Reliability of Strip Line Method for Determination of Conductivity for Lossy Conductive Materials

Abstract

Designing electro-textile antenna requires complex analysis as the material cannot be simply modeled as a conventional antenna with perfect metallic surface. Electro-textile has relatively lower conductivity compared with an ideal conductor. Due to its inhomogeneous and unique thread structure, exact conductivity, $\sigma _{c}$ , is difficult to be determined, which will result in inaccurate modeling during antenna designing. Significant deterioration in antenna efficiency can be observed when low conductivity element is used, and thus, accurate determination of bulk conductivity for customized electro-textile shall be performed. Previously, a strip-line method was proposed, and however, the reliability of the technique in correlation with material’s loss characteristics was not clarified. Furthermore, the practicality of the technique for higher frequency, such as ISM band, was not demonstrated. In this paper, the validity of the technique for lossless, low-loss, and high-loss materials for 2.45-GHz wearable application is clarified. A more practical and straightforward equation to calculate electro-textile’s bulk conductivity is derived, and the accuracy and reliability range of the equation is demonstrated through comprehensive electromagnetic simulation. Through analysis, the correlation between $\sigma _{c}$ , dielectric loss and attenuation loss is shown. For validation, two samples that consist of two conductors, namely copper and SHIELDIT, are fabricated on a low-loss RO5880 substrate. The $\sigma _{c}$ values of the conductors are calculated from transmission loss (S21) measurement. Good agreement of $\sigma _{c}$ is obtained for both cases, particularly when the actual $\sigma _{c}$ is within the threshold range of around 107 S/m.