Measurements of Ankle Specific Absorption Rate and Body-to-Ground Current in a Suit-Protected Human Model for Near-Field Exposures, 2–400 MHz

Abstract

The potential usefulness of a protective-suit ensemble to reduce specific absorption rate (SAR) at submicrowave frequencies was studied using a full-sized, muscle-equivalent human model. In the past, such suits were used predominantly for microwave protection, but some highly conductive suits presented a very real fire hazard when arcing was considered. Suits made from partially conductive fabric were slightly less effective against microwaves but were much less flammable; moreover, their performance against other radiofrequencies (RFs) has not been studied. Recently promulgated exposure standards have imposed theoretical maximum body-currents to limit extremity SAR to 20 W/kg and have impacted certain occupational environments such as those surrounding RF transmitting towers and RF heat sealers. In the present research, reactive near-field irradiation conditions were used at 2.025 and 29.9 MHz, and quasi – near-field conditions were used at 80.0 and 400.0 MHz. Nonperturbing thermal probes were used to measure RF-induced temperature rises from which localized ankle SARs were calculated; a stand-on RF milliammeter recorded RF body-to-ground current over metallic groundplanes. Mean ankle SARs of greater than 23 W/kg were measured for some unprotected conditions, but with full ensemble protection (suit, hood, and overshoes) no mean ankle SAR exceeded 1.1 W/kg. The suit without the overshoes, however, did not reduce ankle SAR. At 29.9 and 80.0 MHz, the presence or absence of the hood caused relatively small SAR changes. We attribute the suit's ability in reducing ankle SAR to the partially conducting nature of the metal/textile material and to the ability of the overshoes to shunt RF current around the ankles. We recommend that further use of partially conducting fabrics be explored to provide practical means of reducing occupational RF-induced extremity SAR to acceptable levels.