Systems and methods for implantable devices that transmit and receive RF transmissions are provided. More particularly, the implantable device includes an antenna encapsulated within a non-hermetic material, wherein the antenna is spaced from the non-hermetic material by an air gap. Preferably, the spacing is provided by one or more enclosures that maintain the air gap surrounding the antenna during and after the manufacture of the device. The one or more enclosures can be in the form or tubing formed from glass, or the like, at least partially surrounding the antenna. By increasing the amount of air encapsulated within the implantable device, and particularly proximal to (e.g., around) the antenna, the susceptibility to changes in RF performance is reduced.