Mutual Coupling Reduction in Circular Polarized

MIMO Antenna Using An Electromagnetic

Bandgap Structure

Yu Dang, Jiaran Qi, Yongheng Mu, Yue xu, and Jinghui Qiu

Department of Microwave Engineering, School of Electronics and Information Engineering

Harbin Institute of Technology, No.2 Yi Kuang Street, Nan Gang District, Harbin, China 150080

Abstract - A two-element directional MIMO antenna with

suppressed mutual coupling whose element is a simple coplanar waveguide fed circular polarized (CP) rectangular monopole antenna with a slotted asymmetric ground plane and an E-type loop. The simulated elements performance indicate that the 10-dB impedance bandwidth reaches 71.8% (2.64 GHz, 1.57–4.21 GHz) and the 3-dB axial-ratio bandwidth is 74% (2.12 GHz, 1.8–3.92 GHz). In order to reduce mutual coupling between the elements an electromagnetic bandgap (EBG) structure is employed to act as a shield which shows remarkable performance to suppress the surface waves. The effect of EBG corrugations results in a mutual coupling improvement of about 12 dB over a frequency band from 1 to 4 GHz.

Index Terms — MIMO Antennas, Electromagnetic Bandgap, Mutual Coupling, AP-related topics.

1. Introduction

Reducing the multipath fading and increasing transmission

quality in wireless communications has challenged many

researchers in recent years [1]. In order to obtain better data

transmission capacity in communication systems and

increased reliability in rich scattering environments,

multiple-input–multiple-output (MIMO) technologies have

been proposed to ease these problems. Since MIMO could

provide a spatial multiplexing gain, diversity gain, improved

link reliability and data rates, many MIMO antennas are

designed for future wireless communications such as

wireless local area network WLAN), World Interoperability

for Microwave Access (WiMAX), Long-Term Evolution

(LTE) and cellular net-work frequency bands over past years,

and the demand for MIMO systems will increase in the

future [2].

Many different types of elements have been presented to

achieve the MIMO antenna. A two-element directional

MIMO antenna whose element is similar to the quasi-Yagi

configuration with the capacity of operating at both the

telemetry L-band 1.27–1.43 GHz and the global system for

mobile communications/long-term evolution band 1.8–2.133

GHz is proposed by using a semi-loop meandered driven

element and a small ground plane in [3].

However, the above-mentioned MIMO unit are all in

linearly polarized operation which has many disadvantages

in numerous communication systems such as GPS, RFID,

WLAN, and WiMAX when compared with circular

polarized (CP) antennas. Since CP antennas are characterized

by insensitivity toward the equipment’s orientation,

resistance to inclement weather, and mitigated multipath

losses a CP element which is a simple rectangular monopole

antenna with an asymmetric ground plane and an open loop

is chosen to achieve MIMO [4].

In consideration of reduction of mutual coupling between

the elements an EBG corrugations is employed to act as a

shield which shows remarkable performance to suppress the

surface waves. The effect of EBG corrugations results in a

mutual coupling improvement of about12 dB over a

frequency band from 1 to 4 GHz while the radiation

characteristic of the element is not affected seriously.

2. Element Design

In order to obtain the CP performance in a wide band and

taking the profile and cost into consideration the printed

monopole antennas whose structure is simple and can be

integrated with many communication systems easily could

be an appropriate candidate.

Fig. 1. Aantenna evolution steps ordered by four prototypes.

S11AR

AR=3dB

S11=-10dB

Frequency (GHz)

S1

1 o

rA

R (

dB

)

Fig. 2. Ssimulated results of the 10-dB impedance bandwidth

and the 3-dB axial-ratio bandwidth.

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

[ThP-13]

691

The general principle of generating CP operation is

obtaining degenerate modes that are 90° out of phase by

changing geometry of the monopole, ground or the feed to

provide extra current path or obstruction..

