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 0.5 dB 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.