Abstract: An ultrawideband transverse electromagnetic mode horn antenna for use at high voltages, comprises a pulse generator (1) and two transmission horns (12, 18) containing different dielectric media. The interface (30) between the dielectric media is configured so that a signal from the generator is incident on the interface at an angle substantially equal to the Brewster angle, thereby maintaining a good impedance match across the interface. A further advantage of the arrangement is that the TEM wavefront is preserved through the antenna section allowing operation at fast pulse risetime (less than 200 ps) for short duration (several ns) at high voltage.