A graded-mesh FDTD code for the study of human exposure to cellular phones equipped with helical antennas

A FDTD code employing a graded mesh has been developed in order to make the FDTD technique suitable for studying cellular phones equipped with helical antennas. The graded-mesh FDTD allows the evaluation of both the radiating properties of the helical antenna, and the power absorption inside the user's head. First, a canonical case has been studied to find the optimal values for the grading factor "q" and the best model for approximating the circular wire section. It has been found that "q" values up to 3 can be used without introducing significant errors in the field solution, and that a good model for the wire circular section is a five-cell cross. Then, the free space radiating properties of a phone equipped with a helical antenna (radiation impedance, radiation pattern, etc) have been evaluated, and compared with those predicted by MoM, showing good agreement. Finally, the power absorption in an anatomical model of the human head has been computed for a radiated power of 250 mW at the frequency of 835 MHz. Peak SAR values of 0.98 W/kg averaged over 1 g and 0.63 W/kg averaged over 10 g have been obtained with the phone held in contact with the ear.