INDUCED FOOT-CURRENTS IN HUMANS EXPOSED TO VHF RADIOFREQUENCY EM FIELDS

In order to limit the specific absorption rate (SAR) in the ankles of a person exposed to an electric field at frequencies below 100 MHz, in duced current limits are prescribed in the 1992 ANSI/IEEE safety stand ard. We have measured the induced currents passing through the feet of nine subjects exposed to vertically polarized electric fields from ne arby antennas, transmitting at frequencies between 90 and 104 MHz (in the FM broadcast band). The experimental results are in excellent agre ement with the results obtained analytically for frequencies up to 110 MHz. The analytical results were obtained by applying the finite diff erence time domain (FDTD) method to an anatomically-based model of an average height male (1.75 m) and to a model of the tallest subject in this study (1.91 m). For the mean height of the nine subjects (1.75 m) , the measured induced foot current for a unit strength vertically pol arized electric field varied with frequency from 4.46 to 3.45 mA/(V/m) for frequencies between 90-104 MHz. For the tallest subject (1.91 m), the corresponding values ranged from 5.42 to 4.45 mA/(V/m). Foot curr ents in excess of the induced current limits in RF safety guidelines f or both the controlled and uncontrolled environments could result even when the vertical component of the incident electric fields comply wi th the corresponding field limits. It is important, therefore, to not only measure the E- and H-fields, but also the induced currents up to the recommended maximum frequency of 100 MHz, and perhaps up to the up per frequency of the FM broadcast hand (108 MHz).