Broadband Reduction in Mutual Coupling in Compact MIMO Vehicle Antennas by Using Electric SRRs

Abstract

Reducing mutual coupling between radiation elements of compact MIMO vehicle antennas is of fundamental importance to achieve simultaneous high capacity and miniaturization. In this work, we propose a commercial vehicle MIMO antenna composed of two inverted-F elements that achieves high isolation of mutual coupling through the incorporation of the electric split ring resonator (SRR). The working mode and frequency band of the SRR are rationally selected based on characteristic mode analysis (CMA). Experimental results validate high isolation below −20 dB across a broadband frequency range from 1.7 GHz to 2.7 GHz, achieving a relative bandwidth of 45.4%, with a maximum reduction of 15 dB of the S21 parameter. Additionally, the MIMO antenna maintains stable performance in both return loss and radiation characteristics, with minimal degradation in gain and radiation pattern. This work provides a compact and bandwidth-enhanced solution for vehicular communication systems.

1. Introduction

In the era of an information society, vehicles have become a smart information terminal rather than merely a mode of transportation. High speed and large capacity for data transmission are of importance for the development of a smart vehicle. Compact MIMO antennas with broadband connectivity and high radiation efficiency are essential to enable next-generation smart vehicles. However, as multiple antennas are placed at a spacing less than λ/2, mutual coupling becomes significant and inevitably deteriorates the radiation performance and the channel capacity of the MIMO system [1]. Therefore, it is of extreme importance for a vehicle MIMO antenna to minimize mutual coupling.

Various methods have been used to enhance the mutual coupling isolation, such as feed network [2], transmission line decoupler [3,4,5], EBG structure [6,7], defected ground [8], etc. However, these methods are complex in design and large in size. Metamaterial, or its two-dimensional counterpart, metasurface, is a class of artificial media composed of subwavelength-sized resonant elements [9,10,11,12,13,14]. The resonant micro and nano-elements can be freely designed for optical properties unattainable in nature, such as negative refraction [11], electromagnetic cloak [15], and metalens [10]. Split ring resonators (SRRs), one of the classical metamaterial microstructures [16,17,18,19,20,21,22,23,24,25,26], are capable of preventing the radiation of electromagnetic waves due to their electric or magnetic resonance in a given band. In this way, electromagnetic wave transmission can be tailored by integrating metamaterials into the radiation system. Another advantage of SRRs is that they provide an in-phase reflection, which enables compact integration. A magnetic SRR array was studied to decrease mutual coupling between dense monopole antennas [16]. Electric complementary SRRs and fishnet were employed to resist surface wave propagation to reduce the adverse effect of mutual coupling between slot antenna array elements [24]. In addition, a metasurface based on SRRs was designed as a supersubstrate to reduce coupling inside a large antenna array [25]. Periodic SRR arrays are more commonly used in antenna isolation to reduce MIMO antenna coupling by suppressing the transmission of surface waves [27,28]. For spatial electromagnetic wave coupling, researchers have suppressed it by introducing metasurfaces or artificial magnetic conductor coverings [29,30,31]. However, these approaches predominantly rely on horizontally aligned gap configurations or multi-layer stacking, which inevitably increase the antenna profile and complicate integration with low-profile vehicle antennas. Moreover, their narrowband operation and sensitivity to SRR orientation limit practical deployment in dynamic vehicular environments.

Based on the above challenges, we propose an integrated electric SRR design to achieve wideband decoupling for in-vehicle MIMO antenna systems. Here, electric SRRs refer to SRRs dominated by electric resonance, where capacitive coupling between the ring gaps and surrounding fields suppresses near-field interactions. In this work, we present a commercial vehicle MIMO antenna incorporating two types of electric SRRs: single SRRs (a single metallic loop with a capacitive gap) and double SRRs (two concentric loops with aligned gaps). Unlike prior studies focused on surface wave suppression via magnetic resonance, our approach leverages vertically integrated SRR architectures to directly attenuate free-space near-field coupling between MIMO radiators. The double SRRs synergize multi-mode electric resonance through coupled inner and outer rings, achieving a 47.6% fractional bandwidth. Significant improvement of mutual coupling is achieved by properly placing an electric SRR between two inverted-F radiation elements without requiring additional clearance layers, reducing integration height by 50% compared to conventional metasurface approaches [32,33]. We have analyzed the working modes of SRRs based on CMA. By selectively exciting the J1 electric resonance mode, we achieve targeted suppression of near-field coupling while maintaining radiation integrity. Furthermore, the high isolation performance remains stable when the SRR is rotated around its symmetry axis, which is a critical advantage over orientation-sensitive designs. More importantly, the return loss and radiation performances of the MIMO antenna remain well.

