This article describes a new technique for determining an appropriate input pulse and its duration, which are important for time-domain studies of ultra-wideband (UWB) antennas. For such studies, the researchers normally prefer EM simulation tools, and use different types of input pulses. The basis of the choice of input pulse is a bit obscure in most of the reports appearing in the open literature. Antenna researchers indeed face a problem in having suitable guidelines for choosing the input pulse, a mathematical approach to generate it, and in estimating the pulse's duration. The width or duration of the input pulse cannot be arbitrary. This plays a significant role in determining the reliable time-domain response of the antenna in both transmitting and receiving modes. This is successfully addressed in this paper, taking a UWB monopole as the antenna under test. The proposed technique should accurately guide and help a researcher to perform the time-domain characterization of a UWB monopole/dipole antenna using mathematical tools and EM simulators commercially available. © 2014 IEEE.