An antenna structure for an implantable medical device (IMD) is provided including a lower dielectric biocompatible antenna portion positioned on a body side of the structure and a high dielectric portion including at least one dielectric substrate having a high dielectric constant positioned on a device side of the structure. The biocompatible antenna portion is derived from an antenna layer, a biocompatible surface layer, and at least one layer of biocompatible dielectric material (e.g., high temperature cofire ceramic (HTCC) material) that provides a matching gradient between the antenna and the surrounding environment. The high dielectric portion may include at least one layer of low temperature cofire ceramic (LTCC) material. The high dielectric portion may be bonded to the biocompatible antenna portion or cofired with the biocompatible antenna portion to form a single bilayer monolithic antenna structure having a lower dielectric HTCC biocompatible antenna portion and a high dielectric LTCC portion.