The proposed antenna for the 2.180–6.410 GHz bands was constructed and tested. The results indicate that the constructed prototype shows good impedance matching within the 2.180– 6.410 GHz bands, which covers the required bandwidths for three ISM-bands operation in the 2.400–2.484, 5.150–5.350, and 5.725–5.825 GHz bands. The proposed antenna’s radiation char- acteristics were also observed. ACKNOWLEDGMENTS This work was supported by an Inha University research grant. REFERENCES 1. K.Q. Lee, K.F. Lee, and J. Bobnchak, Characteristics of a two layer electromagnetically coupled rectangular patch antenna, Elec- tron Lett 23 (1987), 1070–1072. 2. B.L. Ooi, S. Qin, and M.S. Leong, Novel design of broadband stacked patch antenna, IEEE Trans Antennas Propag 50 (2002), 1391–1395. 3. B.L. Ooi, and C.L. Lee, Broadband air-filled stacked U-slot patch antenna, Electron Lett 35 (1999) 515–517. 4. H. Ghannoum, S. Bories, and C. Roblin, Probe fed stacked patch antenna for UWB sectoral applications, in Proceedings of the IEEE International Conference on Ultra-Wideband (ICU), Sep (2005), 97–102. 5. K.F. Lee, K.M. Luk, K.F. Tong, S.M. Shum, T. Huynk, and R.Q. Lee, Experimental and simulation studies of the coaxially fed u- slot rectangular patch antenna, Proc Inst Elect Eng Microwave Antenna Propag 144 (1997), 354–358. 6. M. Sanad, Double C-patch antennas having different aperture shapes, in Proceedings of IEEE AP-S Symposium, Newport Beach, CA, Jun (1995), 2116–2119. 7. F. Yang, X.-X. Zhang, X. Ye, and Y. Rahmat-Samii, Wideband E- shaped patch antennas for wireless communications, IEEE Trans Antennas Propag 49 (2001), 1094–1100. 8. C. Yoon, S.H. Choi, H.C. Lee, and H.D. Park, Small microstrip patch antennas with short-pin using a dual-band operation, Micro- wave Opt Technol Lett 50 (2008), 367–371. 9. C. Yoon, W.K. Park, S.Y. Kang, and H.D. Park, A spiral-shaped monopole antenna for quad-band operation of mobile handsets, Microwave Opt Technol Lett 50 (2008), 2860–2863. 10. C. Yoon, W.J. Lee, S.P. Kang, S.Y. Kang, H.C. Lee, and H.D. Park, A planar CPW-fed slot antenna on thin substrate for dual- band operation of WLAN applications, Microwave Opt Technol Lett 51 (2009), 2799–2802. V C 2012 Wiley Periodicals, Inc. ULTRAWIDEBAND MONOPOLE ANTENNA FOR USE IN A CIRCULAR CYLINDRICAL MICROWAVE IMAGING SYSTEM Khodadad Halili, 1 Mohammad Ojaroudi, 2 and Nasser Ojaroudi 3 1 Department of Electrical Engineering, Aeronautical University of Science and Technology, Tehran, Iran 2 Young Research Club, Ardabil Branch, Islamic Azad University, Ardabil, Iran 3 Department of Electrical Engineering, Ardabil Branch, Islamic Azad University, Ardabil, Iran; Corresponding author: m.ojaroudi@iauardabil.ac.ir Received 18 November 2011 ABSTRACT: A novel ultrawideband printed monopole antenna for use in a circular cylindrical microwave imaging system is presented. The proposed antenna consists of a square radiating patch and a modified ground plane with an open circuit structure slit and an inverted p- shaped parasitic structure, which provides a wide usable fractional bandwidth of more than 130% (2.91–14.72 GHz). By cutting an open circuit structure in the ground plane and also by inserting an inverted p-shaped parasitic structure in the ground plane, additional resonances at 11.5 and 14.2 GHz are excited and hence much wider impedance bandwidth can be produced, especially at the higher band. Unlike other antennas reported in the literature to date, the proposed antenna displays a good omnidirectional radiation pattern even at higher frequencies, and also its radiation efficiency is greater than 87% across the entire radiating band. The designed antenna has a small size of 12 Â 18 mm 2 . Simulated and measured results are presented to validate the usefulness of the proposed antenna structure for circular cylindrical microwave imaging systems. V C 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2202–2205, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27041 Key words: printed monopole antenna; p-shaped parasitic structure; ultrawideband communication systems; circular cylindrical microwave imaging systems 1. INTRODUCTION In general, the microwave imaging system is formed by a circu- lar cylindrical array antenna to detect cancerous tissue. In this approach, circular cylindrical microwave imaging systems require small antennas with omnidirectional radiation patterns and large bandwidth [1–3]. Thus, in circular cylindrical micro- wave imaging system, one of the key issues is the design of a compact antenna while providing wideband characteristic over the whole operating band. It is a well-known fact that planar monopole antennas present really appealing physical features, such as simple structure, small size, and low cost. Because of all these interesting characteristics, planar monopoles are extremely attractive to be used in ultrawideband (UWB) applica- tions and growing research activity is being focused on them [4–8]. This article focuses on a novel UWB printed monopole antenna (PMA) for circular cylindrical microwave imaging sys- tem applications, which combines the ordinary square-patch approach, and the ground plane with an open circuit structure in the ground plane and an inverted p-shaped parasitic structure in the ground plane that achieves a fractional bandwidth of more than 125%. In this article, for the first time, by cutting an open circuit structure in the ground plane, and also by inserting an inverted p-shaped parasitic structure with variable dimensions in the ground plane, additional resonances (third and fourth resonances) are excited. By obtaining these resonances, the Figure 7 Measured gain of the proposed antenna. 2202 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 9, September 2012 DOI 10.1002/mop