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iet-map.2016.1154

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IET Microwaves, Antennas & Propagation
Research Article
Microstrip antenna array mutual coupling
suppression using coupled polarisation
transformer
ISSN 1751-8725
Received on 12th December 2016
Revised 20th June 2017
Accepted on 5th July 2017
E-First on 7th September 2017
doi: 10.1049/iet-map.2016.1154
www.ietdl.org
Kun Wei1 , Jian-Ying Li1, Ling Wang1, Rui Xu1
1School of Electronics and Information, Northwestern Polytechnical University, No. 127, Youyi Road (West), Beilin, Xi'an, People's Republic of
China
E-mail: weikun916@163.com
Abstract: This study presents a periodic defected ground structure (PDGS) to suppress the mutual coupling (MC) between
coplanar antenna elements in a multiple-input and multiple-output (MIMO) communication system. The proposed PDGS
consists of three defected ground structure units. They are etched away from the ground plane between antenna elements. The
coupled polarisation transformer characteristic of the proposed PDGS is utilised to increase the antenna elements isolation. The
increased coupled vertical linear polarisation component and decreased coupled horizontal linear polarisation component
contribute to reduce the MC between antenna elements. By using this method, the authors achieve low MC level and better
MIMO corresponding capacity. This method has little influence on the far-filed main radiation properties of the antenna
elements.
1 Introduction
Microstrip antennas are popularly used in wireless communication
systems, because of its light weight, low profile and ease to form
antenna array. The high antenna isolation in a coplanar antenna
array is very important for many communication applications,
especially for the multiple-input and multiple-output (MIMO)
wireless communication system. The channel capacity of the
MIMO communication system is reduced when the received
signals by different antenna elements are correlated. Therefore, the
mutual coupling (MC) reduction plays an important role in MIMO
wireless communication system.
One of the efficient ways to suppress the MC is using special
band-gap structures. The defected ground structure (DGS) [1] is
one of the most common band-gap structures, which is used to
reduce the MC by suppressing the surface current [2]. The classic
dumb-bell shaped DGSs are studied to reduce the MC between
antenna elements. A high isolation dual-frequency orthogonally
polarised patch antenna utilising microstrip feed line integrated
with a dumb-bell spiral-shaped DGS is presented in [3].
Improvement in port isolation by 20 dB relative to a patch antenna
is observed. The developed dumb-bell shaped DGS inserted
between the patches for MC reduction is presented in [4]. By
suppressing the surface waves, the 19 dB MC reduction is
achieved. The structure consists of a dumb-bell-like pattern etched
in the ground plane, and is proposed in [5]. It is found that more
than 40 dB isolation can be achieved between two parallel
individual planar inverted F antennas (PIFAs). The structure based
on split-ring dumb-bell-like periodic DGS (PDGS) is studied in
[6]. Using the structure, it is possible to achieve a 10 dB reduction
in the MC between two patch antennas.
As the technique develops, some DGSs with simpler structures
are studied to reduce the MC between antenna elements. The
simple ring-shaped DGS is demonstrated to suppress MC between
two cylindrical dielectric resonators. About 5 dB MC suppression
is achieved near the operating frequency [7]. Two kinds of DGSs,
namely back-to-back U-shaped and dumb-bell shaped DGSs, are
analysed and compared in [8]. The analysis indicates that the
simple back-to-back U-shaped DGS is better at suppressing
propagation of surface waves. About 18 dB reduction in the MC
between patch antennas is achieved. Three simple rectangular
DGSs are periodically spaced between the patch antennas. By
suppressing the surface waves through a triple rectangular DGS,
IET Microw. Antennas Propag.
© The Institution of Engineering and Technology 2017
about 16.5 dB MC reduction is observed [9]. The simple periodic
rectangular DGSs are studied in [10]. This proposed DGSs have
reduced the MC for more than 20 dB compared with the case
without DGSs.
