Radiation Efficiency and In-Vivo Link Model of Implantable Antennas in Lossy Media

This work investigates the radiation efficiency and in-vivo link modeling of implantable antennas operating in lossy media such as human tissues. A comprehensive near-field analytical framework is developed to evaluate antenna radiation efficiency, which incorporates intrinsic structural losses (ohmic and dielectric), near-field losses, and compensated far-field attenuation in dispersive lossy media. Through theoretical derivation and full-wave simulations, it is demonstrated that the radiation efficiency advantage of magnetic antennas over electric antennas reaches a maximum when the antenna's physical size is approximately 0.05 effective wavelength. By examining wave-impedance matching and the distribution of radiated power, this study proposes utilizing one effective wavelength serves as the optimal boundary between near-field and far-field regions for deeply implanted antennas in homogeneous tissues, with the wave impedance difference less than 2% and compensated radiation power difference less than 0.5dB beyond this boundary. Based on this boundary, a modified Friis transmission formulation is introduced for in-body link-budget prediction, providing theoretical basis and directional guidance for the optimized design of implanted antennas.