1 A Compact Ultra Wideband Antenna with WLAN (IEEE 802.11a) Band Rejection Y.E. Jalil*, C.K. Chakrabarty Centre for RF and Microwave Engineering, Department of Electronics and Communication Engineering Universiti Tenaga Nasional Kajang, Selangor, Malaysia yantierana@uniten.edu.my, Chandan@uniten.edu.my B. Kasi Department of Electrical and Electronics Engineering Kuala Lumpur Infrastructure University College Kajang, Selangor, Malaysia baskaran@iukl.edu.my Abstract—The problem of interference between ultra-wideband (UWB) system and other narrow band systems has been a major concern for a long time. This paper presents a compact UWB antenna that shows desirable band-notch and gain characteristics. The proposed antenna consists of an octagon- shaped patch with a truncated ground plane that is fed by a 50  microstrip line. The frequency notch characteristic is obtained by inserting two C-shape slots on the radiating patch. The proposed antenna is successfully designed and simulated showing good impedance matching and reasonable radiation patterns over a bandwidth of 2.8 to 11 GHz. Furthermore, the proposed antenna has a compact size of 26 × 32 mm 2 while exhibiting the band rejection performance in the frequency band of 5.15 to 5.85 GHz. Keywords- frequency-notched; microstrip antenna; ultra wideband (UWB) I. INTRODUCTION The Federal Communications Commission (FCC) has authorized the unlicensed use of a bandwidth of 7.5 GHz (from 3.1 to 10.6 GHz) for ultra wideband (UWB) wireless communications in 2002 [1]. Since then, UWB technology has been rapidly advancing as a promising high data rate wireless communication technology for various applications. In spite of having allocated a wide frequency range, UWB devices suffer the consequences of having to share the spectrum with a number of other established narrowband applications such as WiMAX (3.3 – 3.6 GHz) and WLAN IEEE 802.11a (5.15 to 5.85 GHz). The operation of UWB radios is almost "invisible" for other applications attributable to the significantly low emitted power from UWB devices (EIRP -41.3 dBm/MHz) [2]. Nevertheless, the interference from a strong narrowband signal, within the UWB band could affect the overall systems performance of UWB communication systems in terms of increasing pulse distortion and bit error rate. To overcome this unwanted problem, a filtering property is required in an ultra- wideband system in order to prevent sensitive components within the front-end of the receiver from being overloaded by strong signals from other narrowband systems. However, the uses of filters will increase the size, weight and complexity of the UWB system and thus lead to increase in cost [3]. Therefore it is desirable to design UWB antennas integrated with band rejection characteristics in the affected frequency bands. In recent times, numerous researchers have proposed diverse antenna geometries, design methods and structures in order to achieve band-notched features. One of the most popular methods is by embedding complementary resonance elements into either the radiating plane, ground plane or the feed line of the antenna. Some examples on this technique are by inserting half-wavelength slot [4] or a pair of U-shaped slots [5] in the ground plane, by embedding an inverted L-shaped stub [2],[6] or square slot ring [7] on the radiating patch or by utilizing capacitively loaded loops [8]. In contrast to the abovementioned works, two designs of UWB antennas with reconfigurable band notches have been proposed by M. Al- Husseini et al [9]. The first design is based on several nested complementary split ring resonators while the second design has two identical split ring resonators. Both design methods are suitable to be integrated to an antenna in order to produce narrow frequency notches within the UWB range. Nevertheless, the proposed methods only manage to create frequency-notch bands at a random frequency band and not targeting any narrow band services. Additional weaknesses associated with some of other proposed designs are either the antenna’s geometry is complex [10] or not appropriate for integration with compact systems [11]. In this paper, an octagon-shaped microstrip antenna with ultra-wideband impedance matching from 2.8 to 11 GHz is proposed. A frequency band-notched from 5.15 to 5.85 GHz can be achieved by simply employing a couple of C-shaped slots in the radiating patch. The outline of this paper is as follows. The geometry of the proposed UWB antenna is presented in section II while in section III, details of the simulation results and analysis are described. In order to demonstrate the overall performance of the proposed antenna, the radiation patterns and simulated gain results are discussed in Section IV. The conclusions are summarized in Section V II. ANTENNA CONFIGURATION AND DESIGN The proposed UWB antenna configuration and its geometrical parameters are shown in Fig. 1. The antenna is located in the x-y plane and the normal direction is z-axis. This proposed antenna design consists of an octagon-shaped radiator fed by a microstrip line printed on a partial grounded substrate. The microstrip line feed is designed to match a 50  characteristic impedance. The design and optimization of the *Corresponding Author 1st IEEE International Symposium on Telecommunication Technologies 978-1-4673-4786-0/12/$31.00 ©2012 IEEE