## 1 Introduction  
Ultra-wideband (UWB) systems enable high data rate wireless transmission and therefore are not just restricted to wireless communications systems but have application in radar and imaging systems because they enable acquisition of high-resolution images. UWB antennas are an essential component of UWB systems providing low power dissipation and large impedance bandwidth. Characteristics sought after in UWB antennas include small form factor, desired radiation characteristics, and cost effectiveness [1, 2]. The UWB spectrum is shared with other narrowband services, for example, wireless local area network (WLAN) for IEEE 802.11a in the USA (5.15-5.35 and 5.725-5.825 GHz),which can interfere with UWB systems [3]. Therefore, it is important to eliminate such signals from affecting the performance of UWB systems. This is normally achieved by using filters in the RF front-end. Although filters are effective devices, they can introduce extra loss and cost as well as affect the overall size of the systems [4; 5]. To overcome interference between UWB systems and narrowband systems that operate within the UWB spectrum extensive research has been conducted over the past several years on developing UWB antennas that possess band-reject characteristics. Various band-notched UWB antennas have been reported using different design techniques [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. Previous examples of band-notched UWB antennas include reference [19], where two closely spaced but extremely narrow band notches are achieved by adding two stubs on each side of the patch antenna, whose lengths are quarter-guide wavelength at the desired notching frequency. Electromagnetic (EM) analysis reveals that strong rejection is realized when adjacent currents flow in opposite directions and cancel the radiated fields. In fact, at the notch frequencies, the current flowing on the stubs is in opposite direction (out-of-phase) from the current on the antenna edges. In [20], a notched band is realized by placing electromagnetic bandgap (EBG) structures on the coplanar waveguide (CPW) feedline. The EBG structures are constructed on the opposite face of the substrate and aligned with the CPW feedline. Shorting pins are used to connect the EBG structures with the feedline. Essentially, the CPW feedline uses the EBG structures as a ground-plane. The resonant frequencies of the EBG structures are tuned so that they merged to create a rectangular shaped notched band. In [21], a dual-band notch is realized by loading a pair of split-ring resonators (SRR) on the opposite side of the CPW-fed monopole. The SRRs are placed under the feedline. Dimensions of the SRR determine the notching frequency. The pair of SRRs are separated by quarter-guided wavelength to avoid mutual coupling effects. In [22], strategically placing a stepped slot, the antenna can be excited under differential-mode of operation. Dual notched bands are created by introducing two pairs of quarter-wavelength slits, a pair of half-wavelength stubs, and a half-wavelength stub. The bandwidths of the dual notched bands can be controlled by adjusting the lengths of the stubs and slits. In [23], band notch functionality is realized by using two pairs of horizontally placed folded-strip resonators with an inductive coupling scheme. Finally, in [24], a stepped slot antenna is used to realize UWB impedance matching characteristics. By slitting an open-ended quarter wavelength slot and a short-ended half-wavelength ring slot on the ground near the stepped slot, a notched band of 5.15-5.85 GHz is created. The single and dual notched band UWB antennas described above have a relatively narrow bandwidth. Antennas are needed to suppress interference signals from not just narrowband signals but also wideband signals such as the X-band downlink satellite communication band (660 MHz). Therefore, it is necessary to investigate techniques to realize wide rejection bandwidth UWB antennas.