RESEARCH PAPER A Planar Integrated UWB/Reconfigurable Antenna with Continuous and Wide Frequency Tuning Range for Interweave Cognitive Radio Applications Falih Mahdi Alnahwi 1 • Abdulghafor Abdulghafar Abdulhameed 2 • Husham Lateef Swadi 1 • Abdulkareem Swadi Abdullah 1 Received: 2 March 2019 / Accepted: 7 September 2019 Ó Shiraz University 2019 Abstract This paper presents a planar ultra-wide band (UWB) sensing antenna accompanied with a monopole narrow band com- municating antenna for interweave cognitive radio (CR) applications. The UWB antenna bandwidth covers the entire band specified for UWB applications (3.1–10.6 GHz). In addition to the varactor diode, a PIN diode is attached to the com- municating antenna to expand the continuous frequency tuning of the varactor diode up to 3 GHz along the frequency range (3.3–6.3 GHz) including WiMAX, personal area network, and the 5 GHz wireless local area network. The UWB sensing antenna shape and the ground plane size of the communicating antenna are modified to reduce the mutual coupling between the two antennas to values less than - 15 dB. The measurements are well agreed with the simulation, and they verify the antenna suitability for CR systems. Moreover, the results show an omnidirectional power pattern for both sensing and communicating antennas which is compatible to the portable CR devices. Keywords Cognitive radio Á PIN diode Á Reconfigurable antenna Á Ultra-wideband Á Varactor diode 1 Introduction The congestions of the wireless frequency allocation has restricted the deployment of the wireless communication services and limited the response of the wireless technol- ogy to the consumers’ demand. About 70% of the allocated spectrum is identified by the Federal Communication Commission (FCC) to be idle for a sensible amount of time (Tawk et al. 2016). Therefore, researchers have proposed a new wireless technology called cognitive radio (CR) that recruits the idle frequency band of a certain licensed user (primary users) to another unlicensed user called the sec- ondary user. Once the primary user gets back to the service, the secondary user should be switched to another idle band to avoid the interference with the original user. In fact, CR systems are classified into underlay and interweave systems (Tawk et al. 2016). Underlay CR sys- tems rely on ultra-wide band (UWB) antennas with reconfigurable band notches to avert the intrusion to some extreme applications that do not accept any form of interference (Alnahwi et al. 2019; Mohammed et al. 2018(. These systems operate in sensing mode and communica- tion mode alternately and in some cases require switching its operation from wide band sensing mode to narrow band communicating mode (Borhani et al. 2016). The other aspect of CR technology is the interweave CR systems in which a UWB antenna senses the presence and the absence of the primary users and another narrow band frequency reconfigurable antenna used for communication purposes (Tawk et al. 2011, 2012). The communicating antenna of the CR system has the facility of reconfiguring its resonant frequency toward the idle frequency bands (Gahgouh et al. 2016). Similar to the conventional reconfigurable antennas, narrow band communicating antennas modify their reso- nant frequency by PIN diodes (Fallahpour et al. 2014), & Falih Mahdi Alnahwi fma3nc@mail.missouri.edu 1 Department of Electrical Engineering, College of Engineering, University of Basrah, Basra, Iraq 2 Department of Electrical Techniques, Qurna Technique Institute, Southern Technical University, Basra, Iraq 123 Iranian Journal of Science and Technology, Transactions of Electrical Engineering https://doi.org/10.1007/s40998-019-00268-6