Compact Dual-Band MIMO Antenna with Complementary Isotropic Open Resonant Ring for Mutual Coupling Reduction

Article information

J. Electromagn. Eng. Sci. 2026;26(1):10-17

Publication date (electronic) : 2025 August 6

doi : https://doi.org/10.26866/jees.2026.1.r.310

Xuemei Zheng^1,, Ziwei Zhao², Yuwen Pan³

¹KeyLaboratory of Modern Power System Simulation and Control and Renewable Energy Technology, Ministry of Education, School of Electrical Engineering, Northeast Electric Power University, Jilin, China

²School of Electrical Engineering, Northeast Electric Power University, Jilin, China

³Sainty-Tech Communications Ltd., Nanjing, China

^*Corresponding Author: Xuemei Zheng (e-mail: zhengxuemei@neepu.edu.cn)

Received 2024 November 9; Revised 2025 February 16; Accepted 2025 May 9.

Abstract

I. Introduction

Recent advancements in high-data-rate wireless transmission have created an expansive market for planar antennas characterized by high channel capacity [1]. The development of next-generation wireless communication systems necessitates higher data rates and increased channel capacities. However, in complex propagation environments, transmitted signals are limited by multipath fading and interference [2]. At the same time, the multi-band requirements and physical size limitations of most wireless devices and the physical size limitations of the devices require the miniaturization of multi-antenna systems. These conflicting requirements, which were previously impossible to satisfy by employing a single antenna unit, can now be fulfilled using multiple-input multiple-output (MIMO) antenna technology [3]. MIMO antennas can exponentially increase the capacity and spectrum utilization of the communication systems under the condition that the network bandwidth remains unchanged [4].

Owing to the growing demand for miniaturized antenna devices in modern mobile communication technologies, the distance between MIMO multi-antenna systems are usually placed very in close proximity, which induces strong mutual coupling [5]. In other words, it is very difficult to tightly integrate multiple antennas in a size-constrained device while also maintaining a good degree of isolation between the neighboring radiating units [6], since it would affect the compactness of cell spacing. Moreover, excessive mutual coupling can adversely affect the overall diversity performance of a MIMO system.

Various methods have been investigated to reduce mutual coupling between antenna patch units, including the addition of defective ground structures [7] and loading electromagnetic bandgap (EBG) structures [8], supersurface structures [9], or metamaterial structures [10]. Notably, metamaterials are novel artificial materials that can improve isolation without increasing the size and complexity of an antenna system.

The continuous progress of communication technology has gradually driven existing wireless communication technology toward multi-frequency bands. The aim of this paper is to design, simulate, and test a metamaterial-based small-sized high-isolation dual-band MIMO antenna that can cover both the 3.5 GHz WiMAX band (3.46–3.56 GHz) and the 5.3 GHz 5G Wi-Fi band (5.22–5.41 GHz). To achieve low coupling, a complementary dual-band metamaterial decoupling structure consisting of a square ring and a circular ring is introduced on the front side of the dielectric substrate. The passive characterization of the dual-cell MIMO antenna array is conducted using An-soft High Frequency Structure Simulation Software (HFSS). Subsequently, the simulation results are verified through physical processing measurements. The design idea of the dual-band MIMO antenna system and its simulation and measurement results are also presented and analyzed in detail.

II. Dual-Band MIMO Antenna Design

The geometry and detailed dimensions of the proposed dual-band dual-cell MIMO antenna system are presented in Fig. 1. The system was printed on the front side of an FR4 substrate (dielectric constant of 4.4, dielectric loss angle tangent of 0.02) of thickness h = 1.6 mm. As shown in Fig. 1, the designed system comprises two simple rectangular radiating patches placed at a distance of d = 11 mm from each other. Notably, the dimensions of the proposed antenna structure were modeled and simulated for optimization and analysis using HFSS. The finalized antenna parameter values are summarized in Table 1, where d₂ denotes the perpendicular distance from the coaxial feed point to the center of the rectangular patch.

Fig. 1

Geometry of the MIMO antenna: (a) front view and (b) back view.

Table 1

Reference values for antenna structure dimensions (unit: mm)

To achieve better impedance matching for the proposed dual-band MIMO antenna, a coaxial feed was employed to excite it by placing the feed point at the diagonal of the patch, so that the length and width of the patch controlled one resonance point to then obtain the two resonance points of the required design.

III. Metamaterial Unit Design and Analysis

A complementary isotropic open resonant ring structure capable of exhibiting special electromagnetic resonance characteristics near 3.5 GHz and 5.3 GHz was designed as shown in Fig. 2. Its simulation-optimized parameters were a = 9 mm, S₀ = 0.5 mm, g = 0.6 mm, g₂ = 0.4 mm, R₁ = 2.9 mm, R₂ = 2.5 mm, and W₀ = 10 mm.

