Abstract: For the problem that TEM horn antenna used in the radiating-wave nuclear electromagnetic pulse (NEMP) simulator has limited ability of low frequency radiating, two basic types of TEM horn antennas (composed of plates and wires respectively) are analyzed. Based on TEM horn antenna composed of wires that can make simulator lighter, a new type of TEM horn antenna is designed by using the structure of helixes end and attaching the shielded plates circuit to antenna. Results show that compared with the common type of the TEM horn antenna composed of wires, this new type of antenna has improved the ability of low frequency radiating greatly. The pulse width of near field waveform is longer by almost seven nanoseconds, and the peak value of pulse is larger by almost one kilovolt per meter. Increasing the radius of helixes and decreasing the pitch of helixes can further enhance both pulse width and peak value of the field strength. Meanwhile, attaching the shielded plates circuit to antenna can further lower down the wave crest generated by reflected signals, which makes the near field waveforms much better.