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In this letter, we investigate the influence of the output balun for realization of wideband and efficient microwave push-pull power amplifiers (PAs). For this, a push-pull GaN-HEMT PA operating between 1-3 GHz without an output balun has been implemented and a novel push-pull harmonic load-pull measurement setup that allows arbitrary balanced fundamental and second harmonic loads to be presented to the push-pull PA is proposed. The measurement results illustrate that the efficiency may be degraded up to 25% if improper common mode harmonic impedances are presented by the balun. The proposed approach allows the PA and balun to be investigated separately and is therefore expected to be an important tool for further research on balun and PA structures suitable for high efficiency and wideband push-pull applications at microwave frequencies.