A quarter-wave U-shaped patch antenna with two unequal arms for wideband and dual-frequency operation

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 electromagnetically 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 respectively. 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.

I. Introduction

Patch 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 resonators [3], and the use of thick substrates [4], [5]. For an electrically thick substrate patch antenna, coaxial feed is typically 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] [7] [8] have been proposed to solve this problem, including 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] [10]–[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 Fig. 1.

Geometry of the antenna with .

Fig. 2.

Measured SWR of the antenna in Fig. 1.

shorting pins and/or etching slots on a patch [15]–[17], or more recently proposed fractal antennas [18].