SAR reduction of helmet antenna composed of folded dipole with slit-loaded ring

Helmet antennas have been investigated for various applications, such as military, construction, and disaster prevention. In their radio system operated at the low frequency, the half wavelength circular loop antenna, the folded dipole antenna, and the inverted-F antenna have been proposed. Since these antennas are arranged in proximity to the human head, the radiation efficiency is reduced and the value of the specific absorption rate (SAR) increases. To reduce the unwanted radiation toward the human head, the installation of the conductor ring is effective. This paper presents the further improvement of the radiation efficiency and the reduction of the SAR value by loading the slit to the conductor ring. The relative bandwidth of 1.0%, radiation efficiency of 44.0%, gain of ™3.7 dBi, and the 10 g average local SAR of 0.22 W/kg are obtained with the conductor ring.