Abstract
A compact metamaterial-MIMO antenna for WLAN applications is presented in this paper. The MIMO antenna is designed by placing side by side two single metamaterial antennas which are constructed based on the modified composite right/left-handed (CRLH) model. By adding another left-handed inductor, the total left-handed inductor of the modified CRLH model is increased remarkably in comparison with that of conventional CRLH model. As a result, the proposed metamaterial antenna achieves 60% size reduction in comparison with the unloaded antenna. The MIMO antenna is electrically small (30 mm × 44 mm) with an edge-to-edge separation between two antennas of at 2.4 GHz. In order to reduce the mutual coupling of the antenna, a defected ground structure (DGS) is inserted to suppress the effect of surface current between elements of the proposed antenna. The final design of the MIMO antenna satisfies the return loss requirement of less than −10 dB in a bandwidth ranging from 2.38 GHz to 2.5 GHz, which entirely covers WLAN frequency band allocated from 2.4 GHz to 2.48 GHz. The antenna also shows a high isolation coefficient which is less than −35 dB over the operating frequency band. A good agreement between simulation and measurement is shown in this context.
1. Introduction
Recently, social demand on multimedia communication has been rapidly increasing resulting in development of modern wireless communication systems such as Wi-Fi, WiMAX, and 3G/4G. Along with these applications, modern antennas are required to have small size and light weight. However, the typical antennas are usually large in size due to the operating wavelength, so they are difficult to meet the requirements of modern antennas. There are several techniques used to decrease the size of antenna, such as incorporating a shorting pin in a microstrip patch [1], using short circuit [2], and cutting slots in radiating patch [3, 4], by partially filled high permittivity substrate [5] or by Fractal microstrip patch configuration [6]. Besides, transmission line metamaterial (TL-MM) [7] is one of the methods that provides a conceptual way for implementing small resonant antenna [8–15]. The first proposals of using TL-MM structures at resonance to implement small sprinted antennas have been documented in [9, 10].
Wireless LANs have experienced phenomenal growth during the past several years. The new WLANs standard (IEEE 802.11n) promises both higher data rates and increases reliability. This standard is based on MIMO communication technology which has received much attention as a practical method to substantially increase wireless channel capacity without additional power and spectrum. A multiple antenna system is needed for MIMO system. However, it is difficult to integrate two or more antennas in a mobile device. There are two critical factors for MIMO antenna system. One is total size of antenna system with a limited space of mobile device. In such a way the antenna elements must be compact and be put very close. The other factor is the isolation between antenna elements. Due to the close space between antenna elements, the coupling coefficient among radiating elements is very high. This will degrade the performance of MIMO system. Therefore, it will be a real challenging task to design a MIMO antenna with small size while obtaining a very high isolation coefficient.
In this paper, a very compact metamaterial MIMO antenna is proposed. The MIMO antenna consists of two antennas which are based on composite left/right handed (CLRH) transmission lines for reducing the antenna dimension. In the proposed configuration, a defected ground structure (DGS) is employed to increase the isolation between two antenna elements. Thus, a novel metamaterial MIMO antenna is proposed which has a high isolation with only 7.5 mm () distance between antenna elements. This antenna is built on a FR4 substrate with total volume of 30 × 44 × 1.6 mm3 and has very compact radiating elements with total size of 8.92 × 32.6 mm2 and operates at the frequency band of 2.38–2.5 GHz while the values of isolation coefficients are below −35 dB over operating frequency band.
The rest of this paper is organized as follow. In Section 2, detailed designs of the single metamaterial antenna are presented. The proposed MIMO antenna is then introduced in both cases of initial and final design. The simulated and measured results are shown in Section 3, while some conclusions are provided in Section 4.
2. Design of Metamaterial MIMO Antenna
In this work, the design of the antenna is divided into two parts. In the first one, a metamaterial antenna is designed for WLAN frequency ranging from 2.4 GHz to 2.48 GHz. In the second part, the two identical single metamaterial antennas are utilized as elements to form a 2 × 2 MIMO antenna. Finally, the defected ground structure is implemented to diminish the mutual coupling of the antennas.
2.1. Design of Single Metamaterial Antenna
The configuration of metamaterial antenna is shown in Figure 2(a). The antenna is printed on a low-cost FR4 substrate with the thickness of 1.6 mm, dielectric constant of 4.4, and loss tangent of 0.02. As a reference comparison, an unloaded microstrip fed rectangular strip with the length of is chosen as the monopole radiating element. In order to maintain compact electrical length while decreasing the operating frequencies, the monopole antenna is constructed by a modified CRLH single-cell.
The model of conventional CRLH transmission line is shown in Figure 1(a). This is a mushroom-like EBG which can be interpreted by equivalent circuit depicted in Figure 1(b). From this figure, the serial left-handed (LH) capacitor () is created by two adjacent metallic patches placed on the top surface of the structure while the shunt LH inductor () created by the current flows from the metallic patch to ground plane through metallic via. Moreover, the serial right-handed (RH) inductor () is formed by the metallic patch and the shunt RH capacitor () is created due to the parallel arrangement of metallic patch and ground plane.
