Abstract: The radiation mechanism of a TEM horn antenna both in time and frequency domain is researched. The TEM horn fed by the high-frequency signal behaves as an aperture antenna with favorable directional radiation capability and the TEM horn fed by the low-frequency signal behaves as a dipole antenna with omnidirectional radiation field. In time domain, the radiation field of TEM-horn antenna fed by a short impulse consists of four sub-waves. The waveform and comparative time sequence of every sub-wave is different in every direction, and that leads to dissimilar radiation waveform to every direction of antenna. Based on the research result, we propose an end-loading optimizing method for TEM-horn-antenna design. In this way, the radiation performance on boresight and the efficiency of the TEM horn are observably improved.