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UWB is a promising wireless technology that can operate at very low power emission levels while communicating high data rates over short distances. It has attracted much attention as a means of expanding capacity from the already heavily utilized wireless bands. The Federal Communications Commission (FCC) has allocated a bandwidth of 7.5 GHz between 3.1 GHz to 10.6 GHz for commercial UWB communication systems. In this emerging technology, the antenna design is a challenge. Like conventional antenna design, the return loss has to remain higher than 10 dB over the frequency range of operation. For a UWB antenna, the radiation properties should be reasonably satisfactory over the bandwidth. For example, omni-directional radiation patterns are required for indoor and vehicular applications. For a more complete pattern analysis, the total field which includes the coand cross-polarizations of the radiation should be taken into account. The time domain performance such as the group delay and impulse response should also be examined (Chen et al., 2004). Recently, several monopoles have been proposed for various UWB applications. These antennas make use of different structures to meet the requirements of return loss and radiation pattern. A monopole comprised of a square planar structure positioned perpendicular to the ground plane has been proposed, where bevelling or a shorting post is used to optimize and achieve a broad impedance bandwidth (Ammann & Chen, 2003). A biarm rolled monopole (Chen, 2005) which is constructed by rolling a planar monopole has shown that it is capable of achieving broadband and omni-directional radiation characteristics within the UWB band. The UWB antenna in (Behdad & Sarabandi, 2005) consists of half of a coupled sectorial loop above the ground plane. The optimized design is able to achieve an 8.5 : 1 frequency bandwidth for the voltage standing wave ratio (VSWR) < 2.2. Its radiation patterns are relatively consistent within the frequency band. In another design (Liang et al., 2005), a printed circular disc monopole is fed by microstrip line. Its impedance bandwidth covers the UWB frequency band and the radiation patterns are nearly omni-directional. The antenna geometry in (Qiu et al., 2006) is a circular notched ring with an attached element inside the hole. This antenna has band-notched characteristics, which meet the input impedance requirement of the UWB band while avoiding interference

Characteristics of an Ultra-Wideband (UWB) Butterfly-Shaped Monopole Antenna