHIELDIT were fabricated on two low-loss substrates with slightly different dielectric characteristics; Rogers RO5880 and Chukoh CGP-500A at 500 MHz, 2.45 GHz, 3.5 GHz and 5 GHz. For comparison, the applicability of the technique for other samples such as pure laminated copper and self-manufactured electro-textile was also tested as reference. Through analysis, the reliability of the technique for different material’s loss characteristics, material’s conductivity range and operational frequencies was compared through simulation and measurement. The threshold level for conductivity of the test sample was found to be less than 10
⁷
S/m, which has been validated from 0.5 GHz until up to 5 GHz as shown by the measurement results in this paper.