I. Introduction
We have developed an antenna-coupled-electrode electric-field sensor for 28 GHz-band as an electric-field sensor (EO sensor) [1] [2]. For measuring the electric field intensity, we usually use an antenna factor of the EO sensor. We have already developed a far field antenna factor estimation method using the amplitude center modified Friis transmission formula [3] [4]. In the case of using the formula, we can estimate the far-field antenna factor of the antenna under test at the near distance that is less than the far-field antenna distance. We have newly estimated the far-field antenna factor of the EO sensor using the formula. In this paper, the amplitude center modified Friis transmission formula is first explained. Next, the estimated far field antenna factor for the EO sensor system is explained.