(a)
(b)
(c)
(a)
(b)
The equivalent circuit of proposed metamaterial antenna is shown in Figure 1(c). In this design, the metamaterial-loading is carried out in an asymmetric fashion, where serial LH capacitor () is formed between two strips separated by a distance (as shown in Figure 2(a)) while the shunt LH inductor () is formed similarly to the shunt LH one shown in Figure 1(b). Moreover, the additional LH inductor () is built up by meandered strips which connect the structure and the ground plane. Regarding RH components, the serial RH inductor () is formed by the main patch with length of and shunt RH capacitor () is formed similarly to the RH components of conventional CRLH model. As a result, a single metamaterial antenna is proposed with the size of radiating element of 8.92 × 12.6 mm2 ( at 2.4 GHz) and printed in a substrate with two dimensions of 27 × 30 mm2. Finally, the center resonant frequency of proposed metamaterial antenna is defined as follows:
2.2. Design of Metamaterial MIMO Antenna
In this design, a MIMO model is constructed by placing two single antennas side by side at the distance of 20 mm ( at 2.4 GHz) from center-to-center or 7.5 mm ( at 2.4 GHz) from edge-to-edge, making the overall the dimension of this design very compact. The layout of the MIMO antenna is shown in Figure 2(b). In order to increase the isolation between elements of MIMO antenna, a defected ground structure is etched in a part of ground between two elements. Firstly, two parallel slots are etched on the ground plane. As a result, the MIMO antenna satisfies the isolation requirement while the operating frequencies were shifted compared to the WLAN frequency band. Therefore, two I-shaped slots which are used as an impedance matching circuit are etched on the metallic ground (as shown in Figure 2(b)). The final MIMO antenna system is proposed with total size of radiating elements of 8.92 × 32.6 mm2 and satisfies all requirements of MIMO system with very high isolation between antenna elements. All the dimensions of the proposed single metamaterial antenna and MIMO antenna are given in Tables 1 and 2, respectively.
3. Results and Discussions
The performance of the proposed antennas is discussed in detail in terms of simulation and measurement results.
3.1. Single Metamaterial Antenna
As mentioned in Section 2, the resonant frequency of proposed metamaterial antenna depends on the meandered strip length which is controlled by tuning the length as well as the gap between strip steps . The simulated results of the single metamaterial antenna with different values of and are shown in Figure 3. In Figure 3, the resonant frequency reduces with the increasing the value of and . Actually, the increase of and will lead to the increase of the additional LH inductor and therefore making the decrease of the resonant frequency. This is entirely consistent with formula 1. The optimized bandwidth is obtained when the and are set at 2.8 mm and 0.3 mm, respectively. It can be seen from Figure 7 that the bandwidth of the antenna defined by the less than −10 dB entirely covers the WLAN frequency range, which is allocated from 2.4 to 2.48 GHz.
(a)
(b)
The size reduction of the proposed antenna is carried out by taking the simulated of antennas in case of loaded (proposed antenna) and unloaded (conventional antenna). The two antennas are given the same dimensions of substrate layer and radiation elements. As can be seen from Figure 4, the resonant frequency of the unloaded antenna centers at 6 GHz while the resonant one of the proposed antenna is maintained at 2.44 GHz. It is clear that the proposed antenna exhibits smaller resonant frequency than the conventional one. In this case, the proposed antenna achieves 60% size reduction in comparison with the conventional one.
Current distributions of the metamaterial antenna at the center frequency of WLAN are exhibited in Figure 5. As observed in Figure 5, the current distribution on antenna at 2.44 GHz mainly focuses on the meandered strips instead of on the radiating patch as the principle of microstrip antenna.
The radiation pattern of single antenna at the center frequency of 2.44 GHz is plotted in Figure 6. The solid lines display the -plane and the dotted lines represent -plane. It can be observed that the single antenna possesses an isotropic radiation pattern confirming its operation in the fundamental resonant mode. Therefore, its gain is small with the maximum total gain of 1.4 dB.
Finally, the fabricated single metamaterial antenna is presented in Figure 13. The simulated and measured results of of single metamaterial antenna is shown in Figure 7. From this figure, it can be observed that the antenna can operate over the range spreading from 2.4 GHz to 2.48 GHz and from 2.405 GHz to 2.495 GHz in simulation and measurement, respectively.
3.2. Metamaterial MIMO Antenna
The simulated results of reflection coefficients of the initial MIMO antenna (without DGS) are shown in Figure 8. From this figure, it is observed that the initial antenna does not satisfy the impedance matching condition due to the effect of mutual coupling. The -parameters of antenna are changed and could not meet the requirements of MIMO antenna from which and are not below −10 dB and and are not below −15 dB in WLAN band. This fact is clearly demonstrated by the surface current distribution on the initial MIMO antenna in Figure 10(a). As can be observed from Figure 10(a), when the first element (Port 1) is excited, the surface current is strongly induced on the second element (Port 2) resulting in a rise of the mutual coupling ( and ). Actually, the mutual coupling can be reduced by increasing the distance between the elements. However, this will lead to the larger size of the proposed MIMO antenna. These drawbacks of the initial MIMO antenna can be solved thanks to the use of defected ground structure etched on the common ground of MIMO antenna by the following two steps.
