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Rectifying antenna (rectenna) associations can be advantageously used to harvest electromagnetic energy from different directions. When the rectennas are associated in series, their output voltages are added, and the electrical dc energy is more easily exploitable. This letter shows that if a rectenna receives significantly less energy than the others (i.e., unbalanced association), its voltage polarity may reverse. Simulation and experimental results show that when an unbalanced association is operated at maximum power point, voltage reversal can result in a total decrease in efficiency of up to 26%. Possible solutions to reduce these detrimental consequences are also discussed.