With the application requirements of wireless technology in implantable bioelectronics, knowledge of the fundamental limits for implanted antennas becomes critical. In this work, we investigated the variation of maximum power density within simplified body phantoms, caused by changes in the operating frequency, implantation depth, body phantom radius, and source size. Both spherical and planar body models are studied, and the corresponding results are compared to an efficient approximation method. The summarized results are used to derive design rules and can be used as benchmarks for the design of most implanted antennas, especially for shallow implants.