A metamaterial structure placed in the space between two symmetrically printed 5.8 GHz MIMO antenna elements provides an effective means of limiting the surface waves between them in order to reduce mutual coupling. Greater than 24 dB isolation is observed while demonstrating good radiation patterns, efficiency and gain.

MIMO technology has been shown to improve wireless link transmission rates and reliability.1-2 However, when multiple antennas are spaced in close proximity, performance is degraded by mutual coupling. Researchers have introduced different methods to minimize the mutual coupling between antennas. For example, Farahani et al.3 and Coulombe et al.4 used  electromagnetic band-gap (EBG) structures. Others have introduced defecting ground structures (DGS), or slits, in the antenna ground.5-7 Tang et al.8 and Liu et al.9 used decoupling structures between two closely spaced radiating elements to reduce the mutual coupling for UWB applications.

Figure 1

Figure 1 Metamaterial unit cell; 2D view (a), 3D view (b).

Figure 2

Figure 2 Metamaterial permeability, real and imaginary parts.

Metamaterials, possessing distinctive electromagnetic properties, have attracted much interest over the last decade. Metamaterial units are usually repeatedly arranged with a scale of a sub-wavelength. At the macroscopic level, negative values of dielectric permittivity, magnetic permeability or both can be achieved by carefully choosing the shape of the metamaterial units and adjusting structural dimensions.10

Recently, metamaterial structures have been exploited in the design of MIMO antennas. For example, a waveguide metamaterial was inserted between two microstrip patches by Yang et al.,2 and the mutual coupling between the two antenna elements was reduced by about 6 dB from 3.5 to 3.55 GHz. With a rectangular loop resonator, good isolation between two monopoles and three monopoles at 2.45 GHz was achieved by Ketzaki and Yioultsis.10 A complementary split ring resonator (CSRR) was proposed for antenna miniaturization in a 2.45 GHz ISM band application,11 and a metamaterial spiral resonator at 5.5 GHz reduced MIMO system performance degradation caused by strong mutual coupling among four patch elements.12 In this article, a compact MIMO antenna uses a rectangular loop resonator as the metamaterial unit.

ANTENNA DESIGN

Figure 3

Figure 3 Antenna geometry; top view (a), bottom view (b).

First introduced by Pendry,13 the split ring resonator (SRR) has been utilized in many forms in the design of MIMO antennas due to its property of negative magnetic permeability. We use a split rectangular loop structure for this purpose. By altering the geometry and dimensions of this particular kind of metamaterial cell, it exhibits better controllability of the gap capacitance than other resonators.

Rectangular Loop Resonator Design

In Figure 1 a unit cell is etched on a 1.6 mm thick FR-4 epoxy substrate with relative permittivity εr =  4.6 and loss tangent tan δ = 0.019 at 5.8 GHz. Figure 1a shows a two-dimensional view of the unit cell with dimensions: a = 4.2 mm, b = 2.4 mm, c = 1.7 mm, d = 1 mm, e = 4.8 mm, f = 3 mm and s = 0.2 mm. Figure 1b shows unit cell dimensions of 3 × 4.8 × 1.6 mm3 on an xyz axis. An incident plane electromagnetic wave propagates in the x direction towards the unit cell with the magnetic field of the wave oriented along the z-axis and the electric field oriented along the y-axis. The boundary walls along the y-axis (at the xz-oriented sides) are considered perfect electrical conductors. The effective magnetic permeability is determined using the parameter retrieval technique.10,14 As shown in Figure 2, the real part of the permeability at 5.8 GHz is negative.

Metamaterial-Based MIMO Antenna

The microstrip array antenna (see Figure 3) consists of a ground plane with two open L-shaped slots (bottom view) and two U-shaped patch elements (top view) on an FR-4 substrate. Four rows of split rectangular loop resonators are used to minimize mutual coupling between the two radiating elements. Each row contains three elements. The single SRR structure of Figure 1 is sufficient to provide a perfect inductance, but is still small enough to satisfy the condition of having subwavelength dimensions. Table 1 lists the dimensions of the MIMO antenna in Figure 3.

Table 1

Figure 4

Figure 4 Reflection coefficient for various lengths of c.

EXPERIMENTAL RESULTS

The antenna is modeled in HFSS. SRR dimensions are fixed values except for the gap capacitance, which is controlled by the length of c. Figure 4 shows the reflection coefficient for various lengths of c from 1.6 to 1.75 mm. The resonant frequency changes, correspondingly. When c = 1.7 mm, resonance occurs at 5.8 GHz.

Characteristics of the antenna (see Figure 5) are measured in an anechoic chamber and compared with simulation, demonstrating good agreement. The fabricated antenna resonates at 5.8 GHz with a reflection coefficient of -32.7 dB in the simulation and at 5.82 GHz with a reflection coefficient of -26.2 dB in the measurement. The antenna has an impedance bandwidth of approximately 1040 MHz (4.93 to 5.97 GHz) at 10 dB return loss.

Figure 5

Figure 5 Measured and simulated reflection coefficient vs. frequency.

Figure 6

Figure 6 Measured and simulated isolation.

In Figure 6, simulated and measured isolation of the two-element MIMO antenna are shown. At 5.8 GHz, isolation is 15.3 dB without the SRRs. With the SRRs placed between the two antenna elements, isolation is increased to 27.5 (simulated) and 24.6 dB (measured); i.e., coupling is reduced by approximately 12.2 and 9.3 dB, respectively. The small frequency shift between measured and simulated results in Figures 5 and 6 is attributed to manufacturing tolerances.

Figure 7

Figure 7 Measured and simulated radiation patterns at 5.8 GHz; x-z plane (a), y-z plane (b).

In Figure 7, measured and simulated radiation patterns of the MIMO antenna in the x-z (H) and y-z (E) planes at 5.8 GHz are compared when one port is excited the other port is terminated with a 50 Ohm load. The results show that the antenna radiates with a quasi-omni-directional characteristic. Again, measured results agree well with simulation. The measured peak gains of the antenna are shown in Figure 8. The gains are about 1.9 to 3.7 dBi in the 5 to 6.5 GHz band and 2.14 dBi at 5.8 GHz. Measured radiation efficiency (see Figure 9) is 71 percent at 5.8 GHz.

Figure 8

Figure 8 Measured antenna gain.

Figure 9

Figure 9 Measured radiation efficiency.

CONCLUSION

A small sized (25 × 40 × 1.6 mm3) printed microstrip-fed slot antenna has been designed and demonstrated. Within a small available space (0.166λ) between the MIMO radiating elements, a metamaterial-based negative permeability structure is placed as a means to reduce mutual coupling. With this technique, mutual coupling is reduced by 9 dB at the operating frequency. It compares favorably with other decoupling techniques in providing an effective approach for controlling propagation between closely spaced microstrip patches. In addition, full planarity of the MIMO antenna is preserved, while employing a simple and straightforward fabrication process.

References

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