A modified TEM horn antenna which has an asymmetric plates, two plates with different heights, has improved pulse radiation performance. This paper presents an optimized asymmetric TEM (AsyTEM) horn antenna and a same size of conventional exponentially tapered TEM (ETEM) horn antenna to verify the improvement of the impulse radiation performance. The AsyTEM horn antenna is shown to yield an enhanced reflection coefficient at the low frequency of the operation bandwidth. The results show that the AsyTEM horn antenna has reflection coefficient less than-10 dB in the frequencies from 2.6 GHz to over 20 GHz while the ETEM has 2.9 GHz to over 20 GHz. In the case of being compared with the ETEM horn antenna, it also had an improved impulse radiation gain which increased over 0.6.