1082 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 50, NO. 8, AUGUST 2002 A Quarter-Wave U-Shaped Patch Antenna With Two Unequal Arms for Wideband and Dual-Frequency Operation Yong-Xin Guo, Kwai-Man Luk, Kai-Fong Lee, Fellow, IEEE, and Ricky Chair Abstract—A wideband and dual-frequency quarter-wave U-shaped patch antenna with two unequal arms on FR4 substrate is presented. The U-shaped patch antenna is fed electromagnet- ically by another coaxially-fed patch on the same layer, which operates at a higher frequency band. In this paper, we first proposed a new technique to obtain two closely staggered resonant modes to achieve a wide impedance bandwidth using two unequal arms of the U-shaped patch for the lower band. For the substrate thickness 5, 7.5 and 11 mm, the measured bandwidths of the antennas operating around at 900 MHz are 7%, 10%, and 18%, respectively. Then, we modify the coaxially-fed patch so that both bands are wide in bandwidth. The resulting antenna operating at 900 and 1800-MHz bands with the substrate thickness 11 mm has a measured impedance bandwidth of 13.3% and 20%, respec- tively. This thickness corresponds to 0.033 free-space wavelength at 900 MHz. The far-field radiation patterns are stable in the two operating bands. The proposed antenna is simple in structure compared with the regular stacked or coplanar parasitic patch antennas. It is highly suitable for indoor mobile communications. Index Terms—Bandwidth widening, electrically small antennas, multiple band antennas, microscript antennas. I. INTRODUCTION P ATCH antennas are receiving increasing interest in various mobile communication systems since they can provide advantages over traditional whip and helix antennas in terms of high efficiency, low EM coupling to the human head, and increased mechanical reliability [1]. In many applications, the requirements on both bandwidth and physical size are quite stringent. Typically, the antenna is required to have a bandwidth exceeding 10%, a resonant length of about , and thickness not exceeding in one of the bands, where is free space wavelength. During the last two decades, many investigators have dedicated their efforts to creating new design or variations to the original antenna that produce either wider bandwidths or multiplefrequency operation in a single element [2]–[18]. Regarding the bandwidth enhancement of patch antennas, several techniques have been proposed, including the use of an impedance matching network [2], the use of multiple Manuscript received November 7, 2000; revised April 23, 2001. This work was supported by Competitive Earmarked Research Grant (CERG) 9040449, Hong Kong. Y.-X. Guo is with the Institute for Communications Research, National Uni- versity of Singapore, Singapore 119260 (e-mail: yxguo@ieee.org). K.-M. Luk and R. Chair are with the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong. K.-F. Lee is with the School of Engineering, University of Mississippi, Uni- versity, MS 38677 USA. Publisher Item Identifier 10.1109/TAP.2002.801285. resonators [3], and the use of thick substrates [4], [5]. For an electrically thick substrate patch antenna, coaxial feed is typ- ically used. However, the increased inductance introduced by the longer probe will limit the achievable bandwidth to less than 10% of the resonant frequency. For this reason, several other methods [6]–[8] have been proposed to solve this problem, in- cluding etching a small circular slot [6], cutting a U-slot [7] on a patch, and the use of an L-probe feed [8]. Unlike most of the bandwidth-widening methods in the literature, the U-slot patch antenna proposed in [7] retains the major advantage of a single patch on one layer. Subsequently, many published results have confirmed the broadband characteristics of the U-slot patch antenna or its variations [9]–[13]. Both the U-slot patch and the L-probe feed patch can attain over 30% bandwidth. However, the substrate thickness typically is about . Meanwhile, many solutions to achieve multiple-frequency operation were carried out [14]–[18], such as the multilayer stacked-patch antenna [14], the reactive-loading patch antennas by adding shorting pins and/or etching slots on a patch [15]–[17], or more recently proposed fractal antennas [18]. On the other hand, several techniques, including the use of high dielectric constant substrate and cover layers [19], the use of a short circuit [20] and the use of a short pin [21], have been proposed to reduce the size of conventional patch antennas. Significant size reductions, with the resonant length ranging from to , have been achieved by these methods, However, the bandwidth of the proposed antennas are quite narrow, typically less than 5%. Many efforts have recently been devoted to solve the bandwidth limitation of the shorted patch antennas using the above mentioned bandwidth-widening/dual-band techniques. Good results for wideband/dual-band operation have been achieved with the planar inverted-F antennas [22]–[26], the shorted slot-loaded dual-band patch antennas [27], the shorted L-probe [28] or U-slot [29] thick-substrate patch antennas, and the shorted two-layer wideband patch antennas [30]. However, previous investigations either addressed the problem of widening the bandwidth or forming multiple-frequency operation, but seldom both simultaneously. In this paper, a novel shorted U-shaped patch antenna with two unequal arms is proposed to achieve broadband and dual- frequency operation. The design achieves over 10% bandwidth in both bands, with a thickness of less than in the low band. The U-shaped patch antenna is fed electromagnetically by another coaxially-fed patch on the same layer, which oper- ates at a higher-frequency band. For the lower band, two dif- 0018-926X/02$17.00 © 2002 IEEE