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The pyramidal horn and open-ended waveguide (OEG) antennas are widely used on millimeter-wave bands as the standard or wideband antenna. These aperture antennas are normally calibrated in the far-field ranges by using the three-antenna method. However, even if the antenna separation distance satisfies the usual far-field criterion 2D2/λ (D: maximum aperture size), the gain of horn antennas is reduced by the order of 1 dB. The separation distance needed for accurate measurement, i.e., to obtain a gain reduction within 0.05 dB, is 32D2/λ or more. Newell et al. developed the extrapolation technique for reducing the distances required for conventional far-field measurements. As another approach for the same purpose, the efficacy of considering the phase center of an antenna in the three-antenna method has been shown. In this paper, we discuss the determination of the gain of millimeter-wave pyramidal horn and OEG antennas by considering the phase center, and evaluate experimentally the effect of measurement distance on the gain.