56 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) ISSN (Print) 2313-4410, ISSN (Online) 2313-4402 © Global Society of Scientific Research and Researchers http://asrjetsjournal.org/ High Gain UWB Antenna Element Design for Cognitive Radio Systems using Low Cost FR4 Substrate Ahmed A. Kabeel a , Amr H. Hussein b* , Ashraf A. M. Khalaf c , Hesham F. A. Hamed d a Electronics and Electrical Communications Dept., High Institute of Engineering and Technology, New Damietta, Egypt b Electronics and Electrical Communications Engineering Dept., Faculty of Engineering, Tanta University, Tanta 3111, Egypt c,d Electronics and Communications Dept., Faculty of Engineering, Minia University, Minia, Egypt a Email: ahmede_kabeel@hotmail.com, b Email: amrvips@yahoo.com c Email: ashkhalaf@yahoo.com, d Email: hfah66@yahoo.com Abstract In cognitive radio (CR) and Ultra Wideband (UWB) communication systems, one of the keys issues is designing a compact antenna while providing wideband characteristic over the entire operating band. In these systems, printed wide slot antennas have received much attention owing to their wideband matching characteristics and omnidirectional radiation patterns. In addition they present really appealing physical features, such as simple structure, small size and low cost. In this paper, a small antenna with circular radiating patch and defected ground structure for UWB applications is proposed. The proposed antenna provides a wideband frequency range from 1.17 GHz to more than 12 GHz. But, its maximum allowable gain is low and equals 2.78  . In order to increase the antenna gain while maintaining the same impedance matching characteristics, a plane reflector is placed beneath the antenna at a specific distance . The plane reflector reflects the backward radiated power again towards the antenna to be constructively added to the forward radiated power. In this case, the modified antenna radiated power is significantly increased and the maximum allowable antenna gain is also increased up to 7.67  which is 2.75 times greater than the gain of the antenna without reflector. In addition, the modified antenna exhibits 51% to 95% radiation efficiency over the operating frequency range. Good return loss and radiation pattern characteristics are obtained in the frequency band of interest. The antenna is designed using the CST Microwave Studio software using low cost FR4 (lossy) substrate with   = 4.3 and ℎ = 1.6  . Simulated and experimental results are presented to demonstrate the effective performance of the proposed antenna. ------------------------------------------------------------------------ * Corresponding author.