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A TEM horn antenna is usually applied to the air-launching GPR system. Traditionally, the variation of characteristic impedance of a TEM horn antenna is usually set to range from 50 Omega (characteristic impedance of a coaxial cable) to 376.7 Omega (free space wave impedance). However, a difference regularly exists between transmission-line wave characteristic impedance and free space wave impedance. In this paper, we demonstrate that there is no significant difference of performance with the different aperture impedance of the antenna. In addition, the simulated and experimental results both show that the performance of the designed TEM horn antenna matching with impedance of 200 Omega is better than that with free space impedance of 376.7 Omega. For fully understandings, a discussion is provided to explain the possible reasons. At the final aspects of this research, the other components, such as transmitter, receiver, and control units, would work with the TEM horn antennas to complete an air-launching GPR system.