I. Introduction
Helmet antennas have been widely developed and are considered as one of wearable antennas for such as construction and military [1–2]. For disaster prevention, the helmet antenna needs to meet the following four requirements: 1) operating in the VHF band; 2) omnidirectional radiation pattern; 3) low profile so as not to hinder during work; 4) less radiation to the human head. In the previous studies, helmet antennas have been proposed for it [3–4]. However, these antennas operate in horizontal polarized and elliptical radiation pattern in the horizontal plane.