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Wheeler, Introduction to microwave, Prentice/Hall of India Pri- vate Limited, New Delhi, 1974. 21. C.A. Balanis, Antenna theory analysis and design, Wiley, New York, 2008. 22. E.A. Wolf, Antenna analysis, Artech House, Inc, Norwood, MA, 1988. 23. Ansys High Frequency Structure Simulator (HFSS), Ansoft Corp., Pittsburgh, PA. Available at: http://www.ansys.com. V C 2015 Wiley Periodicals, Inc. ISOLATION ENHANCEMENT OF A WIDEBAND MIMO ANTENNA USING FLOATING PARASITIC ELEMENTS Muhammad Saeed Khan, 1,2,4 Antonio-Daniele Capobianco, 1 Muhammad Farhan Shafique, 3,5 Bilal Ijaz, 2 Aftab Naqvi, 2 and Benjamin D. Braaten 2 1 Dipartimento di Ingegneria dell’Informazione, University of Padova, Via Gradenigo 6/b, 35131 Padova, Italy; Corresponding author: Khan@dei.unipd.it 2 Department of Electrical and Computer Engineering, North Dakota State University, Fargo, ND 58102 3 Center for Advanced Studies in Telecommunication, COMSATS Institute of Information Technology, Park, Road, Islamabad, Pakistan 4 Department of Electrical and Computer Engineering, COMSATS Institute of Information Technology, Park, Road, Islamabad, Pakistan 5 Department of Computer Engineering, COMSATS Institute of Information Technology, Sahiwal, Pakistan Received 1 January 2015 ABSTRACT: A Multiple-input multiple-output antenna array with two radiating elements having a wide bandwidth is reported in this work. Spatial diversity has been introduced to achieve the diversity gain and the array was kept compact by introducing five parasitic decoupling ele- ments on the bottom of the substrate; each having a length equal to k/2 at a specific frequency. Each resonant element offers a resonant band- width of 1 GHz starting from 3 GHz to 8.5 GHz. Good agreement between the measured and simulated results shows that the antenna sys- tem performs very well over the frequency range 3 GHz to 8.5 GHz. In addition, an isolation of more than 15 dB is achieved with the help of the parasitic elements, while keeping an edge-to-edge and center-to- center separation of 4 mm and 19 mm, respectively. The proposed antenna measures 26 3 40.5 mm 2 , and it is suitable for handheld devi- ces, personal digital assistants, next generation home entertainment sys- tems, and robots. V C 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1677–1682, 2015; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.29162 Key words: parasitic elements; multiple-input multiple-output; ultrawi- deband; envelop correlation coefficient; Rayleigh fading 1. INTRODUCTION Rapid growth in modern wireless technologies has attracted antenna engineers from all over the world to propose a number of novel techniques and efficient designs. Researchers have shown that the channel capacity of the systems can be increased using wideband Multiple-input multiple-output (MIMO) techniques and not modifying the power limits of the systems [1]. To implement a MIMO antenna, the power dissipation at the coupled port can be reduced with various decoupling methods. Among these tech- niques are electromagnetic band-gaps and neutralizing lines [2,3]. Other techniques involve altering the ground plane by adding stubs or introducing defective ground structures (DGSs) [4,5]. In [4], cone-shaped elements with DGSs have been proposed; and the bandwidth of the antenna is from 3.1 GHz to 5.8 GHz with the dimensions of 60 3 62 mm 2 . Furthermore, the diversity antenna in [5] employs a DGS on a 2 3 2 MIMO array and achieved a 4 GHz bandwidth from 2 GHz to 6 GHz. In another recently proposed diversity antenna, a tree like structure has been introduced on the ground plane [6]. Similarly various stubs have been introduced on the ground plane in other studies to achieve the bandwidth from 3 GHz to 10.2 GHz with the dimensions of 60 3 62 mm 2 and 27 3 47 mm 2 , respectively [7,8]. The afore- mentioned work has been progressive; however, it can be seen that the proposed designs have trade-offs between the operating bandwidth and the antenna compactness. The parasitic elements (which are not directly connected to the transmitter or receiver) are traditionally used in a Yagi-Uda antenna or quasi-Yagi printed elements as a reflector or director [9]. Usually parasitic elements absorb the electromagnetic waves from the nearby driven elements and reradiate them in a desired fashion. The parasitic elements can also be used to improve the antenna functionality such as beam steering, gain enhancement, beam forming, bandwidth enhancement, radiation efficiency, and multiband radiation [10–14]. The parasitic elements are also used for narrow band MIMO antennas [15]; however, these structures are either complex (multidimensional), too large to be used in small devices or in wideband applications and few design guidelines were presented. In this work, the new antenna in Figure 1 is being proposed and can be analytically designed to operate in specific frequency ranges by adding or removing particular parasitic elements, where their lengths are based on the half wavelength of their central frequency. To achieve this novel functionality, five para- sitic elements have been introduced on the bottom side of the MIMO antenna. Each parasitic element is a long horizontal strip having a specific length but their widths are fixed at 2 mm in the initial design. The separation between the parasitic elements was kept at 3 mm, while the antenna edge separation s p was kept at 4 mm and the center to center separation s c was kept at 19 mm. These elements improve the diversity performance and reduce the size of the antenna by keeping the mutual coupling at an appreciable low level. 2. SIMULATION AND MEASUREMENTS The antenna geometry with the l 1 5 l 3 5 2 mm wide parasitic elements in Figure 1 was first designed in full wave EM simula- tor (HFSS). Then, both prototypes with the l 1 5 l 3 5 2 mm wide parasitic elements and with the l 1 5 1 mm, l 3 5 2 mm wide par- asitic elements were designed and fabricated on a Rogers TMM4 substrate with a thickness of 1.524 mm, a dielectric con- stant of 4.5, and a loss tangent 0.002. The fabricated prototypes are shown in Figure 2(a) and the overall dimensions of the final proposed prototype were 26 3 40.5 mm 2 . DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 57, No. 7, July 2015 1677