Increase of Input Resistance of a Normal-Mode Helical Antenna (NMHA) in Human Body Application

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 (R(in)). However, the reason for R(in) increase was not clarified. In this paper, the increase of R(in) is ensured through experiments and the physical reasons are validated through electromagnetic simulations. In the simulation, the R(in) is calculated by placing the NMHA inside a human’s stomach, skin and fat. The dependency of R(in) to conductivity (σ) is significant. Through current distribution calculation, it is verified that the reason of the increase in R(in) is due to the decrease of antenna current. The effects of R(in) to bandwidth (BW) and electrical field are also numerically clarified. Furthermore, by using the fabricated human body phantom, the measured R(in) and bandwidth are also obtained. From the good agreement between the measured and simulated results, the condition of R(in) increment is clarified.