I. Introduction
Ultrawideband (UWB) technology has been widely used in various radars, and attracted much attention recently for communication systems [1]. In these systems, UWB antennas directly transmit narrow pulses instead of a continuous carrier wave to deliver information. Thus a UWB antenna acts as a “wideband band-pass filter” rather than a “spot filter.” Furthermore, successful signal transmission and reception in UWB systems need minimized ringing, spreading and distortions [2]. In order to characterize the time-domain parameters of a UWB antenna, some parameters such as antenna transfer function, pulse energy spread and correlation factor have been introduced. Apparently, the characteristics of a UWB antenna both in frequency domain and time domain should be investigated and optimized for UWB applications.