Recently, LGE and XLIM Laboratories initiated collaboration on high-power microwave (HPM) transient radiation source conception. Ultra-wideband (UWB) antennas have been originally designed for UWB radar demonstrators. These prototypes have been built to support high-voltage (HV) short duration pulses with a 25 kV amplitude. Because of its high gain and its capability to radiate short pulses without dispersion within the 300 MHz–2.5 GHz frequency band, a type of these UWB antennas, the Valentine Antenna, has been chosen to be the radiating element of two new high-power HV UWB sources. This antenna, originally designed for low voltage, is made of two curved metallic strips and a copper coaxial-to-strip transition with a classical N connector. Some modifications were achieved in order to improve the dielectric strength of the antenna. Consequently, it is now able to radiate pulses delivered by HV (up to 300 kV), few hundreds hertz, subnanosecond transient sources. Initially, a 3-D model of the antenna was first computed with a time-domain electromagnetic software so as to study the influence of these modifications on the radiating element main characteristics. Then, frequency and transient domains studies were realised. Presented results demonstrate that the antenna S11 parameter remains relatively unchanged and that in the entire frequency band, gain is 1 dB lower than the expected value with the original low-voltage design. This new HV antenna and its main radiation characteristics are described.