IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008 535 CPW-Fed Aperture-Coupled Slot Dipole Antenna for Tri-Band Operation Shih-Yuan Chen, Member, IEEE, You-Chieh Chen, and Powen Hsu, Senior Member, IEEE Abstract—A uniplanar aperture-coupled slot dipole antenna capable of tri-band operation is presented. Design considerations for the three operating frequencies are extensively discussed. The lower and middle operating frequencies can be obtained by varying the lengths of the lower and middle pairs of the protruded slots, respectively, while the upper frequency can be determined by adjusting the lengths of the upper protruded slot pair and the coupling aperture pair. Within each band, slot-dipole-like radiation patterns, low cross-polarized radiations, and moderate antenna gains can be observed. Index Terms—Coplanar waveguides, multifrequency antennas, slot antennas. I. INTRODUCTION W ITH rapid development occuring in wireless communi- cations, the integration of heterogeneous wireless sys- tems into a single platform has recently received considerable attention. As a result, the demand for antennas with multi-fre- quency operation capabilities, compact size, low cost, and high efficiency has increased drastically. The coplanar waveguide (CPW)-fed slot antenna has become a promising candidate due to its uniplanar structure, wide bandwidth, and easy integra- tion with RF front-end circuitry. To our knowledge, however, only a few slot antenna elements using CPW as feed lines have been proposed to possess dual- or multi-frequency capabilities [1]–[8]. Slot rings and slot loops are the fundamental structures commonly used in [3]–[6], but they inherently produce high cross-polarized radiations. On the other hand, when designing multi-frequency antennas, the coupling effects among the reso- nant or radiating parts for different operating frequencies often complicate the design process. Therefore, it is ideal to have inde- pendent design parameters for allocating different operating fre- quencies. In this paper, a CPW-fed aperture-coupled slot dipole antenna based on the authors’ previous dual-frequency design [8] is proposed to be capable of tri-band operation and to ex- hibit satisfactorily low cross-polarization levels throughout each band. Moreover, the three operating frequencies can be allo- cated almost independently. When measured against the radial Manuscript received August 01, 2008; revised September 05, 2008. First published September 19, 2008; current version published nulldate. This work was supported by the National Science Council, Taiwan, under Contracts NSC 96-2752-E-002-002-PAE and NSC 97-2221-E-002-058. S.-Y. Chen is with the Department of Electrical Engineering and the Graduate Institute of Communication Engineering, National Taiwan University, Taipei 10617, Taiwan (e-mail: sychen@cc.ee.ntu.edu.tw). Y.-C. Chen and P. Hsu are with the Department of Electrical Engineering and the Graduate Institute of Communication Engineering, National Taiwan Univer- sity, Taipei 10617, Taiwan (e-mail: phsu@cc.ee.ntu.edu.tw). Digital Object Identifier 10.1109/LAWP.2008.2006071 Fig. 1. Geometry of the tri-band aperture-coupled slot dipole antenna fed by a CPW. slot antenna presented in [7], the advantages of the proposed aperture-coupled slot dipole antenna are its tri-band operation capability and wider tuning range for the ratio of the upper res- onant frequency to the lower one, while the main disadvantage is the slightly higher cross-polarization levels due to the less sym- metric structure. II. ANTENNA STRUCTURE AND DESIGN The geometry of the proposed aperture-coupled slot dipole antenna fed by a CPW is depicted in Fig. 1. A pair of collinear rectangular apertures with length and width is placed trans- verse to the CPW feed line. Three parallel pairs of thin slots, namely the upper, middle and lower pairs, protrude outward from the ends of the rectangular aperture pair. Among them, the middle and lower pairs, which are chosen to be longer than the rectangular aperture pair, are the main radiators for the middle and lower operating frequencies and , respectively. Since the rectangular aperture pair acts as open circuits as well as cou- pling apertures for the radiators of and , the middle and lower pairs of the protruded slots are open-ended at the pro- truding junctions. With one end open and the other short-cir- cuited, the lengths of the middle and lower protruded slot pairs and correspond to quarter-guided wavelengths of the slots resonating at and , respectively. In this paper, it is assumed for simplicity that the lower pair of protruded slots is longer than the middle pair or . However, the above discussion also applies for , with the resultant oper- ating frequencies for the middle and lower protruded slot pairs modified to and , respectively. For the upper operating frequency to be practically higher than and , the upper pair of the protruded slots is chosen to be much shorter than the rectangular aperture pair as well as the lower and the middle protruded slot pairs. At , the rectan- gular aperture pair functions as not only the coupling apertures but also a portion of the radiator. The reason may be that the rect- angular aperture pair, which is longer than the upper pair of the 1536-1225/$25.00 © 2008 IEEE Authorized licensed use limited to: National Taiwan University. Downloaded on February 18, 2009 at 03:13 from IEEE Xplore. Restrictions apply.