Design of Half-Ring MIMO Antenna to Reduce the Mutual Coupling

Abstract

We propose a novel, dual-polarized MIMO radiator, which consists of half rings, and included in the ring a square shape at the top side and half circular shape at bottom side operating from 2 to 10 GHz with a microstrip feeding. The suppression of mutual coupling is obtained by maintaining the separation between the patches around 0.25 λ₀. The half-ring MIMO antenna is resonating at dual band of frequencies at 7.24 and 8.16 GHz with impedance bandwidths of 430 and 230 MHz, has |S₁₁| < −10 dB in the MIMO range from 2.0 to 10.0 GHz and has a mutual coupling with |S₂₁| < −20 dB. The radiator has a low ECC (envelope correlation coefficient) with values equal to approximately less than 0.025, which will prove that the half-ring MIMO radiator shows better diversity performance. The half-ring MIMO radiator has improved the parameters of reflection coefficient, mutual coupling, realized gain, group delay and real/imaginary impedances.

8.1 Introduction

The major challenge in the design of any multiple input-multiple output (MIMO) antenna is when the antenna elements are at close proximity, it produces a mutual coupling. A few isolation techniques like using split ring resonators reduced the isolation to 20 dB [1], and by using common mode suppression of ultra-wide band antenna (UWB)-MIMO radiator reduces the isolation to 15.5 dB [2] and maintains an ECC value of 0.04. To reduce the mutual coupling, the design structure Minkowski produced multi-band [3] characteristics having S₂₁ of 42 dB. A compact antenna having an area 23 × 39.5 mm² and polarization diversity array produced an isolation of 20 dB operated between 2.5 and 12 GHz [4] whose ECC value maintained at less than 0.55. In [5] a parasitic approach like a UWB-MIMO radiator used a carbon film black operated from 2.5 to 11 GHz with reflection coefficient (S₂₁) of 20 dB. A floating parasitic decoupling structure [6] with a size of 33 × 45.5 produced a UWB system whose isolation ≤ 20 dB is achieved. An extendable MIMO array antenna has a low ECC around 0.025 and its peak gain 4 dBi was observed [7]. A wide-band MIMO antenna with band rejection [8], tri-band MIMO for defence and radio astronomy applications [9] was observed. For the mobile terminal applications, a compact G-shaped design [10] having an area of 50 × 82 mm², operating in the frequency ranges from 2.2 to 13.3 GHz with isolation of ≤ 15 dB and a maximum gain of 4.78 dBi is achieved. Finally, it is interesting to observe that the proposed half-ring design structure reduced the mutual coupling to 40 dB. By using the techniques of optimization procedure finalized the dimensions. Table 8.1 shows the comparison with previous designs.

Table 8.1 Comparison with existing systems

8.2 Antenna Construction and Its Geometry

The proposed structure having dimensions 46 × 35 mm² is designed with a thickness of 1.5 mm on FR-4 substrate. The loss tangent (tan δ) is 0.02 and relative permittivity (ε_r) of the substrate is 4.3. The half-ring MIMO system produces a dual-band structure using the C-band applications. The step-by-step process of analysis is indicated in Fig. 8.1. Antenna # 1 shows that a strip is added to a circular ring which produced a reflection coefficient of 22 dB. Antenna # 2 represents the cutting of the half of the circles on both sides of total ring with S₁₁ 13 dB. Antenna #3 represents adding half semi-circles on both sides of the patch and S₁₁ is reached to 34 dB. Antenna #4 having a square patch is added to half circles on both sides to form a new structure whose S₁₁ is 10 dB. Finally, Antenna #5 represents the proposed structure having the reflection coefficient greater than 25 dB and transmission coefficient of 40 dB. The design dimensions of the half-ring MIMO structure are listed in Table 8.2.

Fig. 8.1

Evolution stages of half-ring antenna

Table 8.2 Dimensions of half-ring MIMO antenna (in mm)

Finally, the designed structure having the dual band of operation produces the parameters S₁₁ and S₂₁ maintained less than 10 and 20 dB. In this design, strengthening the parameter mutual coupling between the two symmetrical radiators distance can be considered as 4 mm.

