Trident-shape strip loaded dual band-notched UWB MIMO antenna for portable device applications

Abstract

In this paper, a novel compact dual band-notched ultra-wideband (UWB) multiple-input multiple-output (MIMO) antenna of size 22 × 26 × 0.8 mm³ is proposed for portable devices. The antenna comprises of two stepped slot UWB antennas fed by 50 ohms microstrip line, T-shape slot and narrow slot. Dual band-notches from 5.4 to 5.86 GHz and 7.6–8.4 GHz are achieved by loading trident-shape strips on microstrip line. A T-shape slot is used on the ground plane to enhance impedance matching characteristics and to minimize mutual coupling above 4 GHz. To improve isolation further at 3–4 GHz, a narrow slot is used on ground. The proposed antenna is giving a good bandwidth ranging from 3.1 to 11.8 GHz with |S₁₁| > 10 dB and mutual coupling larger than 20 dB in the entire operating band except at two rejected bands. The simulation and measurement results demonstrate that the antenna is more suitable for portable device applications.

Introduction

Multiple-input multiple-output (MIMO) is a promising system for increasing communication data rate without extra bandwidth or total transmission power. Thus, MIMO system has been grabbing more attention in recent times. MIMO system uses multiple transmitting and receiving antennas [1]. Several MIMO antenna designs have been proposed for different wireless communication systems like UMTS and WLAN for uses in portable devices [2], [3], [4]. It is well known that use of MIMO system technology in UWB system significantly provides high channel capacity, high rate of transmission, and better immunity to multipath fading as compared to that in narrow-band system [5]. As space is limited in portable devices, maintain less mutual coupling between antennas is always big challenge. Some experiments were done to mitigate the effects of coupling in UWB MIMO antenna systems such as use of small strip [6], tree-like structure [7], annular slot [8], and protruding ground structure [9].

Ultra-wideband technology has been developing rapidly, since Federal Communications Commission (FCC) officially authorized unlicensed band of frequencies from 3.1 to 10.6 GHz for UWB applications with less emission power level (less than −41.3 dBm/MHz) [10]. However, UWB band easily interfere with existed narrow-band systems like wireless local area network (5.15–5.35 and 5.725–5.825 GHz) and X-band communication services (7.9–8.395 GHz). Therefore, to minimize the interference problem with other systems, UWB antennas with band notched characteristics has to be designed. UWB MIMO antennas to achieve single notch at WLAN band were proposed in [11], [12], [13], [14], [22], which includes the use of two stubs along the ground edges [11], placing of split ring resonators on the patch [12], loading of strips on the ground plane [13], receded steps with protruding ground structure [14], and etching of λ/2 and λ/4 resonators [22]. Several compact UWB MIMO antenna designs exhibiting dual notched functions to reject the interference at WLAN and WiMax/X-bands were presented in [15], [16], [17], [18], [19]. In [15], pair of inverted L-shape strips added on ground to obtain notch at WLAN band and slots on the radiating patch to get notch at WiMax band. Two metal strips having lengths of λ/3 and λ/4 are used on the radiator in [16], two metal strips are loaded on ground plane in [17], meandered slots are etched on the patches and C-shape strips near to the feeding lines in [18], open -ended slots and split ring resonators are used in the ground in [19], use of C-shaped slot on radiating element and incorporating an L-shaped strip on ground plane in [20], and etching two nested C-shaped slots on the patch in [21] were proposed to achieve dual band notches. Thus, band-notch filter structures incorporated in [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22] is more complex and also it occupies more space.

In this paper, an ultra-wideband MIMO antenna with dual band-stop functions is proposed. The MIMO antenna has a size of 22 mm × 26 mm = 572 mm², which is very small compared the antenna proposed by Zhu in [19]. Two defected ground structures are used to improve isolation in the UWB band. T-shape slot extends the current path and suppress the surface currents. Hence, the mutual coupling of <−20 dB is obtained which is very less compared to the mutual coupling obtained by the antennas in [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. A narrow slot further minimizes the mutual coupling between antennas from 3 to 4 GHz. A simple notch filter is implemented by trident-shaped strips which are loaded on microstrip lines to obtain dual notched characteristics at 5.7 and 8.2 GHz with efficiency of about 9% at the notch frequencies.