At first, two parallel slots are added to central ground plane between two ports (as shown in Figure 2(b)). The length slot is varied to find out the value from which the impedance matching and mutual coupling have the best solution. The optimized length of is chosen as 12 mm. Simulated -parameters of MIMO antenna with dual slots are shown in Figure 8. From this figure, it can be seen that the isolation coefficient is lower than −18 dB for all frequency in WLAN band. However, the impedance is not matched enough in this band so that the is below −10 dB over the frequency ranging from 2.42 GHz to 2.5 GHz, and therefore the antenna could not cover the WLAN frequencies.
The current distribution of MIMO antenna with the implementation of dual slots at 2.44 GHz is shown in Figure 10(b). It can be seen from Figure 10(b) that the surface current partly focuses on the slots and somewhat coupling to the radiation strips of the adjacent antenna element.
In order to solve this problem, in the second step, two I-shaped slots are etched on the ground plane and used as an impedance matching circuit. The effect of I-shaped slots to impedance matching of the MIMO antenna is investigated via the length . Figure 9 shows the simulated -parameters of the MIMO antenna for the different values of . It can be observed that the isolation coefficient is below −15 dB for all cases and the return loss is changed with the various sizes of slots. When the length of I-slots increases, the input impedance decreases make the impedance highly matched. The final MIMO antenna with full DGS is formed as the value of fixed at 5 mm. As a result, the full DGS MIMO antenna achieves high isolation coefficient which is less than −35 dB over all frequency of WLAN band while the operating bandwidth covers from 2.38 GHz to 2.52 GHz. The current distribution of the final MIMO antenna at 2.44 GHz is focused on the defected ground structure shown in Figure 10(c). Therefore, the effect of the surface current to the second element is significant reduced.
(a)
(b)
(c)
Simulated radiation patterns of final MIMO antenna in the , , and planes at 2.44 GHz when the antenna is fed; each port in turn is shown in Figure 11. The antenna displays good omnidirectional radiation patterns in the and planes (-plane) while the separate far field patterns are produced in the plane (-plane). Therefore, the diversity for the antenna is achieved. Thanks to this characteristic, the antenna is a promising candidate for MIMO system.
(a)
(b)
Figure 12 gives the simulated radiation efficiency of the proposed antenna. This figure indicates that the proposed antenna shows good radiation efficiency, which has the average value of 85% in over the operating bandwidth of WLAN system.
(a)
(b)
Figure 14 presents the measured and of the fabricated initial and final MIMO antenna shown in Figure 13. From this figure, it is observed that the final MIMO antenna can operate over the range spreading from 2.38 to 2.5 GHz which is covering the WLAN band. Meanwhile, the mutual coupling between two elements () is less than −35 dB over WLAN range. It should be noted that the measured results are in good agreement with the simulated results.
(a)
(b)
3.3. MIMO Characteristics
MIMO antennas are required to be characterized for their diversity performance. In each system, the signals can be usually correlated by the distance between the antenna elements [16]. The parameter used to assess the correlation between radiation patterns is so-called enveloped correlation coefficient (ECC). Normally, the value of ECC at a certain frequency is small in case of the radiation pattern of each single antenna differently from each other. Otherwise, the same patterns of these antennas will exhibit the larger value of enveloped correlation coefficient. The factor can be calculated from radiation patterns or scattering parameters. For a simple two-port network, assuming uniform multipath environment, the enveloped correlation (), simply square of the correlation coefficient (), can be calculated conveniently and quickly from -parameters [17], as follows:
The calculated ECC of proposed antenna by using the simulated and measured -parameters is shown in Figure 15. From this figure, the proposed MIMO antenna has the simulated ECC lower than 0.01, while the measured one has lower than 0.02 over the operating frequencies. Therefore, the proposed antenna is suitable for mobile communication with a minimum acceptable correlation coefficient of 0.5 [18].
4. Conclusions
The compact 2 × 2 metamaterial MIMO antenna is designed to operate in WLAN frequency band. By using the modified CRLH model, the proposed metamaterial antenna achieves 60% size reduction in comparison with the unloaded antenna. The defected ground structures are inserted to suppress the effect of surface current on the elements of the proposed antenna for reducing the mutual coupling. The antenna offers the compact size with the diversity radiation patterns. The fabricated MIMO antenna shows isolation less than −35 dB over its operating frequency band spreading from 2.38 to 2.5 GHz. The proposed MIMO antenna has also a minimum correlation coefficient which is less than 0.02 over the WLAN frequency range. Summing up the result, it can be concluded that the proposed antenna is a good candidate for WLAN applications.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.