Published in IET Microwaves, Antennas & Propagation Received on 12th February 2008 Revised on 16th October 2008 doi: 10.1049/iet-map.2008.0210 ISSN 1751-8725 Compact wideband Koch fractal printed slot antenna D.D. Krishna M. Gopikrishna C.K. Aanandan P. Mohanan K. Vasudevan Center for Research in Electromagnetics and Antennas (CREMA), Department of Electronics, Cochin University of Science and Technology, Cochin 682022, India E-mail: dasdeepti@yahoo.co.in Abstract: A compact wideband printed slot antenna, suitable for wireless local area network (WLAN) and satisfying the worldwide interoperability for microwave access (WiMAX) applications, is proposed here. The antenna is microstrip-fed and its structure is based on Koch fractal geometry where the resonance frequency of a conventional triangular slot antenna is lowered by applying Koch iterations. The antenna size inclusive of the ground plane is compact and has a wide operating bandwidth. The proposed second iteration Koch slot antenna operates from 2.33 to 6.19 GHz covering the 2.4/5.2/5.8 GHz WLAN bands and 2.5/3.5/5.5 GHz WiMAX bands. The antenna exhibits omnidirectional radiation coverage with a gain better than 2.0 dBi in the entire operating band. Design equations for the proposed antenna are developed and their validity is confirmed on different substrates and for different slot sizes. 1 Introduction Rapid developments in wireless technologies demand the integration of the various standards like 2.4/5.2/5.8 GHz wireless local area network (WLAN), and 2.5/3.5/5.5 GHz worldwide interoperability for microwave access (WiMAX) into a single unit. Such units would require antennas that are not only low profile and lightweight but also simultaneously operational at the desired frequency bands. In addition, recent years have witnessed great interest in wideband antenna designs primarily to realise high data rate wireless transmission demanded by the continuously expanding range of wireless communication services. There has been particular emphasis in printed slot antennas, especially wide-slot antennas, as they are completely planar and have a wide operating bandwidth [1, 2]. Several studies on printed wide-slot antennas indicated that the impedance bandwidth of the antenna is controlled by the coupling between the tuning stub and the slot. Bandwidth enhancement is achieved by employing a feeding scheme that generates multiple resonances. Then, by optimising the distance between the tuning stub and ground surrounding it, the impedance change from one resonant mode to the other is minimised, resulting in wideband operation. Fork-like, circular, elliptical, rectangular and inverted cone shapes of the tuning stubs have been reported. Microstripline-fed slot antennas implementing these stub designs in combination with different types of slot geometries, like rectangular, circular, elliptical, triangular, rotated square, rhombus, inverted cone and so on, are reported for wideband operation [3–10]. In spite of the improved feed designs and slot geometries, these wideband slot antennas (inclusive of ground planes) are relatively large when scaled for operation at the 2.4 GHz WLAN band [3–5] and often do not cover the 2.4 GHz (2.4 – 2.484 GHz) and 5.2/ 5.8 GHz (5.15 – 5.35 GHz/5.75 – 5.825 GHz) WLAN the 2.5/3.5/5.5 GHz (2.5 – 2.69 GHz/3.4 – 3.69 GHz/5.25 – 5.85 GHz) WiMAX bands simultaneously [6–10]. The objective of this paper is to design a compact wideband slot antenna covering WLAN/WiMAX frequency bands. The lower edge of the operating frequency band of a wide- slot antenna is observed to be dependent on the polygonal slot and more specifically its perimeter [5–7]. Hence, the concept of space-filling characteristics of fractal curves, which is used in the design of compact and multiband patch 782 IET Microw. Antennas Propag., 2009, Vol. 3, Iss. 5, pp. 782–789 & The Institution of Engineering and Technology 2009 doi: 10.1049/iet-map.2008.0210 www.ietdl.org