Time dependence of dielectric properties of human body phantoms

Standard methods for measurement of the specific absorption rate (SAR) including recipes of tissue-equivalent dielectric liquid have been discussed by international standard organisations in order to evaluate mobile telecommunication equipment. Standards, and draft documents of standards recommend glycol type liquids as issue-equivalent liquids for frequencies above 1 GHz. Although the amounts of the ingredients are specified in recipes, possible difficulties such as stability of dielectric properties are not clearly described. We measured the time dependence of the relative permittivities and conductivities of glycol-based dielectric liquids, and discussed their influence on SAR. The experimental results suggest that relative permittivities decreased with time and conductivities were almost constant. SAR, however, was not affected significantly by the change in relative permittivities.