Deterministic Equation of Self-Resonant Structures for Normal-Mode Helical Antennas Implanted in a Human Body

Recently, helical antennas had been used as a very small implantable antenna in a capsule endoscopy system. For a helical antenna, a normal-mode helical antenna (NMHA) is promising because of its small size and the completeness of an antenna design method, which has already been established. However, in the human body, the effects of human tissues on the electrical characteristics of an NMHA were not clarified. The most important aspect is to determine the self-resonant structures. In this letter, a deterministic equation is derived for the self-resonant structures of an NMHA in the human body. By utilizing simulation results of an electromagnetic simulator, a capacitive reactance equation is deduced based on the electric field distributions around an antenna. An inductive reactance equation is obtained by modifying the equation of free space to the condition of inside a dielectric material. Finally, by equating the capacitive and inductive reactance equations, the self-resonant equation is determined. The accuracy of the equation is ensured through the agreements of the antenna structures given by equations and electromagnetic simulations. Moreover, by fabricating an NMHA and a human body phantom, the measured results are obtained. The self-resonant structure is ensured through the measured results.