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 modular header core includes a header core module including at least one bore hole configured to receive a lead, an antenna attachment module coupled to the header core, and an antenna engaged with the antenna attachment module. The antenna attachment module is configured to locate the antenna in a selected position with respect to the header core module.