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We have proposed a method for evaluating the free-space antenna factor of a log-periodic dipole antenna (LPDA) with a time-domain technique. The proposed method is based on a time-domain analysis and a pulse-compression technique. The proposed method enables the direct wave to be isolated from reflected waves on the ground plane of an open-area test site (OATS) and evaluation of the free-space antenna factor. In a measurement, two LPDAs are held on masts at a height of 6 m above the ground and are separated by 10 m. The direct wave is separated using a Wiener filter obtained from frequency-domain data at 3 m antenna separation. The proposed method can completely separate the direct wave from other waves from the ground plane of an OATS. The expanded uncertainty (k = 2) of LPDAs was estimated to be from 0.38 to 0.45 dB over the frequency range from 300 to 1000 MHz.