IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 70, NO.

2, FEBRUARY 2023 501

A Compact Quadruple-Band Circular Polarized

MIMO Antenna With Low Mutual Coupling
Asif Khan , Yejun He , Senior Member, IEEE, Zhou He , and Zhi Ning Chen , Fellow, IEEE

Abstract—This brief presents a compact multiband multiple such as total gain and radiation efficiency, other characteris-
input multiple output (MIMO) antenna, where two robot- tics of the MIMO antenna are reduced due to the compactness
character-shaped patches are closely positioned on the top of of multiple antennas [3]. Common isolation enhancement
the substrate for circular polarization. The dimensions of the
approaches including the isolation network [4], neutraliza-
proposed MIMO antenna are 0.34λ × 0.13λ × 0.01λ at 4 GHz
and the distance between two symmetrical elements is 0.05λ tion line [5], and defective ground structure (DGS) [6] have
mm. The robot-character-shaped patch generates four different been discussed. Typical isolation enhancement approach such
frequency bands of 3.85-4.25 GHz, 4.95-5.1 GHz, 6.94-7.35 GHz, as decoupling structure attained an isolation improvement of
and 8-8.3 GHz. The simulation and measurement results show 7-18 dB in [7]. Low mutual coupling between antenna ele-
that the designed antenna has better S-parameters |S11 | ≤ ments is obtained by putting a multi-band antenna with PIN
−10 dB, and an isolation of |S12 | ≤ −25 dB resulting from diodes and a dielectric resonator [8]. Apart from these isolation
the irregular parasitic element to reduce mutual coupling, an
axial ratio (AR) of no more than 3 dB, radiation pattern, total
improvement strategies, the high isolation is achieved without
gain, and efficiency over all operating bands when compared to introducing external decoupling structures in [9]. All of the
the existing two-port MIMO antennas. MIMO antennas diver- structures previously documented have larger dimensions.
sity characteristics are also calculated to evaluate the proposed Several MIMO antenna designs were also proposed to cover
antennas performance. The proposed antenna is a good con- different frequency bands for Wi-Fi, LTE, and WLAN applica-
tender for 5G pioneer band, 5 GHz WLAN band, and satellite tions with MIMO [10] and without MIMO [11] configuration.
communication applications concomitantly. However, most of these MIMO antennas have a multi-layered
Index Terms—Multi-band antenna, multiple-input-multiple- 3-D configuration [10], very large in size [12], inadequate
output (MIMO) antenna, parasitic element, circular polarization, decoupling between MIMO components [13], limiting their
decoupling. applications for future compact, low-profile, and multi-band
MIMO antennas.
I. I NTRODUCTION Circularly polarized MIMO antennas have recently become
ITH the rapid advancement of wireless communication
W systems, higher transmission rate and larger channel
capacity are highly desired to maintain the coverage area
very popular in wireless communication systems because they
eliminate mutual coupling issues and the negative impact on
overall system performance caused by combining multiple cir-
and obtain the high-definition (HD) video streaming without cularly polarized antennas to cover different frequency bands.
any interference [1]. Furthermore, MIMO antenna technology, Circular polarized (CP) antennas provide significant benefits
which improves data rate without increasing input power or over linear polarized antennas because reflected CP signals
bandwidth [2], is one of the most vital features of future result in polarization reversal, therefore CP antennas are highly
wireless communication systems. Although MIMO technol- useful in fighting multi-path interference. As a result, fol-
ogy offers several advantages over competing technologies lowing reflection, a right-hand circularly polarized (RHCP)
incident signal will exhibit left-hand polarization (LHCP),
Manuscript received 28 July 2022; revised 30 August 2022 and
27 September 2022; accepted 4 October 2022. Date of publication which will be attenuated by an RHCP antenna. Furthermore,
6 October 2022; date of current version 9 February 2023. This work compared to linearly polarized antennas, circularly polarized
was supported in part by the National Natural Science Foundation of antennas have better mobility, better alignment between trans-
China under Grant 62071306, and in part by the Shenzhen Science mitter and receiver, and better weather penetration [14]. The
and Technology Program under Grant JCYJ20200109113601723,
Grant JSGG20210420091805014, and Grant JSGG20210802154203011. authors suggested a three-element compact MIMO antenna
This brief was recommended by Associate Editor K.-F. Tong. with polarisation (linear/circular) diversity in [15]. In [16],
(Corresponding author: Yejun He.) a dual-band MIMO antenna with polarization diversity is
Asif Khan and Yejun He are with the Guangdong Engineering Research
Center of Base Station Antennas and Propagation, the Shenzhen Key
proposed. A circularly polarized dielectric-resonator (DR)
Laboratory of Antennas and Propagation, and the College of Electronics two-port MIMO antenna (DRA) was proposed in [17], but
and Information Engineering, Shenzhen University, Shenzhen 518060, China it has limited isolation and a huge dimension, and operates
(e-mail: asifm20019@gmail.com; heyejun@126.com). at one band only. The designs in [18] have high isolation but
Zhou He is with the Department of Mechanical Engineering, University of
Maryland, College Park, MD 20742 USA (e-mail: zhe12@umd.edu). suffer from the large size, small gain, narrow bandwidth, and
Zhi Ning Chen is with the Department of Electrical and Computer Tri-band only.
Engineering, National University of Singapore, Singapore 117583 (e-mail: The primary contributions of this research are as follows.
eleczn@nus.edu.sg).
Color versions of one or more figures in this article are available at
i) Comparitive research is conducted to explain the originality
https://doi.org/10.1109/TCSII.2022.3212618. of the proposed MIMO-CP antenna. In terms of size, band-
Digital Object Identifier 10.1109/TCSII.2022.3212618 width, axial ratio, gain, efficiency, isolation, and other MIMO
1549-7747 
c 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See https://www.ieee.org/publications/rights/index.html for more information.
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502 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 70, NO. 2, FEBRUARY 2023

