Calibration of specific absorption rate (SAR) probes in waveguide at 900 MHz

The radiation safety tests for hand-held mobile phones require precise calibration of the small electric field probes used for the measurement of specific absorption rate (SAR) in phantoms simulating the human body. In this study, a calibration system based on a rectangular waveguide was developed for SAR calibrations at 900 MHz. The cross-sectional dimensions of the waveguide are a=190 mm and b=140 mm. The waveguide is loaded with a rectangular liquid slab where the dielectric parameters of the medium simulate human muscle and brain. The precise SAR reference is derived from the temperature rise during a short-term (10-15 s) microwave heating of the lossy slab by measuring with sensitive thermistor-type probes equipped with highly resistive lines. The thermistor probes are calibrated against a calibrated mercury thermometer. To improve the uniformity of the electric field at the calibration position, the thickness of the tissue equivalent slab was varied to adjust the standing wave pattern. This resulted in an almost threefold reduction in the positioning error of the E-field probe. The absolute uncertainty of the calibration is estimated to be ±5% (2σ) not including the uncertainty of the conductivity. The difference between the thermally measured SAR and a value computed with the FDTD method was well within this limit of uncertainty. This kind of closed waveguide system is more compact and requires less microwave power than open field calibration systems. Moreover, no radio-frequency interference is generated