Wireless energy transfer has been demonstrated using microwave electromagnetic support. Significant efficiencies are reported in the case of large dimension systems. Lots of embedded systems require a small power supply but with a large degree of integration where standard contactless energy transfer techniques suffer from poor efficiency. In such systems, RF input energy is rectified using rectenna circuits. The latter circuits are optimized for a given input RF power and cannot accommodate the two possible ways of energy transfer: the dedicated transfer (high power) or the harvesting of ambient energy (low power). This paper presents a novel rectenna architecture tunable for 900 MHz-2.45 GHz operation, able to process RF input power in the $-$30 to +30 dBm (dB miliwatt) range with a peak efficiency of 80%.