Analysis of Input Resistance Increase by Human Body Conductivity at Embedded Normal-Mode Helical Antenna

A normal-mode helical antenna (NMHA) is widely used as a human body sensor for human health care applications. It is gaining interest in integration with 5G/6G technologies due to its compact size and high efficiency. The conductivity of the human body significantly affects the antenna's electrical performances, such as input resistance and radiation power. Previously, the increase in input resistance was related to a reduction in the NMHA electric current. However, the effect of electric power absorption by conductivity is crucial and has not been clarified in previous research. This letter clarifies the relationship between antenna input power and the surrounding power through electromagnetic simulations in the self-resonant condition of NMHA at 402 MHz. Notably, the Poynting vector on the antenna surface decreases as conductivity increases. In the case of self-resonant, the radiation power represented by the Poynting vector corresponds to the antenna input power. The dependence of input resistance on conductivity in the human stomach, skin, and fat is obtained by analyzing the relationship between input power and input resistance. The proposed method aligns well with previously obtained results. Moreover, measurement results of NMHA in the stomach and fat phantom are provided to validate the simulation results.