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In this paper, an analysis of a power amplifier manipulated using a second harmonic (PA-2HM) is described using a remarkable correlation between the fundamental and second harmonic impedances. The output loading condition, made up of an optimum fundamental impedance mapped to the conditional second harmonic reactance, allows us to achieve a high-efficiency power amplifier (PA) with a simple output matching circuit. For the analysis, the output voltage and current waveforms are modeled in terms of their output loading in order to extend the output power and efficiency. Specific PA-2HM cases, inverse Class-F and Class-J modes, are analyzed using different second harmonic reactance conditions. The allowable second harmonic reactance for maintaining maximum efficiency has been found to be spread throughout a wide range. To justify the analysis, a harmonic load-pull simulation and measurement are conducted and compared with analysis results. For verification, a commercially available 60-W gallium-nitride (GaN) device was used for different types of PA (inverse Class-F and Class-J) with appropriate second harmonic impedance. In terms of the output power and drain efficiency, the measured results are in good agreement with not only computer-aided design simulations, but also with our analysis and load-pull results.