2. Characteristics of Vehicle MIMO Antenna

In this work, the MIMO antenna under investigation is widely applied in Roewe RX5, SAIC MOTOR, and can be readily purchased. The MIMO antenna, as schematically illustrated in Figure 1, is composed of two inverted-F radiation elements standing vertically on a 105 mm × 42 mm ground substrate with 43 mm inter-element spacing. While operating in the same frequency band, the two elements are slightly offset in frequency to enhance system bandwidth. The dielectric substrate is made of FR4 with moderate permittivity and dielectric loss. As shown in Figure 1, two radiation elements are fabricated on separate substrates and placed in the x-y plane.

A commercial electromagnetic solver based on the finite-difference-time–domain (FDTD) method is used for numerical simulation in our study. In the experiment, scattering parameters of each radiation element are measured using an AV3629D vector network analyzer, manufactured by the China Electronics Technology Group Corporation (CETC 41st Institute) in Bengbu, China.

The simulated and measured return loss and mutual coupling spectra of the MIMO antenna are shown in Figure 2. As seen from S11 and S22, the vehicle MIMO antenna operates effectively from 1.7 GHz to 2.6 GHz with respect to the return loss below −10 dB. Obviously, such a configuration of MIMO provides a relatively broad bandwidth. As depicted in Figure 2, the mutual coupling between two radiation elements, referred to as S21, fluctuates around −15 dB, indicating a rather strong correlation between MIMO radiation elements.

As demonstrated in previous reports, in addition to surface waves inside the substrate and ground plane, electromagnetic radiation in free space plays an important role in mutual coupling. As two radiation elements are placed vertically to the ground substrate, electric field coupling in free space is expected to be visible. It is therefore reasonable to design a metamaterial microstructure to tailor spatial electric field coupling.

3. Mutual Coupling Suppression by Electric SRR

3.1. Design and Experiment of SRR

Two types of SRRs, including single rings and double rings, as illustrated in Figure 3a,c, are designed to suppress mutual coupling. SRR samples are prepared by standard PCB technology to print metallic patterns on a teflon substrate with a relative permittivity of 2.65 and a dielectric loss tangent of 0.001. SRR is fabricated from 0.03 mm-thick copper. Geometry parameters of each SRR are optimized to achieve an electric dipole-dominated resonance around the central operation frequency of the MIMO antenna. Single square SRR was designed with linewidth w = 1 mm, gap g = 1 mm, and side length a = 40 mm. The entire dielectric substrate has a sidelength of A = 44 mm and a thickness of 0.8 mm. The design of SRR has minimal space requirements and consideration for practical installation environments, though future implementation may require compatible PCB slot designs.

Figure 3 illustrates the SRR under z-axis normal incidence, with x-polarized excitation aligned to the continuous side opposite the gap. The electromagnetic response of SRRs is tested by using a pair of monopole antennas in free space, which is similar to Ref. [24]. The measured S21 parameters are calibrated by first characterizing the baseline coupling between the monopoles and then normalizing this effect to isolate the intrinsic electromagnetic response of the SRR. As the incident magnetic field is parallel to the surface of SRRs and the electric field is parallel to the SRR side without a gap, neither magnetic resonance nor magneto-electric resonance can be excited. It is therefore expected that only electric resonance can be excited. The shallow transmission dip (−8 dB) of the single SRR indicates inadequate energy localization, primarily caused by the limited structural capacitance, which prevents effective electric field localization, leading to weakened resonance strength. To address this issue, we propose a double SRR design through two structural optimizations. Unlike conventional magnetic SRRs, the identical gap orientations of the inner and outer rings form cascaded electric field hotspots, enhancing energy storage near the gaps. By introducing a concentric inner ring, a secondary resonance peak is generated, interacting with the primary resonance peak to broaden the isolation bandwidth while stabilizing the central frequency. This configuration achieves a significantly deeper transmission dip, as the design leverages cooperative resonance rather than isolated resonance, simultaneously optimizing suppression depth and bandwidth. Although the double SRR exhibits a narrower −10 dB bandwidth in controlled tests, its high Q-factor and multi-mode coupling effects synergistically could broaden the effective isolation bandwidth when integrated with the MIMO antenna system.