Fig. 1. Evolution process of the proposed antenna.

TABLE I
D IMENSIONS OF THE P ROPOSED A NTENNA AND D ECOUPLING
S TRUCTURE

Fig. 2. (a) Simulated |S11 | of all antennas, (b) simulated |S21 |, (c) axial
ratio of all the iterative structures, and (d) axial ratio without/with parasitic
element for the proposed antenna.

antenna aspects, the comparison indicates that the developed

The proposed MIMO antenna consists of two robot-character-
antenna outperforms the other designs. ii) To review, none
shaped radiation patches where each patch is stimulated by
of the proposals combine the advantages of a unique design,
a coaxial feed line from the ground plane and a parasitic
multi-band, high isolation, broadband CP characteristics, and
element. The antennas in this MIMO context have the feed
MIMO capabilities in one set, such as our proposed design.
ports at different points, that’s why it slightly affects |S11 |
To the best of the authors’ knowledge, this brief is the first
and |S22 | subsequently. The general assembly is designed on
quad-band MIMO-CP antenna by executing the parasitic ele-
a widely available low-cost FR-4 substrate. The simulated and
ment and multi-band characteristic along with axial ratio less
fabricated models are designed with dimensions of 0.34λ ×
than 3 dB is obtained over entire operating bands.
0.13λ × 0.01λ at 4 GHz. Two coaxial lines with 50  are
used to feed the two patches using lumped port excitation and
II. D ESIGN P ROCESS OF THE A NTENNA the two patches are spaced by a small gap of 0.05 λ. High-
In this section, seven different designs have been simu- frequency structure simulator software (HFSS) has been used
lated and compared with each other to obtain the desired for simulations.
multi-band antenna. And then, a novel robot-character-shaped
MIMO antenna (the best one among all seven different designs B. Decoupling Structure Design and Analysis
in terms of their parameters) operating at four different High isolation between antennas is achieved in this strategy
frequencies is obtained. Finally, an irregular dotted parasitic by adding a decoupling element. Dimensions of the decou-
element is placed between two identical patches for low mutual pling structure are given in Table I. In this brief, an irregular
coupling and circular polarization. dotted-shaped parasitic element has been used to obtain high
isolation. Firstly, a rectangular shape is executed and sup-
A. MIMO Antenna Configuration ported by the other two sub-rectangular shapes which create
an irregular shape. Due to this irregular-shaped decoupling
A rectangular monopole antenna is suggested (Ant-1). The structure, mutual coupling of |S21 | ≤ −20 dB has been signif-
radiator is cut off with a circle slot (Ant-2). Following that, icantly reduced at two operating bands (3.85–4.25 GHz and
another semicircle structure is loaded onto the 2nd iterative 4.95–5.1 GHz). However, in other two operating bands (6.94–
structure to create the 3rd, 4th, 5th, 6th, and then the proposed 7.35 GHz and 8–8.3 GHz), |S21 | remains above −20 dB which
iterative structure is shown in Fig. 1. Dimensions of the should be further reduced. Finally, the parasitic element is cut
proposed antenna are given in Table I. off with numerous small square-shaped holes, resulting in a
Fig. 2(a) presents the reflection coefficient of all the reduced mutual coupling (|S21 | and |S12 |≤ −25 dB) in the
extended iterative MIMO antennas. Ant-1 doesn’t oper- overall operating bands as shown in Fig. 2(b).
ate at any desired frequency as |S11 | ≥ −10 dB. Ant-2
(6.5 GHz and 7.5 GHz), Ant-3 (4.7 GHz and 6.6 GHz), Ant-4
(4 GHz and 5 GHz), Ant-5 (5.6 GHz and 7.6 GHz), Ant-6 C. Axial Ratio
(5.2 GHz and 7 GHz) operate at two different frequencies The parametric study of the axial ratio for the proposed
due to the iterative structures |S11 | ≤ −10 dB, followed by MIMO antenna is studied from two perspectives (by the vary-
Ant-7 (proposed novel antenna) which operates at four center ing structure of the given antenna and by the addition of the
frequencies in three different frequency bands which are C- different irregular structures). The AR of all the proposed
band, S-band, and X-band, respectively. Antenna-7 is slightly structures is given in Fig. 2(c). It can be observed that the
different from antenna-6 because of the small cuts in the cir- polarization is not affected by varying the structure of the
cle which look like a jaw resulting in a quad-band antenna. antenna from a simple rectangular shape to a robotic shape.

