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An EMI antenna calibration method is newly developed which requires measurements of antenna impedance above a metal ground plane. The paper reviews the obsolete reflection method for determining the antenna gain, and then proposes an improvement to make it applicable to mismatched antennas. In addition, the multiple-scattering is accounted for by introducing mutual-coupling impedance between an antenna and a ground plane. Thus, the antenna input impedance is formulated in terms of the height-dependent radiation impedance and the effective length of the antenna. Using this formula, an algorithm is developed to determine the free-space antenna factor from measured data on the antenna impedance for various heights. Theoretical and experimental investigations clearly demonstrate that the newly developed calibration method can yield the free-space antenna factor very accurately with an error around 0.3 dB or less at almost all frequencies.