In this paper, an impedance matching network is designed for an RF energy harvester used in the context of Body Sensor Area Networks. It is a fully passive circuit and allows compensating the antenna impedance variations linked with the presence of the body. The designed RF energy harvester has an average RF-to-DC conversion of 40 % for an incident power of -10 dBm for a variation of 30 to 120 Ω for the real part and of 0 to 150 Ω for the imaginary part of the antenna impedance