The radio frequency (RF) performance of implantable antennas is significantly affected by the lossy biological tissues surrounding the implant. This paper introduces an an-alytical method for assessing the gain and radiation patterns of implantable antennas using planar body models, which are particularly suitable for large host bodies like the human body. The closed-form expressions derived in this method reveal that the antenna gain primarily depends on a few key parameters such as host body complex permittivity, implantation depth, operating frequency, and encapsulation dimensions. A numerical case study featuring an ingestible smart capsule within an anatomical human body model demonstrates the practical application of this method, offering a streamlined approach for assessing RF performance in the early stages of implantable antenna design.