When designing a planar band-gap structure filter, the
minimisation of circuit dimension is desired. The implementation
of a fractal DGS is presented in [11] to reduce the interaction
between two planer monopole patch antennas. The design achieves
a 7 dB reduction in MC level. The novel fractal DGS to suppress
MC between coplanar spaced antenna elements is presented in
[12]. More than 35 dB MC reduction is obtained using the fractal
DGS.
For the MIMO wireless communication system, high antenna
isolation is needed [13]. A T-shaped DGS is presented in [14] to
suppress the MC between PIFA in a MIMO wireless
communication system. The high antenna isolation is achieved,
which will contribute to achieve better MIMO system correlation
and better channel capacity performance. With the etched DGS, the
high antenna element isolation (which is less than −25 dB) between
printed antennas is obtained. This helps to achieve the better
MIMO channel capacity performance [15]. The design of high
isolation using DGS between antenna elements in a MIMO system
is presented in [16]. The isolation of the antenna elements is about
28 dB. Some relevant comparisons with earlier works are shown in
Table 1. For most of researches, the reason for reducing the MC by
DGS is suppressing the surface currents. While the reason for
suppressing the MC in this paper is using the DGS coupled
polarisation transform characteristic.
This paper presents a PDGS to suppress the MC between
antenna elements in a MIMO wireless communication system.
Three DGS units are etched from the ground plane between
antenna elements. The coupled polarisation transform
characteristics of the proposed PDGS are studied to suppress the
MC between antenna elements. Using this method, we achieve low
MC level and better MIMO corresponding capacity. This proposed
method has little influence on the far-filed main radiation
properties of the antenna elements.
2 Geometries of the designed antennas and
PDGS
The geometries of the proposed PDGS and antenna elements are
shown in Fig. 1. The antenna elements have the resonant frequency
1
Table 1 Comparison of MC reduction using DGSs
Ref.
MC reduction
Structure
[4]
[5]
[6]
[8]
[9]
[10]
[12]
[15]
[18]
this work
19 dB
40 dB
10 dB
18 dB
16.5 dB
20 dB
35 dB
25 dB
19.6 dB
15 dB
dumb-bell shaped DGS
dumb-bell-like DGS
dumb-bell-like PDGSs
U-shaped and dumb-bell shaped DGSs
rectangular DGSs
periodic rectangular DGSs
fractal DGSs
rectangular DGSs
microstrip parasitic isolator
spiral-shaped PDGSs
Explanation
suppress the surface waves
suppress the surface waves
suppress the electric fields normal to the ground plane
suppress the surface waves
suppress the surface waves
suppress the surface waves
suppress the surface waves
suppress the surface waves
control the polarisation
transform the coupled polarisation
Fig. 2 MC reduction with DGS
between antenna elements is observed. It shows that the frequency
after the PDGS is etched. As the resonant antenna resonant
frequency shifts about 15 MHz to upper frequency of the antenna
can be shifted by simply adjusting the antenna patch dimension, the
proposed PDGS is efficient in application.
The envelope correlation is one of the key parameters for
MIMO system, since the high level of the antenna elements MC
may degrade the system corresponding capacity performance. For
two elements antenna array in the MIMO communication system,
the envelope correlation equation is given as [18]
ρe =
Fig. 1 Geometries of the designed
(a) Antenna elements with PDGS, (b) DGS unit
band from 2.245 to 2.3 GHz (centre frequency is about 2.27 GHz).
They are placed collinearly along the E-plane with the centre-tocentre distance d1. The substrate has relative dielectric constant 6,
thickness 3.18 mm and loss tangent 0.0023. The antenna element
has outline size s = 35 mm, patch length l = 30 mm. Three DGS
units are placed along the y-axis periodically, and they have centreto-centre distance d2. The single DGS unit, as shown in Fig. 1b,
consists of two spiral arms symmetric around the centre point and
interlace with each other. The gap width of the etched structure is
g, length of each arm is l0 = l1 + w1 + l2 + w2 + l3 + w3.
