Body-implanted bioelectronics rely on antennas to interface with external on- or off-body equipment. For a given input power, the antenna type and operating frequency significantly affect the power dissipation in tissues. This study addresses the exposure by realistic magnetic and electric equivalent sources using fullwave modeling techniques. The effect of equivalent source configuration on the power dissipation, exposure, and radiation performance is here quantified for the first time. The results demonstrate the existence of mode-dependent optimal operating frequency that maximizes the achievable radiation efficiency and minimizes Specific Absorption Rate (SAR) for a given input power.