Rectennas operating as microwave electromagnetic energy harvesting system has low input and output power characteristics. Also, the output power efficiency is greatly dependent on the input power level. In this paper is investigated the behavior of output power of two rectennas according to the level of input power at the rectifiers circuits. The rectenna are composed of printed antennas which were manufactured using a low loss substrate to increase efficiency of the systems. Rectifiers were built using fiber glass substrate. The antennas simulations were carried out using Computer Simulation Technology software, and rectifier circuits were designed using the Advanced Design System software. The project sought the impedance matching between rectifier circuits and antennas at the operating frequency of 2.45 GHz. The measurement results of both rectennas, operating with a 50 Ω load, are compared and discussed.