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Over the past decade, extensive research on broad dual- or multi-band antennas, with low-profile, lightweight, flush mounted and single-feed to fit the limited equipment space of portable wireless devices, has contributed to the development of modern communication technology. In the past, a whole host of dual- or multi-band planar antennas fed by either a microstrip line or a probe have been developed for various communication-related applications. These antenna designs include planar inverted-F antennas (PIFA),^1,2 a microstrip-fed antenna³ and planar monopole antennas.^4,5 However, a new coplanar waveguide structure, used to design a dual- or multi-band planar antenna, has recently received much attention due to its potential in providing various required radiation features, good impedance matching, dual- or multi-band operation, easy integration with system circuits, omnidirectional radiation pattern, etc.^6-8

In this article, a dual-band monopole antenna, with both a simple structure and simultaneous wide operating bands, is proposed and investigated, which is suitable for the international mobile telecommunication (IMT)-2000 and wireless local-area networking (WLAN) communication systems. The proposed antenna is fed by a CPW structure and has a simple uni-planar geometry. The dual-band characteristic is achieved by adding lateral patches to the pentagonal radiating monopole. In addition to the details of the antenna design, the dual-band impedance matching, the radiation patterns and gains are also presented and discussed.

Antenna Configuration and Design

The geometrical configuration of the proposed lateral pentagonal CPW-fed monopole antenna for dual-band operation is depicted in Figure 1, with dimensions in millimeters. The antenna, including the ground plane, is printed symmetrically with respect to the longitudinal direction (z-axis) on only one side of an FR4 dielectric substrate (thickness = 1.6 mm and relative permittivity = 4.4); the other side is without any metallization. The basis of the antenna structure is a pentagonal radiating patch, which has the selected dimensions of 9 x 10 mm and is cut with a triangle notch of 3 x 3 mm at each of its two lower corners. The feed is a CPW structure with two sections for the central feeding strip. The top end of the upper section with length = 10.5 mm and width = 3 mm is connected to the pentagonal patch, while each side of the lower section with dimensions of 14.5 x 4 mm has a spacing of 0.8 mm away from a rec-tangular ground of height = 12 mm and width = 10.5 mm. In this design, to obtain good dual-band operation, a rectangular patch of length = 8.5 mm and width = 5.8 mm is added on each side of the top end of the feed strip as a lateral element. The spacing between the lateral patch and the central feed strip is chosen to be 1.5 mm; the width of the thin strip used to connect this patch and the feed strip is 0.5 mm. These antenna dimensions were obtained from theoretical analysis using the IE3D^TM simulation software to achieve dual-band operation in the 2.45 and 5 GHz WLAN bands.

Figure 2 shows the simulated frequency responses of the return loss for the proposed antenna (denoted as curve (i)), of the proposed antenna without the central pentagonal monopole (denoted as curve (ii)) or without the two lateral patches (curve (iii)). Obviously, for the proposed antenna, three resonant modes are excited at 2.45, 5.4 and 6.4 GHz. The two higher modes are close and thus form a broad operating band. The two wide separate operating bands have been obtained, with 10 dB impedance bandwidths of 250 MHz (2.32 to 2.57 GHz) and 3.04 GHz (4.52 to 7.56 GHz) at 2.45 and 5.4 GHz, respectively. However, for the case without the central pentagonal monopole, the lower resonant mode band vanishes; only the higher resonant mode at approximately 5 GHz exists. As for the case without the two lateral patches, though it seems that there exists two resonant modes at the lower (2.3 GHz) and upper (7 GHz) bands, the impedance matching is very poor. These results clearly show that the pentagonal monopole principally controls the lower resonant mode, while the added two lateral patches significantly affect the higher frequency modes as well as improve the impedance matching condition to thus enhance the bandwidths.