Electromagnetic dosimetry in a Crawford cell: 225 to 500 MHz.

A Crawford cell exposure chamber (main section 30 cm square) was used for electromagnetic dosimetry in the 225 to 500 MHz frequency range. Electric field patterns were measured within the cell. No higher-order modes were observed at any of the frequencies studied below 500 MHz, but significant field perturbations were seen at 2450 MHz. Energy absorption in a prolate spheroidal hamster phantom and in saline-filled spheres of various sizes was measured. The normalized whole-body specific absorption rate (SAR) in watts/kg per mW/cm2 increased from 0.11 (at 225 MHz) to 0.42 (at 450 MHz) for the hamster phantom in E polarization, from 0.06 (at 325 MHz) to 0.20 (at 500 MHz) for K polarization, and from 0.03 (at 325 MHz) to 0.12 (at 500 MHz) for H polarization. Although these values exceed predictions from the long-wavelength far-field approximation for power absorption, the measured SARs were corroborated directly using calorimetry. In addition, calorimetric data obtained using saline-filled spheres suggests that absorbed power in a Crawford cell converges to the far-field plane wave theoretical values only for objects occupying less than 20% of the vertical dimension from the septum to the outer wall.