3 MC suppression performance
The model of the proposed PDGS and the microstrip antennas are
analysed using Ansys HFSS [17] (high-frequency structure
simulator). The optimised parameter values are: s = 35 mm, l = 25 mm, d2 = 25 mm, a = 2 mm, l1 = 20.5 mm, l2 = 14.5 mm, l3 = 5.5 mm, w1 = 16 mm, w2 = 9 mm, w3 = 2 mm and g = 1 mm.
Fig. 2 shows the return loss (S11) and MC (S12) of the antenna
array with and without the PDGS when d1 = 70 mm, which equals
to 0.53 free-space wavelength. More than 15 dB MC reduction
2
2
1 − S11
2
∗
∗
S11
S12 + S21
S22
2
− S21 1 − S22 2 − S12 2
(1)
Equation (1) is used to calculate the envelope correlation of this
MIMO antenna array. Fig. 3 illustrates the array envelope
correlations without and with the proposed PDGS. The envelope
correlation value of the MIMO array with the PDGS is much lower
than that without the PDGS. The envelope correlation reduction
will increase the MIMO communication system corresponding
capacity. That is to say, high antenna isolation indicates better
corresponding capacity performance for MIMO communication
system.
The main and cross-polarisation (XP) radiations of the antenna
element 1 with and without the PDGS are plotted in Fig. 4. Only
element 1 is excited while the other element is 50 Ω loaded. No
significant difference is observed between the simulated and
measured main polarisation radiation patterns in the upper-sphere
radiated space. However, the XP radiation patterns with the PDGS
get worse comparing to that without the PDGS. The reasons of
antenna isolation improvement and XP level degradation when the
proposed PDGS is etched in the ground plane will be provided in
the next section.
4 Coupled polarisation transform characteristic
For better understanding why the MC reduction is achieved using
the PDGS, the antenna coupled polarisation is studied. Fig. 5
shows the surface current vectors graphs of the antenna element 2
without the PDGS at time T = 0, π/2, π and 3/2π from Figs. 5a–d.
IET Microw. Antennas Propag.
© The Institution of Engineering and Technology 2017
Fig. 3 Envelope correlation of the MIMO antenna array without and with
the proposed PDGS
Fig. 6 Coupled surface current vectors of coupled antenna with the DGS
Fig. 4 Radiation patterns with and without the proposed FDGS on
(a) E-plane, (b) H-plane
Fig. 5 Coupled surface current vectors of coupled antenna without the
DGS
IET Microw. Antennas Propag.
© The Institution of Engineering and Technology 2017
Only antenna element 1 is excited, while the other one is 50 Ω
loaded. Element 1 is linearly polarised at the horizontal direction,
the element 2 has the same coupled linear polarisation mode. Fig. 6
shows the coupled surface current vectors graphs of antenna
element 2 with the PDGS at time T = 0, π/2, π and 3/2π from
Fig. 6a–d, when only element 1 is excited and element 2 is 50 Ω
loaded. The element 1 is horizontal linear polarised, coupled
polarisation of element 2 rotates 90° from horizontal direction to
vertical direction. Thus, the received energy of element 2, which is
radiated by element 1, is reduced comparing to that without the
PDGS. In this way, the MC level between antenna elements is
decreased and XP level is increased.
The current vectors on ground plane under antenna element
change back and forth along the horizontal direction, while the
current vectors of the proposed PDGS are along the vertical
direction. The PDGS is couple-fed by antenna element and the
current vectors are along the vertical direction. This may cause the
coupled polarisation reversal on the antenna patch. There is some
other method to reduce the MC. Using the special band-gap
structures is one of the efficient ways to increase antenna isolation.