3.2. CMA of SRR

Figure 4a presents the characteristic mode analysis (CMA) results of the double SRRs, where the first three modes (J1, J2, J3) are calculated. The J1 and J2 modes operate as electric resonance modes with distinct orientations and frequency bands: the J1 mode exhibits an X-axis-oriented current distribution with peak modal significance (MS) at 1.95 GHz, while the J2 mode features a Y-axis-oriented current distribution peaking at 2.05 GHz; the J3 mode, identified as a magnetic resonance, displays negligible MS values (<0.2) across 1.5–3 GHz. Notably, the J1 mode of the double SRRs demonstrates a broader bandwidth compared to the J1 mode of the single SRR, validating the superior broadband isolation performance of the double-SRR configuration. To enhance isolation within the 1.7–2.7 GHz operational band, the J1 mode is selected as the dominant working mode. By orienting the SRRs with their gaps facing downward, the induced X-axis currents of the J1 mode align orthogonally with the vertically polarized near-field coupling of the MIMO antennas, thereby generating counteractive currents that suppress mutual coupling and improve isolation.

To optimize the electromagnetic response of the SRR, a parametric study was conducted to analyze the effects of varying side length a (36 mm to 40 mm) on resonance frequency and modal significance as shown in Figure 5. The results show that as a increases, the resonance frequency of the SRR decreases from 2.5 GHz to 2.05 GHz, while the modal significance (MS) of the dominant mode (J1) significantly improves. At a = 39 mm, the J1 mode achieves a peak MS of 1 at 2.05 GHz and maintains high values (MS > 0.6) across the 1.7–2.5 GHz band. This demonstrates that a larger a extends the SRR’s resonance bandwidth and enhances its electric field suppression capability, thereby providing an optimal design for broadband isolation.

3.3. Broadband Decoupling Performance of SRRs

To validate the isolation effectiveness of mutual coupling, an SRR is inserted in the middle between two radiation elements. In order to minimize the influence of SRR and its substrate on the MIMO antenna, the SRR is placed with its plane orthogonal to that of the radiation elements, as shown in Figure 6.

Figure 7 shows the influence of SRR insertion on the return loss of the MIMO antenna. Experimental results demonstrate that return losses of the MIMO antenna with single SRR and double SRRs are nearly the same. Compared to that of the reference, the MIMO antenna without the SRR, a slight shift towards the lower frequency of S11 and S22 is observed, which is presumably caused by the minor change in the dielectric surrounding environment. Even though the −10 dB operational bandwidth remains nearly unchanged, it clearly demonstrates that the influence of SRR insertion on return loss is rather weak.

The reduction in mutual coupling characteristic of radiation elements is also analyzed. Experimental transmission spectra between MIMO radiation elements are monitored by VNA and plotted in Figure 8. The transmission spectra of the reference MIMO without SRRs are reproduced. From the comparison, a single SRR offers an isolation enhancement to mutual coupling. A clear isolation improvement by 2 dB to 5 dB is observed in the operation regime of MIMO, although the electric resonance of a single SRR is not so strong. On the other hand, double SRRs provide a more visible isolation improvement in a relatively broad bandwidth from 1.6 GHz to 2.7 GHz, which totally covers the operating bandwidth of the MIMO antenna. It is noted that mutual coupling can be improved by 15 dB in the lower frequency region and 5 dB to 8 dB in the higher frequency region. Importantly, such high isolation below −20 dB is preserved from 1.7 GHz to 2.7 GHz, accounting for a bandwidth up to 45.4% with respect to the central frequency, which is the maximum bandwidth mutual coupling reduction, according to the best knowledge. The shift between the SRR’s standalone resonance (2.05 GHz) and the system peak isolation frequency (1.8 GHz) may arise from parasitic capacitance/inductance introduced by near-field coupling, slightly tuning the SRR’s resonance.

Table 1. Performance comparison of SRR decoupling structures.

Reference	Size (λ)	Coupling Reduction Bandwidth (%)	−20 dB Coupling Bandwidth	Orientation to Ground
16	0.06λ × 0.27λ	12%	8%	Vertical
25	0.24λ × 0.96λ	25%	25%	Vertical
26	2.32λ × 2.32λ	5%	N/A	Parallel
28	0.67λ × 0.67λ	~15%	7.3%	Parallel
29	0.09λ × 0.26λ	4.8%	4.8%	Parallel
This work	0.3λ × 0.3λ	47.6%	45.4%	Vertical

compares our SRR decoupling design with existing approaches. The proposed 0.3λ × 0.3λ SRR achieves 47.6% coupling-reduction bandwidth, which is higher than previous works while maintaining a compact size. This vertical-orientation design demonstrates superior broadband performance with a 45.4% −20 dB bandwidth, making it ideal for space-limited vehicle MIMO systems.

