This disclosure describes antenna structures for use in an implantable medical device. The implantable medical device may include a housing that hermetically encloses electronic components of the implantable medical device and a fixation mechanism that affixes the implantable medical device to a target location, such as a wall of a vessel. The fixation mechanism functions as a radiating element of an antenna of the implantable medical device. The housing of the implantable medical device may include a conductive loop that electrically couples to a telemetry module and magnetically couples to the fixation mechanism. The telemetry module may provide signals to be transmitted to the inner loop and those signals are magnetically coupled between the inner loop and the fixation mechanism, which radiates the signals.