Fig. 2

Metamaterial cell structure.

To verify the electromagnetic characteristics of the designed complementary isotropic open resonant ring unit, this chapter uses HFSS was employed to analyze its performance in detail. The metamaterial cell patch was placed on the upper surface of the FR4 dielectric substrate of thickness 1.6 mm, following which the entire setup was placed in an air box. For simulation analysis, the upper and lower surfaces perpendicular to the z-axis were set as wave port excitations, the front and rear surfaces perpendicular to the x-axis were set as ideal magnetic conductors (Perfect H), and the left and right surfaces perpendicular to the y-axis were set as ideal electric conductors (Perfect E). Effectively, the magnetic field was perpendicular to the surface of the metamaterial unit used to simulate magnetic resonance when the magnetic field passed through the unit. The overall structure of the unit is illustrated in Fig. 3.

Fig. 3

A 3D view of the metamaterial cell structure.

The values of the S₁₁ and S₂₁ parameters of the designed metamaterial unit structure, as shown in Fig. 4(a), were obtained by HFSS simulation. As evident from the figure, the proposed metamaterial unit structure generated transmission resistance bands due to magnetic resonance near the two frequency points of 3.5 GHz and 5.3 GHz.

Fig. 4

(a) Plot of the S-parameter structure and (b) plot of the extracted equivalent parameter values.

Notably, to design a metamaterial unit structure that satisfies a specific resonance frequency, the unit structure needs to be verified by theoretical simulation experiments after modeling it in HFSS software. In this study, equivalent values for permeability and permittivity were obtained using the S-parameter extraction technique [11]. The results in Fig. 4(b) show that the proposed structure attained negative values for equivalent permeability μ and positive values for equivalent permittivity ɛ at the two frequency bands of 3–4 GHz and 5–6 GHz, thus satisfying the minimum requirements for electronegative metamaterials.

IV. Overall MIMO Antenna Structure Design

To reduce electromagnetic mutual coupling between the two-cell MIMO antenna array, the waveguide metamaterial structure in Fig. 2, in the form of 1 × 3, was introduced between the two radiating patches. The overall MIMO antenna array structure, along with the metamaterials, is presented in Fig. 5. Notably, to make the experimental results comparable, the same structural dimensions d₃ = 0.2 mm, L₂ = 30 mm, and W₂ = 10 mm were maintained for the antenna structures with and without metamaterials.

Fig. 5

Front view of the overall antenna structure with metamaterial.

1. S-Parameter Simulation Analysis

A comparison of the S-parameters obtained through simulation is shown in Fig. 6. Fig. 6(a) indicates that although the central operating frequency shifts slightly to the low frequency, the overall deviation of the operating frequency band is not large. Meanwhile, Fig. 6(b) clarifies that the coupling degree at the two center frequencies reduced further after loading the metamaterial, with the scattering parameter S₂₁ being below −18 dB. The isolation degree at the low and high frequencies improved by 6.16 dB and 3.62 dB, respectively, which further indicates that the proposed dual-band metamaterial cell structure achieved reduced antenna coupling.

Fig. 6

S-parameter comparison diagram: (a) S₁₁ and (b) S₂₁.

2. Antenna Surface Current Distribution

To understand the decoupling principle of the metamaterial structure acting on the MIMO antenna system more intuitively, the surface current distributions of the antenna without and with metamaterials at 3.5 GHz and 5.3 GHz were investigated. The results shown in Fig. 7 suggest that the addition of the metamaterial structure significantly reduced the coupling current between the MIMO antenna arrays at the two operating frequencies.

Fig. 7

Surface current diagram of the proposed antenna without and with metamaterials: (a) 3.5 GHz and (b) 5.3 GHz.

3. Gain and Efficiency

Fig. 8 illustrates the gain and efficiency results of the MIMO antenna equipped with metamaterials. The gain is in the 1.63–1.74 dBi range and efficiency is in the 46%–48% range for the low-frequency band of 3.46–3.56 GHz. Meanwhile, for the high-frequency band of 5.22–5.41 GHz, the gain is in the range of 4.07–5.05 dBi and efficiency is in the range of 59%–64%.

Fig. 8

Gain and efficiency of the proposed antenna.

4. Radiation Direction Diagram of the Antenna

Fig. 9 displays the radiation direction diagram of the original antenna and the antenna loaded with metamaterials at 3.5 GHz and 5.3 GHz in the main radiation direction. Owing to the structural symmetry, one antenna element was actively driven at Port 1, while the counterpart element was passively terminated into a 50 Ω matched load at Port 2. Both port numbers are explicitly annotated in Fig. 5. Fig. 9 shows that the E-plane and H-plane radiation patterns of the MIMO antenna exhibit nearly identical trends before and after loading the metamaterial. This implies that the incorporation of the metamaterial does not compromise the far-field radiation characteristics of the MIMO antenna array.

