Abstract: In this paper, the radiation characteristics of constant impedance TEM horn of various geometrical structures are calculated. In each calculation, peaktopeak value of axial electric field obtained from an equivalent ideal aperture is used to normalize electric fields along axial of practical antennas. The specific value is defined as feeding coefficients and then applied to determine how the radiation properties of the horns can be affected if the horn structure is adjusted. These results provide a basis to design and fabricate practical ultrawideband antenna.