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An original measurement system for nonlinear RF power-transistor characterization is presented. This new setup enables the measurement and optimization of output power and/or power-added efficiency (PAE) using active harmonic tuning and six-port reflectometers as vector network analyzers. Two active loops are inserted at both ports of transistors in order to independently control the source and load impedances at the fundamental and at the second harmonic frequency. To the authors' knowledge, this is the only active technique that allows a complete automated multiharmonic load-pull/source--pun measurement system. Experimental results are shown for a commercial GaAs MESFET power transistor at 2 GHz. Index Terms--Load-pull, MESFET, multiharmonic, nonlinear, power amplifiers, six-port, source-pull.