Radiation efficiency of planar implantable antennas at ISM band

Most attentions regarding the performance of implantable antennas have been paid to the reflection coefficient (S11 with respect to 50 Ω), bandwidth, and far-field radiation pattern, while little effort has been dedicated to investigate the radiation efficiency during the implant design process. Analysis of two slot-type (annular and meandered) and two strip-type (ring and meandered PIFA) antennas embedded in muscle is presented in this paper at 2.4 GHz ISM band. It reveals that the slot types have superior performance in terms of their radiation efficiency. Also, the radiation efficiency is improved by operating away from the resonance, mostly where the admittance is inductive. The radiation efficiency may vary over an order of magnitude or more depending on the choice of the antenna and its resonance point, a fact that deserves further attention.