Abstract: TEM horn antenna with constant characteristic impedance(CCI TEM horn antenna) as one kind of ultra-wideband(UWB) antennas has been widely used. As the characteristic impedance of such antenna is a constant value along the axial direction, the reflection loss in the horn is very low. In order to elevate the CCI TEM horn antenna’V_pp (peak to peak value of the far-field pulse in the axial direction), the relationship between the V_pp and the aperture as well as characteristic impedance is analyzed by numerical simulations and experimental measurements. If the CCI TEM horn antenna is fed by a 50Ω coaxial cable and its length along the axial direction is a constant value, the maximum V_pp of such antenna can be reached with optimal apertures. Also, when optimal aperture is chosen for the antenna with different impedance values, the V_pp of such antenna with characteristic impedance 280Ω is the maximum.