An antenna system for an implantable device like a cardiac pacemaker or a cochlear implant. The antenna system includes at least two coil units coupled with their terminals to a control circuit which can selectively connect the coil units in series or in anti-series, corresponding to a “operational mode” and a “safety mode”, respectively. In the operational mode, magnetically induced voltages in the coil units add, while they subtract and therefore completely or partially compensate in the safety mode. Thus the implantable device can be protected from damage due to extraordinarily large changing rates of external magnetic fields as they exist for example during MRI examinations.