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KHAN et al.: COMPACT QUADRUPLE-BAND CP MIMO ANTENNA WITH LOW MUTUAL COUPLING 503

Fig. 3. (a) Simulated model and (b) fabricated model of the proposed antenna.

Actually, using different shapes of the irregular parasitic ele-

ment will help to generate the circular polarization. As shown
in Fig. 2(d), without any decoupling structure, the MIMO
antenna doesn’t work as a circularly polarized antenna. After
appending the irregular structure, the value of axial ratio tends
to 3 dB but the proposed MIMO antenna is still not circu-
larly polarized as the AR > 3 dB. Further modification in
the structure (from an irregular structure to an irregular dotted
structure) provides an AR band < 3dB of 3.8-4.2 GHz, 4.75-
5.2 GHz, 6.9-7.15 GHz, and 8-8.4 GHz, respectively. It is
because the parasitic element with dots serves as a supplemen-
tary radiating source, which generates additional resonances in
axial ratio bandwidth. This dotted slot influences the current
distribution and the electric field.
Fig. 4. Surface current distribution to generate CP currents excited by port-1,
(a) current scale, (b) 4 GHz at 0◦ , (c) 4 GHz at 90◦ , (d) 5 GHz at 0◦ ,
D. Surface Current Distribution (e) 5 GHz at 90◦ , (f) 7 GHz at 0◦ , (g) 7 GHz at 90◦ , (h) 8.1 GHz at 0◦ ,
Surface current distribution demonstrates the performance (i) 8.1 GHz at 90◦ , respectively.
analysis of mutual coupling and other performance of the
designed antenna over the desired frequency band and fur-
ther justified. Fig. 4 depicts the current distributions utilized
to analyze the circular polarization generation of the proposed
antenna. Fig. 4(a) is a current scale. Fig. 4(b) and Fig. 4(c)
illustrate the current distribution at 4 GHz, where the current
is distributed on the patch-1 and the vector rotates clockwise
when the phase is altered from 0◦ to 90◦ . As a result, the
MIMO antenna could radiate LHCP in the broadside direc-
tion at 4 GHz. The same situation also happens when the
antenna-1 is stimulated at 7 GHz, as illustrated in Fig. 4(f) and Fig. 5. Simulated and measured (a) S-parameters and (b) axial ratio.
Fig. 4(g). Meanwhile, Fig. 4(d) and Fig. 4(e), Fig. 4(h) and
Fig. 4(i) depict the antenna-1 being excited at 5 and 8 GHz,
respectively. However, the vector turned anti-clockwise when simulation and measurement results due to manufacturing tol-
the phase changed in this case. erance. Fig. 5 also shows the comparison between simulated
and measured results of axial ratio as AR ≤ 3 dB within the
antenna bandwidth.
III. FABRICATION AND M EASURED R ESULTS The proposed MIMO antenna’s simulated and measured
Fig. 5 shows the proposed antenna’s simulated and mea- radiation patterns for left hand circular polarization and right
sured S-parameters and axial ratio. The simulated results are hand circular polarization in the four operating frequency
in good agreement with experimental measurements of the bands are illustrated in Fig. 6. Due to cable connections and
fabricated antenna. The MIMO antenna covers the frequency measurement errors, there is a small variation between the
ranges of 3.85-4.25 GHz, 4.95-5.1 GHz, 6.94-7.35 GHz, and simulation and experimental results.
8-8.3 GHz. The measured coupling values for the operational Simulated and measured gain and efficiency of the proposed
resonant bands at 4 GHz, 5 GHz, 7.05 GHz, and 8.17 GHz antenna are shown in Fig. 7, where the gain is just above 4 dBi
are less than −35 dB, −45 dB, −40.54 dB, and −26.5 dB, in the first and second band, 3 dBi in the third band, and above
respectively, indicating that the MIMO antenna achieved high 5 dBi in the fourth operating band. Similarly, the radiation
isolation. There is a slight deviation of S-parameters between efficiency endures above 80% overall bands, respectively.

