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In recent years, the development of healthcare monitoring devices requires high performance and compact in-body sensor antennas. A normal-mode helical antenna (NMHA) is one of the most suitable candidates that meets the criteria, especially with the ability to achieve high efficiency when the antenna structure is in self-resonant mode. It was reported that when the antenna was placed in a human body, the antenna efficiency was decreased due to the increase of its input resistance (<i>R_in</i>). However, the reason for <i>R_in</i> increase was not clarified. In this paper, the increase of <i>R_in</i> is ensured through experiments and the physical reasons are validated through electromagnetic simulations. In the simulation, the <i>R_in</i> is calculated by placing the NMHA inside a human&#8217;s stomach, skin and fat. The dependency of <i>R_in</i> to conductivity (<i>&#963;</i>) is significant. Through current distribution calculation, it is verified that the reason of the increase in <i>R_in</i> is due to the decrease of antenna current. The effects of <i>R_in</i> to bandwidth (BW) and electrical field are also numerically clarified. Furthermore, by using the fabricated human body phantom, the measured <i>R_in</i> and bandwidth are also obtained. From the good agreement between the measured and simulated results, the condition of <i>R_in</i> increment is clarified.