Antenna assemblies for an implantable medical device are disclosed. The implantable medical device comprises a hermetically sealed housing, typically formed of titanium materials, and electronics, including a transceiver, disposed therein. An antenna is disposed in an air, gas or plastic dielectric filled compartment within a header, which is attached to the housing. The header is premolded so as to create the compartment. The antenna is then placed within the compartment, which is then sealed.