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504 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 70, NO. 2, FEBRUARY 2023

Fig. 9. Simulated (a) TARC and MEG, (b) multiplexing efficiency and
channel capacity loss of the proposed MIMO antenna.

Similarly, low ECC leads to high diversity gain which shows

good MIMO performance, as shown in Fig. 9(b).
The total active reflective coefficient (TARC) is a source
that further describes the data rate and efficiency of the MIMO
Fig. 6. Simulated and measured radiation patterns (LHCP and RHCP) of antenna. It can be given by [17]
the proposed antenna at (a) 4 GHz, (b) 5 GHz, (c) 7 GHz, and (d) 8.1 GHz 
when port-1 is excited and port-2 is terminated. N
i=1 |hi |
2
a = 
t
(3)
N
i=1 |gi |
2

where |gi |and |hi | are the incident signals and reflected signals,
respectively. The value of TARC ≤ 0 means that the whole
amount of power is radiated competently, where this designed
antenna value of TARC is less than zero in all operating bands
as shown in Fig. 9(a).
The suggested MIMO antenna’s multiplexing efficiency was
Fig. 7. Simulated and measured (a) gain and (b) efficiency of the proposed
computed using the following relation [9].
antenna.

ηMux = 1 − |ρ|2 η1 η2 (4)
where η1 , η2 and ρ denote the efficiency of element-1, the
efficiency of element-2, and the complex envelope correlation
coefficient (ρ ≈ |ECC|2 ) between the two elements. As shown
in Fig. 9(b), ηMux for all operating bands ≥ −1dB, which is
reasonable for a MIMO operation.
Mean effective gain (MEG) evaluates the function of the
Fig. 8. Simulated and measured far field ECC and diversity gain. MIMO antenna in a wireless environment. It can be calculated
by using S-parameters as
(1 − |Sii |2 − |Sij |2 )
IV. D IVERSITY C HARACTERISTIC A NALYSIS MEG = (5)
(1 − |Sij |2 − |Sjj |2 )
The ECC describes the amount of radiation pattern
dependent between the two antenna components and shows The value of MEG ≤ 3 dB shows a better MIMO antenna
the isolation between them. ECC may be calculated using far performance [17]. The simulated MEG of the proposed MIMO
field as antenna is given in Fig. 9(a). It can be noticed the value is
 smaller than 3dB, which means that the projected antenna has
|  
4π [F1 (θ, φ) × F2 (θ, φ)d] |
2
ρe =    (1) improved diversity parameter.
|  
4π [F1 (θ, φ)] | d | 4π [F2 (θ, φ)] | d
2 2 The CCL provides information about the upper limit of the
communication that can be flourished executed without any
where F  1 (θ, φ) is far field of the MIMO array when port-1 loss. CCL can be determined by
is excited. The solid angle is denoted by . Meanwhile, in
contrast to a single antenna, the enhancement in multiple CCL = log2 Det(ai ) (6)
 
antennas’ signal-to-noise ratio (SNR) is described by diversity. aii aij
a =
i
(7)
It can be calculated for a MIMO antenna as [20] aji ajj

DG = 10 1 − β (2) aii = 1 − (|Sii |2 + |Sji |2 ) (8)
ajj = 1 − (|Sjj | + |Sij | )
2 2
(9)
In a MIMO system, larger channel capacity requires low
ECC ≤0.5 [9], where ECC in the proposed MIMO antenna aij = (Sii ∗ Sij ) + (Sji ∗ Sjj ) (10)
is less than 0.5 almost equal to 0 in all operating bands. aji = (Sjj ∗ Sji ) + (Sij ∗ Sii ) (11)

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KHAN et al.: COMPACT QUADRUPLE-BAND CP MIMO ANTENNA WITH LOW MUTUAL COUPLING 505

TABLE II
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