Figure 8.2 indicates the S-parameter evolution of each stage of half-ring MIMO design which shows all the cases S₁₁ ≤ −10 dB. Figure 8.3 shows the comparison of S parameters (S₁₁ and S₂₁), which shows that a greater improvement in the mutual coupling observed at 7.2 GHz is 21.89 dB and at 8.1 GHz it almost touches to 40 dB reduction. Figure 8.4 shows the S₂₁ comparison at different distances. This result shows that a greater interference between the patches is reduced. Figure 8.5 depicted the distribution of surface current vectors at 7.2 GHz as well as at 8.1 GHz. Figure 8.5a perceived that port #1 is excited and port #2 is terminated with a characteristic impedance of 50 Ω load. In this case we observed that the maximum current flows through half-ring structure and observed that within a half ring a semi-circle is added with a square patch maximum current flowing. Figure 8.5b perceived that port #2 is excited and port #1 is terminated with 50 Ω load observed that half-ring MIMO structure at bottom of the ring structure patch maximum current is flowing. For both the ports, the analogous distribution of surface current perceived that two homogenous elements were positioned side by side which can lead to create first resonant mode at 7.2 GHz. Similarly, the second resonant mode is at 8.1 GHz. Figure 8.5c shows the distribution of surface current vectors, maximum current flows at the left side of the half-ring resonator and a partial current flows through the edge of the microstrip connector and a little amount of current is observed at the left side of the half-ring patch. Figure 8.5d identified that maximum current flows through the left-side patch, a partial current is observed at the edge of the strip and a minimum surface current on right-side patch having a half-ring added to the square patch.

Fig. 8.2

Step-by-step analysis of S parameters

Fig. 8.3

S parameter comparison

Fig. 8.4

S₂₁ Graph at different spacing between antennas

Fig. 8.5

Surface current distribution of the design

8.3 Results Analysis and Discussion

The simulated representation of S parameters satisfies the condition at dual band of operations of |S₁₁| < −10 dB and |S₁₁| < −20 dB. These parameters indicate that the proposed structure has a greater improvement in the parameters of transmission coefficient as well as reflection coefficients at the dual-band resonant frequencies of 7.2 and 8.1 GHz for the applications of C-band and X-band. Figure 8.6 shows the real/imaginary part of impedance verses frequency from 2.0 to 10 GHz. Figure 8.7 indicates the co- and cross-polarization effects with respect to the xz and yz planes having a dual-band operation at 7.2 and 8.1 GHz when one port of the antenna is excited and another port of the system is terminated with characteristic impedance of 50 Ω. It is observed that the pattern of radiation in xz plane is approximately analogous to pattern in the yz plane. It is important to identify that co-pol patterns are omnidirectional in nature in xz plane. Similarly, it is also to identify that the cross-pol component magnitude increases as the response of the frequency increases with horizontal direction component. Here, we observed that the co-pol is always outer region of the radiation pattern and cross-pol is inner side of the co-pol is a reasonably good agreement. Therefore, it can say that in order to placing half-ring antennas placing linearly the radiation patterns of separate elements lies on surface of a plane similar to each other by properly choosing the separation between the two radiating patches. It is also realized that the proposed structure helps to reduce the factor of mutual coupling.

Fig. 8.6

Real/imaginary impedances

Fig. 8.7

Radiation patterns at dual-band (a) f₁ = 7.2 GHz and (b) f₂ = 8.1 GHz

Figure 8.8 shows the realized gain variations of the half-ring structure produced dual-band operation when port #1 is excited and port # 2 is terminated with a 50 Ω load. When port #1 is excited, peak gain in first resonant band is 6 dBi and at another band is 4.8 dBi. Figure 8.9 shows the parameter of ECC which is lower than the 0.03. In time-domain analysis process group delay is a major important parameter. It shows how much time is considered for antenna to receive a pulse. There is a variation in group delay when placing the antenna front to front and side by side for 1 n. It validates the linear phase over entire far-field region. Figure 8.10 indicates the simulated result of group delay of 7.85±1 n, −0.98±1 ns over a resonant band of frequencies at 7.2 and 8.1 GHz. From the graph it is interesting to observe that the factor of negative group delay (NGD) increases, the signal attenuation increases and the bandwidth decreases.

Fig. 8.8

Comparison of realized gain

Fig. 8.9

ECC Simulation at 2.5 and 6.4 GHz

Fig. 8.10

Group delay of proposed design

8.4 Conclusion

A half-ring MIMO antenna, which consists of semi-half-ring added to a square patch, has been produced with dual-band characteristics at 7.2 and 8.1 GHz for C-band and X-band applications, respectively. The ineluctable suppression of the isolation has been attained by selecting the distance between the patches nearly 0.25 λ₀. We observed that from the simulation results for the dual-band region |S₁₁| < −10 dB has been achieved and impedance matching maintained at |S₂₁| < −20 dB in the dual resonant frequencies. The group delays (GD) at the dual-band frequencies are 7.85±1 ns and − 0.98±1 ns, and ECC value lower than the 0.03 has been identified for the half-ring design. Thus, this system arrangement improves the parameter of antenna and good radiation patterns have been observed.