5. R. Mittra, Computer techniques for electromagnetics, Pergamon Press, Oxford, 1973. 6. Hao Ling, R.C. Chou, and S.W. Lee, Shooting and bouncing rays: calculating the RCS of an arbitrarily shaped cavity, IEEE Trans Antennas Propag 37 (1989), 194–205. 7. S.K. Jeng, Near-ﬁeld scattering by physical theory of diffraction and shooting and bouncing rays, IEEE Trans Antennas Propag 46 (1998), 551–558. 8. D.L. Mensa, High resolution radar cross-section imaging, Artech House, Norwood, MA, 1991. 9. K.T. Kim, D.K. Seo, and H.T. Kim, Efﬁcient classiﬁcation of ISAR images, IEEE Trans Antennas Propag 53 (2005), 1611–1621. V C 2010 Wiley Periodicals, Inc. A HIGHLY COMPACT SEMIELLIPTIC SHAPE ULTRAWIDEBAND MONOPOLE ANTENNA M. Naser-Moghadasi, 1 H. Rousta, 1 and B. S. Virdee 2 1 Faculty of Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran; Corresponding author: mn.moghaddasi@srbiau.ac.ir 2 Faculty of Computing, London Metropolitan University, 166-220 Holloway Road, London N7 8DB, United Kingdom Received 30 March 2010 ABSTRACT: This article describes a compact microstrip-fed planar monopole antenna, which comprises of a semielliptic shape radiation element on a partially modiﬁed ground-plane. This antenna is different from the traditional monopole antenna as its ground-plane is arc-shaped near the vicinity of the radiating element, and it embodies two square slots disposed on either sides of the feed-line. The modiﬁcations implemented to the ground-plane improve the antenna’s input impedance bandwidth and its radiation performance across the ultrawideband (UWB) frequency range. The resulting antenna prototype exhibits a return-loss lower than 10 dB between 3.1–11.5 GHz, and a radiation pattern that is comparable with the traditional monopole antenna. V C 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:229–231, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25674 Key words: UWB; semielliptic; microstrip 1. INTRODUCTION Until recently, much effort has been expended by researchers in academia and industry to overcome the technical difﬁculties in the development of ultrawideband (UWB) system, in particular antennas that satisfy the stringent regulation of spectrum mask deﬁned by Federal Communication Commission (FCC). Also of particular importance in UWB systems is the development of techniques to mitigate EM interference with other communica- tion systems sharing the same frequency spectrum. UWB spec- trum is deﬁned by FCC to be between 3.1 and 10.6 GHz and is intended mainly for short-range peer-to-peer ultrafast communi- cations that enable high-capacity data transmission [1]. The spectrum allocation has excited antenna designers to seek vari- ous solutions to satisfy the challenge of developing low-cost and compact UWB antennas. The task of developing antennas for UWB systems to meet the aforementioned requirements is very demanding. Several monopole-like antennas over the ground- plane have been proposed to support UWB communications [2, 3]. However, these antennas are relatively large and their struc- tures make them difﬁcult for low proﬁle system integration. In addition, UWB antenna designs have been reported to have high electric near ﬁelds that easily cause unwanted coupling to nearby objects [4]. Broadband planar monopole antennas have received considerable attention owing to their attractive merits, such as ultrawide frequency band, good radiation properties, simple structure, and the ease of fabrication. The typical conﬁg- urations of these antennas are in the form of half-disc [5], circle, ellipse [6, 7], and rectangle [8]. In this article, a microstrip-fed planar UWB antenna is pro- posed. The semielliptic-shaped radiation patch has a ground- plane, which has an arc-shape in the vicinity of the radiating patch and two square-shaped slots on either side of the feed- line. The modiﬁcation to the antenna enables it to provide an UWB performance bandwidth between 3.1 and 11.5 GHz. In addition, the antenna has a compact size of 22  22 mm 2 . The proposed antenna’s measured performance exhibits a return-loss greater than 10 dB across a frequency range from 3.1 to 11.5 GHz. 2. ANTENNA CONFIGURATION In this section, the conﬁguration of UWB antenna is described. Figure 1 shows the geometry of the proposed antenna. It con- sists of a semielliptic radiation patch with a partially modiﬁed ground-plane comprising of two square slots. This change ena- bles the antenna to achieve a broad bandwidth characteristic. The primary radiating element and the microstrip feed-line are etched on one side of substrate with a ground-plane on the other side. The radiation patch is of a semiellipse conﬁguration with major axis of length 15.8 mm and the axial ratio of 1.16. The antenna performance can be further enhanced in the high-fre- quency band by creating an arc-shape in the ground-plane near the radiating patch that follows approximately the curvature of the radiating patch. The gap between radiation patch and ground-plane is denoted as g. In the design, the microstrip-line feeding the antenna element has a characteristic impedance of 50 X that has a corresponding width of 2 mm. The antenna was fabricated on FR4 substrate with thickness of 1.6 mm, relative permittivity of 4.4, and loss tangent of 0.02. The fabricated antenna is shown in Figure 2. 3. SIMULATION AND MEASUREMENT RESULTS In this section, the proposed antenna was analyzed using Ansoft’s high-frequency structure simulator, and the fabricated antenna’s performance was measured. The measured and simu- lated return-loss characteristics of the optimized microstrip-fed Figure 1 Geometry of the proposed antenna deﬁning its physical pa- rameters in both top and bottom layer. (Units: mm) Where g ¼ 1.27 mm and L 2 ¼ 3.5 mm. [Color ﬁgure can be viewed in the online issue, which is available at wileyonlinelibrary.com] DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 1 January 2011 229