Attenuation Characteristics of the SAR in a COST244 Phantom with Different EM Source Locations and Sizes

where E [V/m] is the amplitude of effective value of the electric field, σ [S/m] and ρ [kg/m] are the electric conductivity and the density of human tissue, respectively. In general, an approximation of a plane wave for the electromagnetic field has been applied to E in eq. (1) in order to represent the SAR characteristics in the internal parts of the head [1], [4]. For example, attenuation characteristics of the SAR inside a phantom are determined from an empirical formula in [1]. However, the electric field radiated from the antenna is considered to be a spherical or a cylindrical wave since a mobile phone is used close adjacent to the human head. The authors have proposed a new measurement system of the SAR that makes use of a noble algorithm that predicts the SAR from the measurement of the magnetic field distribution in the vicinity of a cellular radio in free space [5]. In this procedure, the average SAR is calculated by the 3-dimensional SAR distribution in a cubic shape that is estimated from the SAR on a phantom surface using an attenuation constant of the liquid in the phantom. Thus, an accurate evaluation of the attenuation constant is essential for this measurement system. This paper presents a formula to provide more accurate attenuation characteristics of the SAR, which takes a spatial attenuation factor of the electromagnetic field of a spherical or a cylindrical wave into consideration. Attenuation characteristics of the SAR in a COST244 phantom that simulates the human head are evaluated with different electromagnetic source locations and sizes using the FDTD technique. From this, some physical pictures regarding the attenuation characteristics in a lossy media of the COST244 phantom are clearly understood. The analysis shows that excellent agreement between predictions using the formula and the calculated quantities is obtained.