The MC reduction is achieved by suppressing the surface waves in
the ground plane. It is possible to use this method without breaking
the ground plane. A parasitic isolator studied in [19], which is
printed between two patches, controls the polarisation of the
coupling field to reduce the antenna coupling. Comparing to the
method in [19], the main novelty of this paper is using the coupled
polarisation transformer DGS for achieving MC reduction. The
DGS is able to transform the coupled polarisation from horizontal
direction to vertical direction. Moreover, the MC reduction
performance and the coupled polarisation transform characteristic
are studied in this paper to illustrate the working principle of the
proposed PDGS.
The simulated S12 between antennas elements with and without
the proposed PDGS are plotted in Fig. 7, where d1 is given
different numerical values. The distance varies from 0.53 freespace wavelength to 1.1 free-space wavelength in a step of 0.076
free-space wavelength. As the distance between antenna elements
increases, the MC of antenna array without PDGS decreases from
−16 to−21 dB. However, the MC with PDGS increases from −30 to
−23 dB when d1 increases. That is to say, MC reduction
characteristic of PDGS gets worse as distance d1 increases.
The surface current vectors of element 2 with different antenna
element distances are indicated in Fig. 8 when antenna element 1 is
excited and element 2 is 50 Ω loaded. Fig. 8a shows the surface
current vectors of element 2 when d1 = 70 mm. The coupled
polarisation of element 2 with the PDGS rotates 90° from
horizontal to vertical direction. Due to that, the MC is greatly
reduced. Fig. 8b shows the surface current vectors of element 2
when d1 = 80 mm, horizontal linear polarisation component
increases and vertical linear polarisation component decreases. The
increased horizontal linear polarisation component degrades the
MC reduction performance. The horizontal and vertical linear
polarisation components co-exist in Fig. 8c when d1 = 90 mm.
3
Fig. 7 MC reduction with different antenna elements distance
Fig. 10 Simulated and measured main polarisation
than 15 dB MC reduction between antenna elements is achieved in
both simulation and measurement. Note that the measured S11 is
slightly shifted to lower frequency than that of the simulated result.
The main reason, except for the system and measuring errors, for
these different between simulated and measured results is that the
performance of the antenna is very sensitive to the patch
dimension.
Fig. 10 shows the measured and simulated main polarisation
radiation patterns of antenna element 1 with the proposed PDGS.
Only element 1 is excited and the other one is 50 Ω loaded. The
measurements meet well with simulations. No significant
differences are observed between the measurement and simulation
in the upper-sphere radiated space.
6 Conclusion
Fig. 8 Coupled surface current vectors with different antenna elements
distance
This paper presents a PDGS for suppressing the MC between
antenna elements in a MIMO communication system. The coupled
polarisation transform characteristic of this proposed PDGS is
studied to verify the performance enhancement in MC reduction.
The increased coupled vertical linear polarisation component and
decreased coupled horizontal linear polarisation component
contribute to reduce the MC between antenna elements. Both
simulated and measured results are presented to validate that the
MC effect between antenna elements. More than 15 dB MC
reduction is achieved. The MIMO system corresponding capacity
performance gets better because of MC reduction. The far-field
radiation patterns with and without PDGS show that proposed
PDGS has little effects on the antenna upper-sphere radiation
patterns.
7 Acknowledgments
This work was supported in part by the National Natural Science
Foundation of China (no. 61301093, no. 61601372 and no.
61601373).
Fig. 9 Simulation and measurement S-parameters
8 References
There is pure horizontal component when d1 = 100 mm. No MC
reduction is achieved when there is no coupled vertical linear
polarisation component. In general, the increased coupled vertical
linear polarisation component and decreased horizontal linear
polarisation component contribute to reduce the MC between
antenna elements. As distance d1 increases, the coupled vertical
linear polarisation component decreases and horizontal linear
polarisation component increases. This explains why the MC
reduction performance using the proposed PDGS gets worse when
d1 increases.
[1]
5 Simulation and measurement results
The antenna elements and the proposed PDGS are fabricated based
on the optimised parameter values. The simulated and measured
S11 and S12 of the antenna array with the PDGS are shown in Fig. 9
when d1 = 70 mm, which is to 0.53 free-space wavelength. More
4
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