Abstract: Based on the consideration of power capacity and aperture matching to extend the low-frequency operating bandwidth, an ultra-wideband TEM horn antenna with a distributed impedance end-loading structure was designed. First, the gradual coaxial-flat balun structure was optimized, which expands the working bandwidth of the feed structure and improves the feed efficiency. Secondly, the distributed impedance matching design was performed on the exponential TEM horn antenna, and its port characteristics and radiation characteristics were significantly improved. The power pattern and energy pattern were used to evaluate the radiation effect of the antenna. The experimental results show that, compared with the exponential TEM horn antenna, after the distributed impedance matching structure is loaded, the antenna low-frequency bandwidth is widened by 330 MHz, the peak-to-peak value of the antenna main axis radiated electric field is increased by 10%, and the feed efficiency is increased by 17%.