Design of a Compact Tri-band Mimo Antenna With Reduced Mutual Coupling

Abstract

The mutual coupling analysis in a tri-band MIMO antenna is the subject of this article. The suggested antenna has three frequencies of resonance: 5.8 GHz, 7.2 GHz, and 8.6 GHz. This mutual coupling study is performed in three ways: (1) without the use of any additional structure; (2) with the use of additional structure (2) Polarization diversity (3) Spatial diversity The mutual coupling is negated in the first technique because the current flow in adjacent arms is in the opposite direction. The second method involves altering the distance between them to do mutual coupling analysis. In the third method, mutual coupling analysis is performed for various antenna orientations. Finally, all three approaches are compared in terms of mutual coupling. The MIMO antenna's performance was further tested in terms of radiation pattern, ECC, gain, and efficiency. It's also worth noting that the simulation results match the measured ones.

1. Introduction

There is one transmitting antenna and one receiving antenna in a typical communication system. The system's fundamental restriction is channel capacity. To tackle this difficulty Foschini [1] proposed MIMO antenna. Multiple antennas are used at both the transmitter and receiver to increase channel capacity. When these antennas are placed at a distance, however, they operate well. Unfortunately, with portable equipment, it is necessary to install antennas near together, resulting in mutual coupling. To reduce the coupling effect, different strategies have been proposed in the literature. Mutual coupling is reduced when stubs are used in the ground plane [2]. [3] proposes a dual polarized antenna with shared radiating aperture using DGS. DGS is used to reduce mutual coupling between two monopoles covering UWB bands that are situated close together. In [5,] a CPW-fed double-stepped antenna for diversity applications is proposed. Wideband neutralization is proposed in [6] to eliminate mutual coupling throughout the whole UWB spectrum. [7] uses EBG structures to reduce mutual coupling between two UWB monopoles. To eliminate reciprocal coupling in the L and S bands, a novel DGS is adopted in [8]. In a dual wide band MIMO antenna, flag-shaped stubs are employed to reduce mutual coupling [9]. To lessen reciprocal coupling between the patches, a novel construction called parasitic elements with vias is adopted [10]. To reduce mutual coupling between patches, L-shaped strips with vias are utilized [11]. In the UWB range, a decoupling structure is utilized to reduce mutual coupling [12], while a neutralization line is used to reduce mutual coupling between two rectangular monopoles in [13].

This paper presents a 22 MIMO antenna with minimized mutual coupling. The space between the pieces is kept to a minimum, at 0.0380 just. The antenna is 5225 mm2 in size and resonates at tri-band frequencies, making it appropriate for WLAN and X-band applications. It achieves an MC reduction of -26 dB at the appropriate resonant band without the need of any extra structure. To reduce MC, the surface current cancellation principle is applied. Section 2 covers the antenna design, while Section 3 covers critical characteristics such as S-parameters, radiation pattern, ECC, and diversity gain. The intended work is concluded in Section 4.

2. Basic Configuration Of Proposed Antenna

The suggested triple band antenna's geometry and dimensions are shown in Figure 1. The suggested antenna is made from FR-4 with a thickness of h=1.6 mm and a relative permittivity of 4.3. The antenna's entire size is 24 × 25mm². The parameters of the proposed antenna are listed in Table 1.

Table 1

 Parameters of the proposed antenna

Figure 1 shows the basic layout of the proposed antenna. The proposed antenna has three frequencies of resonance. Figure 2 shows the surface current distributions for the proposed antenna at three resonating frequencies.

The proposed antenna resonates at three frequencies 5.8 GHz, 7.2 GHz, 8.6 GHz. From Fig.2.the surface currents responsible for resonant frequencies are derived by using Equation (1). The surface current responsible for resonating frequencies 5.8 GHz, 7.2 GHz, 8.6 GHz are L_r1 = 13.5mm, L_r2  = 10.8mm, L_r3 = 9.02mm. The difference between the design equation and full wave simulation are shown in table.2.   

