An implant (110) includes an antenna unit (130) and an encapsulation. The antenna unit includes an elongated ferrite core (142) having a first length and an antenna coil (146) wound around the ferrite core, and is configured to communicate with an external unit (120) using inductive coupling of a magnetic field. The encapsulation encapsulates the antenna unit, and includes one or more openings (134) that are aligned with the ferrite core and have respective second lengths that are equal to or greater than the first length of the ferrite core.
