An implantable medical device that includes an enclosure, an electrical module for the desired stimulation, sensing and communications functions, a power source, an air-core antenna, and supporting structures therefore. The antenna includes a quarter-elliptical shape that maximizes the antenna area to facilitate transmission of electromagnetic waves through the enclosure. A support structure is provided so that the antenna, the module and the battery self-align with each other and the enclosure. The support structure contains contacts to facilitate electrical connection of the antenna to the module and positions the module so that contacts thereon are in close proximity in plan and elevation to external electrical feed-throughs in the housing and the antenna contacts on the support structure. The support structure positions the antenna between the module and the housing, separated therefrom by a predetermined minimum distance to facilitate efficient coupling of electromagnetic waves to and/or from the antenna.