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Our cities are surrounded by a large number of radio frequency (RF) signals broadcasted by various wireless systems. In order to enhance the efficiency of energy usage in addition to the purpose of communication, ambient RF energy harvesting systems are designed to harvest and recycle wireless energy for many applications such as battery chargers, sensor devices and portable devices. The main element of the ambient RF energy harvesting system is a rectenna which is the combination of an antenna and a rectifying circuit. Even though the ambient RF energy is widely broadcasted by many systems, the energy is extremely low. Therefore, high performance antenna and rectifying circuits have to be designed for supporting small incident power; also the number of frequency channels of the rectenna can enhance the performance and support different harvesting locations. This paper proposes a dual-band rectifier for RF energy harvesting which is designed to operate at 2.1 GHz and 2.45 GHz. The first channel can provide the maximum efficiency of 24% with 1.9 V of the output voltage at 10 dBm of input power. On the other hand, a maximum efficiency of 18% and 1.7 V of the output voltage can be achieved by the second channel at 10 dBm of input power.