Systems and methods for implantable medical devices and headers are described. In an example, an implantable medical device includes a device container including an electronic module within the device container. A header is coupled to the device container. The header includes a header core including a conductive member electrically coupled to the electronic module within the device container. A header shell is disposed around the header core and attached to the device container. An antenna is coupled to the header core and electrically coupled to the electronic module. A first portion of the header is proximate the antenna. The first portion includes a first dielectric constant that is lower than a second dielectric constant of a second portion of the header.
