Abstract: The ultra-wideband TEM horn antenna fed by coaxial cable and loaded with lumped resistor for the application of imaging radar system is presented. The low-frequency characteristic of the antenna and its directional radiation ability are largely improved with the smoothly tapered structure and loading technology instead of the original design taking account of the radiation characteristics, the reflection from the feed section and the dimension limitations. Finally, a pair of antennas are fabricated and measured based on the theoretical analysis and simulation. It is concluded that the antenna has a good impedance bandwidth of about 11∶1, good radiation characteristics and moderate gain for voltage standing wave ratio (VSWR) below 2 by comparing the simulated results with the measurements, which may be suitable for the requirements of the radar system.