Fig. 9

Radiation direction diagrams of E-plane (left) and H-plane (right) of the antenna: (a) 3.5 GHz and (b) 5.3 GHz.

5. Diversity Characteristics of the MIMO Antenna

The practical application of MIMO antennas necessitates careful analysis of its diversity performance. Notably, a number of diversity characteristics and parameter indices can be employed to judge whether a designed antenna performs well in terms of important indicators of good diversity in a MIMO system.

In this study, the envelope correlation coefficient (ECC) was employed to further evaluate the performance of the proposed MIMO antenna. This parameter (ρ_e) is useful for estimating the diversity performance of MIMO systems since, in general, low envelope correlation always leads to high diversity gain (DG). It was calculated using Eq. (1) based on the S-parameters obtained from the simulations [12], as below:

(1) ρe=|S11*S12′+S21*S22′|2(1-(|S11|2+|S21|2))(1-(|S22|2+|S12|2)),

where S11* and S21* respectively represent the real parts of parameters S₁₁ and S₂₁, while S12′ and S22′ respectively refer to the real parts of parameters S₁₂ and S₂₂.

Furthermore, DG [13] can be used to measure the spatial diversity of a MIMO antenna the larger the DG, the better the MIMO antenna technique’s ability to improve diversity. Notably, DG can be calculated using Eq. (2):

(2) DG=1-(ECC)2.

The simulated ECC and DG plots of the proposed MIMO antenna are shown in Fig. 10, demonstrating that the ECC value satisfies the minimum requirement of the ideal value over the entire operating band the ECC is less than 0.02 in both operating bands. This confirms that the designed MIMO antenna offers good isolation and optimal performance, even in a multipath fading environment. Moreover, the proposed MIMO antenna achieved a DG of 10 dBi, exhibiting good diversity characteristics in both operating bands.

Fig. 10

ECC and diversity gain of the proposed antenna.

6. Comparative Analysis of Antenna Array Elements

All the MIMO antenna arrays whose performances are compared in Table 2 utilize the same decoupling technique [14–19]. Notably, compared to [14] and [15], the proposed antenna has some disadvantages with regard to isolation but offers advantages in terms of bandwidth and size. Furthermore, although its bandwidth is relatively narrower than [19], the two bands of the proposed antenna are more suitable for the needs of modern mobile communication, the antenna structure is smaller in size, and it offers better isolation and gain in the lower band. In addition, compared to [16–18], the proposed antenna’s ECC value is smaller, which implies that it allows for better isolation. Therefore, the proposed design meets the minimum requirements of a dual-band MIMO antenna in terms of isolation, size, and diversity characteristics.

Table 2

Comparison of the proposed structure with MIMO antenna arrays reported in the literature

V. Antenna Field Measurement and Results

To validate the proposed design, an antenna prototype was fabricated based on the antenna parameters optimized by HFSS, following which its S-parameters were measured using an Agilent Vector Network Analyzer (VNA). The antenna fabricated based on the above parameters is depicted in Fig. 11, while the measured and simulated values of the S-parameters are shown in Fig. 12, indicating a small frequency shift in the high-frequency band. This shift may be attributed to SMA connector losses, cable losses, or radiation boundaries during the measurement process. Overall, the simulated and measured data agree well with each other.

Fig. 11

Physical processing diagram: (a) front view and (b) back view.

Fig. 12

Measured and simulated S-parameters of the designed MIMO antenna.

VI. Conclusion

In this study, a metamaterial-based symmetric dual-band dual-cell MIMO antenna system is proposed for WiMAX and 5G Wi-Fi applications. The electromagnetic coupling between the dual-band array antennas was reduced by adopting a 1 × 3 complementary dual-band metamaterial cell structure. According to the simulation and measured results, the proposed MIMO antenna system achieved a bandwidth of 3.46–3.56 GHz in the low-frequency band and 5.22–5.41 GHz in the high-frequency band, while the isolation of the MIMO antenna array exhibited an increase of 6.16 dB and 3.62 dB in the two frequency bands, respectively, compared to antennas without metamaterials. Furthermore, low ECC values and high DG were attained, implying that the antenna system has good diversity characteristics. In addition, a comparison of the simulation and measurement results clarified that the introduction of metamaterials did not deteriorate the impedance matching characteristics of the antenna, and further improved the coupling degree between the antenna elements. Therefore, the proposed MIMO antenna structure is applicable in the field of dual-band miniaturized antennas.

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