We developed an in-situ direct measurement technique for the electric field antenna impedance using onboard signal processing. Antenna impedance is an important parameter for observed electric field calibration. Because it changes depending on local plasma parameters, in-situ measurements along the orbit are essential for obtaining precise electric field calibration. We installed the onboard signal calibration function (SWCAL) as a part of the onboard software of the plasma wave experiment (PWE) aboard Arase and performed more than 5,500 measurements in the inner magnetosphere from March 2017 to March 2020. The measured the antenna capacitance along the orbit of Arase, which is important for AC electric field calibration. We found that the measured antenna capacitance varies between 70 and 160 pF depending on the ambient electron density. Further, the effects of the antenna impedance on the AC electric field calibration were evaluated. Results indicated that the observed electric field data include ambiguity of up to 7.8 dB and 17° in the main target frequency range of the waveform capture/onboard frequency analyzer of PWE. This ambiguity can be calibrated by considering the real antenna impedance effects measured using SWCAL.