Table 2

 Comparison between design equation and full wave simulation

3. Mutual Coupling Analysis In Mimo Antenna

The geometry of the proposed MIMO antenna is as shown in Fig.3.The area of single antenna is 24×25 mm² and MIMO antenna is 56×25 mm². Two identical patches are placed at equal distance on either side of origin. The end-to-end distance between two patches is 0.038λ_0. Microstrip line feeding is used for exciting the antenna. Table.3 shows parameters of the proposed MIMO antenna.   

Table 3

Parameters of MIMO antenna 

Figure 4 shows the simulated and measured return loss curves (S11). Antennas are found to resonate at 5.8 GHz, 7.2 GHz, and 8.6 GHz. Figure 5 shows the simulated and measured MC (S21). At 5.8 GHz, MC is -26 dB, -14 dB at 7.2 GHz, and -13 dB at 8.6 GHz. The relevance of the proposed design is that it reduces MC without the need of any additional structures such as EBG, DGS, and so on. Surface current distribution is seen in Fig.6. Surface currents in adjacent arms are found to flow in opposite directions, cancelling each other out. The proposed MIMO antenna structure is built around this notion.

The MC analysis in MIMO antenna is further verified by using spatial and polarization diversities. 

3.2 Mutual coupling analysis using Spatial Diversity

Fig.7. shows the spatial diversity among two patches. The Spatial diversity is one in which the space between the elements is varied and observed the changes in MC as shown in Fig.8.

The main disadvantage of spatial diversity is requiring more space. It may be implemented at base station but not in mobiles. It is observed that as the edge-to-edge distance between the patches increases, MC decreases. 

3.3 Mutual coupling analysis using Polarization Diversity

Two antennas with distinct polarizations are referred to as polarization diversity. Figure 9 depicts the polarization variety associated with various orientations. Figure 10 depicts the MC curves. Pos-1, Pos-2, and Pos-3 correspond to each other. Diversity in polarization is more effective than diversity in space. It was discovered that the least MC was achieved for POS-3

Figure 11 depicts the fabricated MIMO antenna. Figure 12 depicts the ECC. Equation [14] is used to calculate the correlation coefficient (1). Diversity performance is determined by ECC. If the ECC is low, diversity gain is better, and vice versa. At all three resonating frequencies, the ECC values are less than 0.01.       

The gain plot is shown in Fig.13. It is observed that the gain is 3.5 dB, 3.4 dB and 3.6 dB at 5.8 GHz, 7.2 GHz and 8.6 GHz respectively. The efficiency of the antenna is shown in Fig.14. The efficiency of the antenna above 80% at all three frequencies.

The VSWR plot is shown in Fig.15. The VSWR is below 2 at all three frequencies.

The radiation patterns in YZ-plane and XZ-plane at three resonating frequencies are shown in Fig.16.  

The performance comparison of the proposed MIMO antenna with antennas reported in the literature is shown in Table.4.

Table 4

4. Conclusion

This proposal is for a tri-band MIMO antenna that resonates at 5.8 GHz, 7.2 GHz, and 8.6 GHz. It is later expanded to a total of 22 MIMO antennas, and mutual coupling analysis is performed in three different ways. (1) There is no additional structure used in the mutual coupling analysis. Mutual coupling is decreased by -26 dB at 5.8 GHz, -14 dB at 7.2 GHz, and − 13 dB at 8.6 GHz in this 22 MIMO antenna since both antennas are situated in close proximity 0.0380. (2) For varying values of the distance between the antennas, mutual coupling analysis is performed using spatial diversity. Mutual coupling reduces as the distance between the antennas grows, with the largest reduction occurring at d = 7.5 mm. (3) For three sites, mutual coupling analysis is performed using polarization diversity. Position 3 (POS-3) has the most mutual coupling reduction of the three. The MIMO antenna's performance is further tested in terms of VSWR, gain, ECC, radiation pattern, and efficiency. The simulation and measured results are also found to be in good agreement.

Declarations

Data availability statement: 

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Competing Interests

Author 1, 2 and 3 declare they have no financial interests. 

Findng

No Funding.

Code

 Not applicable

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