3.4. Rotation Effects on Decoupling Performance

The dependence of mutual coupling suppression on SRR rotation is investigated. Figure 9 shows the transmission spectra as a function of double SRRs’ rotation with respect to their symmetry axis, namely, the x-axis. The initial state is defined with ϕ = 0° when the SRRs’ plane is parallel to the xy plane. As shown in Figure 8, when double SRRs rotate gradually by a step of 30°, it is observed that the mutual coupling suppression remains rather stable except for a slight frequency shift and minor magnitude variation. Similar phenomena are also observed for single SRRs. As mentioned previously, both single SRRs and double SRRs are excited by an incident electric field. Despite the complex electromagnetic field distribution of the MIMO antenna, the electric field is always polarized parallel to the SRR surface. This directly results in stable excitation for electric resonance during SRRs’ rotation. Therefore, the electric resonance of SRR, as well as the mutual coupling reduction, is independent of its rotation. It is noted that such insensitivity of mutual coupling suppression on SRRs’ rotation is extremely important as it provides more convenient methods for insertion operation in real RF operations.

The influence of opening resonance rotating around itself on antenna isolation is further studied. When the double SRRs are from 0° to 90°, the coupling between antenna elements changes from broadband isolation to narrowband isolation, but the amplitude of the isolation increases. As shown in Figure 10, there are three resonant valleys in the working frequency band of MIMO antenna, which greatly improves the isolation of MIMO antenna at the corresponding frequency points, and realizes the isolation improvement of 20.48 dB, 24.94 dB, and 19.44 dB at 2.17 GHz, 2.54 GHz, and 2.70 GHz, respectively. Based on this, it is found that different modes of SRRs can be used in the design of different antenna isolations.

4. Comprehensive Analysis of Decoupling Mechanism

In order to explore the underlying mechanism of mutual coupling suppression by SRRs, a detailed illustration is given by monitoring the local electric field when the left antenna 1 is fed. Figure 11 depicts the local electric field distribution in the xz plane for vehicle MIMO at 2.0 GHz. Three cases, including the reference MIMO with and without SRRs, have been analyzed. As can be seen from Figure 11a, when the left radiation element is fed, an intense electric field concentration is observed in the right radiation element, indicating that mutual coupling is rather visible. When a single SRR perpendicular to the ground plane is inserted, the local electric field becomes weakened inside the right radiation element, whereas an intense electric field can be observed inside the single SRR. Moreover, the incorporation of double SRRs directly leads to a further decrease in local electric field concentration inside the right radiation element, while more localized field enhancement can be observed inside double SRRs, as shown in Figure 11c. This directly coincides with the mutual coupling suppression inside Figure 8.

We also investigate the radiation performance of the reference MIMO antenna with and without SRRs. Far field radiation patterns were measured in an anechoic chamber. Figure 12 shows the far field radiation pattern in the H-plane for the MIMO antenna under excitation by port 1 and port 2, respectively. Experimental and simulated results agree well. In comparison with the radiation pattern of the reference MIMO, neither single SRRs nor double SRRs change most of the radiation characteristics of the MIMO. However, backlobe levels, which are attributed directly to mutual coupling, were suppressed by 2 dB and 5 dB by single SRRs and double SRRs, respectively. This coincides with the transmission characteristic between two radiation elements.

5. Conclusions

In this paper, an effective approach is proposed to suppress spatial mutual coupling between radiation elements of vehicular MIMO antennas over a broad bandwidth. Two types of electric SRRs are employed to achieve high isolation. As two radiation elements are placed vertically to the ground substrate, electric field coupling in free space is visibly significant. It is therefore reasonable to design a metamaterial microstructure to tailor spatial electric field coupling. The working mode and frequency band of the SRRs are rationally selected based on CMA. By integrating SRRs into the MIMO antenna, mutual coupling is significantly reduced below −20 dB across the 1.7–2.7 GHz band, achieving a relative bandwidth of 45.4% and a maximum coupling reduction of 15 dB. The return loss and radiation characteristics of the MIMO antenna remain stable without significant degradation. Notably, the broadband isolation performance is independent of the SRRs’ rotation orientation. This work provides a practical and efficient methodology for broadband mutual coupling suppression in vehicle communications and the sensing field.