A coplanar waveguide fed CP printed monopole antenna is

proposed. The main radiating element of the antenna is a

rectangular monopole whose right part is bigger than the left

to associate with the E-type loop in a wide band. The

asymmetric ground plane make the CP radiation in upper

band possible, and the slotted right part becomes an

obstruction of surface current with a chaotic phase. Then, a

parasitic E-type loop is employed to achieve wide-

impedance bandwidth and broadband CP characteristic. In

Fig. 1, antenna evolution steps are explained by four

prototypes.

By optimizing the geometry parameters the simulated

results show that the 10-dB impedance bandwidth reaches

71.8% (2.64 GHz, 1.57–4.21 GHz) and the 3-dB axial-ratio

bandwidth is 74% (2.12 GHz, 1.8–3.92 GHz) as plotted in

Fig. 2.

3. MIMO antenna with EBG

Since reducing the antenna size in wireless systems has

caught many attention, the space left between elements is

limited. However, mutual coupling is always stronger in

closer elements and worsens the input impedance of each

array element which may result in reducing the antenna gain,

bandwidth and affecting the radiation pattern. The surface

wave between adjacent elements is usually a main

disadvantage which enlarges the coupling between elements.

Fig. 3. An electromagnetic bandgap (EBG) structure with

unit cell boundary in CST studio.

Fig. 4. Schematic of the MIMO antenna with EBG.

In consideration of reduction of mutual coupling between

the elements an electromagnetic bandgap (EBG) structure as

shown in Fig. 3, which arranges periodically and shows

remarkable performance to improve antenna performance by

suppressing the surface waves is employed to act as a shield.

EBG structures are based on the photonic bandgap

phenomena in optics and in this work we use a 2D EBG

printed between the above-mentioned coplanar waveguide

fed CP printed monopole antenna.

The mechanism behind EBG structures is that it can

increase the surface impedance and act as a high impedance

layer at any frequency bands. Changing shape and size of

this structure will result in tuning the frequency band.

Suppression of surface-wave propagation, and the in-phase

reflection coefficient can be attributed to the presence of

bandgap in this structure.(EBG) The goal in this work is

reducing mutual coupling between units by utilizing the

surface-wave suppression capacity of the 2D EBG structure.

The schematic of the MIMO antenna with EBG is depicted

in Fig. 4. The field distribution of MIMO antenna depend on

whether there is EBG as shown in Fig. 5. Simulated S21 is

depicted in Fig. 6.

Fig. 5. The simulated two-dimensional electric field intensity

profiles of the MIMO antenna with EBG or not.

Frequency (GHz)

S2

1 (

dB

)

Fig. 6. Simulated S21 of the MIMO antenna with EBG or not.

4. Conclusion and discussion

A two-element directional MIMO antenna with suppressed

mutual coupling with the help of EBG is proposed. The

simulated elements results are that the 10-dB impedance

bandwidth reaches 71.8% and the 3-dB axial-ratio

bandwidth is 79.3%. The effect of EBG corrugations results

in a mutual coupling improvement of about 12 dB over a

frequency band from 1 to 4 GHz.

References

[1] T. S. See and Z. N. Chen, “An ultrawideband diversity antenna,” IEEE

Trans. Antennas Propag., vol. 57, pp. 1597 1605, 2009. [2] Z. Ying, “Antennas in cellular for mobile communications,” Proc. IEEE,

vol. 100, pp. 2286–2296, 2012.

[3] S. S. Jehangir1, M. S. Sharawi, A. Shamim, “Highly miniaturised

semi-loop meandered dual-band MIMO antenna system,” IET Microw.

Antennas Propag., vol. 12, pp. 864-871, 2018.

[4] K. Ding, Y. X. Guo and C. Gao, “CPW-Fed Wideband Circularly

Polarized Printed Monopole Antenna With Open Loop and

Asymmetric Ground Plane,” IEEE Antennas & Wireless Propagation

Letters, vol. 16, pp. 833-836.

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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