In an embodiment, an antenna for a medical device, e.g., an implantable medical device (IMD), comprises an electrically conductive wire that spirals to form a three-dimensional shape of a rectangular cuboid. In another embodiment, the antenna comprises an electrically conductive wire that spirals to form a three-dimensional shape of an elliptical cylinder, an oval cylinder, an elongated pentagonal prism, an elongated hexagonal prism, or some other shape where the longitudinal diameter of the antenna is greater than the lateral diameter of the antenna. The antennas are sized to fit within a portion of a header of the medical device. Such antennas are designed to provide increased antenna gain and antenna bandwidth.