We have already proposed a single antenna method for an
antenna factor estimation [1]. Our method makes it possible
to measure separately the reflection wave at the ground plane
and the intrinsic self-reflection wave of the antenna under
test (AUT) using time-domain subtraction. However, our
previous proposed method uses the fixed antenna distance.
For this reason, main source of measurement uncertainty of
far field antenna factor estimation is dependence of antenna
distance. In order to decrease a measurement uncertainty, we
propose a new estimation method for determining the
antenna distance at each frequency that to estimate the far
field antenna factor. Our proposed method is based on the
techniques of a time domain analysis and a time-frequency
analysis using a short time Fourier transform. This method
can determined the antenna distance at each frequency. In
this paper, we explain the single antenna method to estimate
a free space transmission wave of a log-periodic antenna
(LPDA), which is commercially available. Next, difference
between the estimated antenna factor and a far field antenna
factor using phase center modified Friis transformation is
explained.