An implantable medical device is stored in a container prior to implantation in body tissue. The IMD includes transmitter/receiver circuitry and at least one antenna. The storage container (packaging) includes an impedance altering substance positioned in proximity to the IMD when stored in the container, the substance having electrical material properties that alter the input impedance of the antenna to improve the reception and transmission properties of the antenna when the IMD is stored in the container. A container for storing an IMD having an antenna has a packaging tray for housing the IMD, the packaging tray having a support for supporting the IMD and the container including a substance positioned in proximity to the support that has electrical material properties that after the input impedance of the antenna of the IMD supported by the support, so as to improve the reception and transmission properties of the antenna. A method for packaging an IMD prior to implantation in body tissue includes the step of providing such an impedance altering substance in the pre-implantation IMD packaging, in proximity to the packaged IMD.