An Accurate Probe for Mapping Strong HF Magnetic Fields

Design, calibration, and performance of an HF magnetic field probe based on photoammeter principles are discussed. The probe is minimally field perturbing, measures an average value of magnetic field over a 2 cm2 area, has precision to ±1 percent of full scale values, and has an accuracy of ±5 percent. The probe can measure orthogonal components of a field, and in that sense is a vector probe. Relative phases of orthogonal field components are not measured, however. The probe is useful in measuring and mapping relatively intense magnetic fields of known frequency (e.g., 80 A/m at 13.56 MHz) such as those used in hyperthermia for cancer treatment with magnetic induction techniques. Accurate magnetic field measurements permit a determination of the absorbed power densities that result in tissue heating.