Examples of implantable devices having a hermetically-sealed, metal container are disclosed. In one example, the implantable device has a Bluetooth Low Energy (“BLE”) beacon inside the hermetically-sealed, metal container. The hermetically-sealed, metal container may have a micro-strip type transmission line on an internal side of the container such that the container acts as the ground plane of the micro-strip transmission line. A slot at a specific location and having specific characteristics (such as shape, length, and/or width) may be cut into the ground plane of the micro-strip to act as a radiator. Thus, the size of implantable devices can be reduced by using a slot antenna in the hermetically-sealed, metal container. Such a device implanted in a suitable location and at a suitable depth in a user can communicate with an external computing device, such as a smartphone held in the user's hand.