This paper presents the design of a robust and large range RF energy harvester with a control loop. The harvester is based on an optimally mismatched antenna-rectifier interface that offers a large passive voltage boost to increase the sensitivity of the energy harvester while still extracting energy from the antenna. A control loop is proposed that maximizes the voltage on the storage capacitor by keeping the rectifier at resonance during charging continuously. The basic principle of this loop has been implemented and verified with simulations. The loop is able to optimize the antenna-electronics interface for a ±33% change in antenna reactance. A comparison with state-of-the-art RF energy harvesters shows major improvements in rectified output voltage at power levels lower than -25 dBm.