A practical method of estimating the effective impedance of electric dipole antennas aboard the scientific spacecraft Geotail has been developed. The principle of the newly proposed method is to derive the complex impedance of the dipole antennas immersed in the space plasma by means of the analysis of signal waveforms captured when a calibration signal was imposed directly onto the antennas. The derived complex antenna impedance is expressed as frequency responses of its real and imaginary parts. During the flight of Geotail in the Earth's magnetosphere, the impedance measurements were conducted in various regions of the magnetosphere. It has been confirmed, from the analysis of the measured data, that the impedance of the Geotail antenna forms an equivalent electric circuit consisting of a resistance and a capacitance connected in parallel. It has been found that the resistance value is easily changed by the extremely rare ambient plasma density in the magnetosphere. The validity of the measured values have been examined along a theory of probes in a sheath region, taking into account antenna potentials which are lifted up due to the photoelectron emissions from the antenna surface. The analyzed result shows that the measured values of the antenna resistance are consistent with those within the sheath calculated from the theory. Correlation plots of the capacitance values obtained from both the measurements and the calculations have shown that the major parts of the measured capacitance represent the “sheath capacitance,” but other parts might result from a combination of the sheath impedance and the plasma impedance, in addition to the free-space capacitance.