Mutual coupling reduction in microstrip antennas using defected ground structure

A Multiple Input Multiple Output (MIMO) antenna with high isolation is proposed in this paper. The proposed antenna includes two sets of four elements (2 × 2) and it is yielded at the central frequency of 5.5 GHz for Wireless Local Area Network (WLAN) applications. Based on RT5880 with height of 1.575 mm, the overall size of MIMO antenna is 140 × 76 × 1.575 mm3. To get high isolation between antenna elements, a Defected Ground Structure (DGS) is integrated on ground plane. Besides, the MIMO antenna witnesses a large bandwidth of 9.1% and an efficiency of 90% while the pick gain is 8.5 dBi. The measurement results are compared to simulation ones to verify the performance of the proposed antenna

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Mutual coupling reduction in microstrip antennas using defected ground structure
gn of MIMO Array Antenna
 The model of the MIMO array antenna is shown
in Figure 4. The proposed antenna consists of two
symmetrical sets of four elements (2 × 2) on the top of
the substrate. The distance between radiation patches
in the MIMO antenna is approximately λ/2 while the
closest gap from edge to edge of the two arrays is 6.5
mm. The antenna is realized on Roger5880TM substrate
with the dimension of 140 × 76 × 1.575 mm3.
 In order to enhance isolation for MIMO antenna, the
DGS is integrated into the ground plane. The ground
plane includes 3 cells of DGS with the distance from
center to center between them of 42.5 mm. Here, the
size of the DGS is 32 × 57 mm. By adjusting some
parameters such as wdgs2, ldgs2, ldgs3, ddgs, lcut, wcut,
we can obtain the desired resonant frequency. Table II
shows some parameters of the MIMO array antenna.
 Figure 5. The reflection coefficient of the single array.
3 Results and Discussions
A. Simulation Results
 the antenna at the frequency of 5.5 GHz is -28 dB while
3.1 Single array antenna the bandwidth is 830 MHz. In this case, the bandwidth
 Figure 5 shows the reflection coefficient of the single is extended by making at least two consecutive resonant
array. From Figure 5, we can see that the return loss of modes. It is clear that use of DGS on ground plane
N. N. Lan: Mutual Coupling Reduction in Microstrip Antennas using Defected Ground Structure 41
 Figure 6. The xz and yz planes of the proposed array antenna.
 (b) (c)
 Figure 8. Simulated gain, S11 and S21 for the different widths of DGS.
 disposal of antenna. The principle of gain enhancement
 as well as mutual coupling reduction, the author is pre-
 sented more detailed in [16]. Then, we can adjust this
Figure 7. The simulated results of S-parameters with and with- distribution by changing the dimensions of DGS. As a
out DGS. result, the most currents are concentrated an identified
 place while the other places are limited. Therefore,
 the isolation of antenna is enhanced. In addition, by
made consecutive cavity resonators and this leads to
 making parasitic inductances and capacitances, the size
creating resonant modes. Utilizing DGS not only en-
 of an element is also reduced when DGS is used (the
hances bandwidth for antenna, but also keep efficiency
 dimensions of an element are 19.5 × 19.5 mm without
at high level. Here, the gain and efficiency of the
 DGS and 19 × 12.5 mm with DGS). This shows that
antenna reach 7.3 dBi and 87%, respectively.
 using DGS not only enhances isolation for antenna,
 Figure 6 illustrates the xz and yz planes of the
 but also reduces size for antenna. However, there is
proposed. The antenna has the directivity of 7.8 dBi
 always a tradeoff in techniques which are used for
while the angular width (3 dB) is 40.5 degree.
 improving parameters of antenna. In this case, utilizing
 DGS changed the position of the main lobe. Normally,
3.2 MIMO Array Antenna if the place of the main lobe is at 0 degree, the main
 As mentioned above, the goal for using DGS in lobe place is 18 degree. However, this is acceptable.
MIMO array antenna is to reduce mutual coupling. This Using DGS not only reduces mutual coupling, but
characteristic is illustrated in Figure 7, which displays also enhance bandwidth for antenna and this is also
a comparison of S-parameters in two cases of without illustrated in Figure 7. If the bandwidth at -10 dB
and with the DGS. of antenna 620 MHz with DGS this data is only 240
 As shown Figure 7, the isolation of antenna without MHz without DGS. Here, there are at least two created
DGS is only 10 dB while with case of DGS, this value resonant modes and as a result, the bandwidth of an-
is greater than 20 dB although the distance between tenna is improved, Moreover, the efficiency of antenna
elements is 30 mm (the distance between elements with is remained at high level with 90%.
DGS is 27 mm). It is clear that there is a significant Figure 8 illustrates simulated gain, S11 and S21 for
improvement in mutual coupling between antenna el- the different widths of DGS (wdgs2 in Table II). Al-
ements when DGS is used. This can be explained as though the S21 values are guaranteed under -20 dB
following: The use of DGS causes a disturbance in in three cases, gain and S11 achieve the best values
current distribution [15] and this leads to current re- with wdgs2 = 32.
42 REV Journal on Electronics and Communications, Vol. 10, No. 1–2, January–June, 2020
 Figure 11. Current distribution of the proposed antenna: (a) MIMO
 antenna including 1st element (left) and 2nd element (right); (b) single
Figure 9. The xz and yz planes of the proposed MIMO array antenna. antenna.
 Figure 10. The ECC of the proposed array antennas.
 Figure 9 shows the xz and yz planes of the proposed
MIMO array antenna. The gain of the proposed array
antenna gets 8.5 dBi while the angular width (3 dB) is
37.4 degree. Besides, another important parameter in
MIMO system to determine diversity performance is
the envelope correlation coefficient (ECC). Here, ECC
is defined as follows [17]: (b)
  ∗ ∗  Figure 12. The prototypes of the proposed MIMO antenna: (a) single
 S11S12 + S21S22
 ρe = . (1) array antenna; (b) MIMO array antenna.
  2 2  2 2
 1 − |S11| − |S21| 1 − |S22| − |S12|
 Figure 10 and Figure 11 display the ECC and current
 Rogers RT/DuroidTM 5880 substrate with thickness of
distribution of the proposed antenna. From Figure 10
 1.575 mm, εr = 2.2 and tan δ = 0.0009. The overall
we can see that the ECC of the antenna is very small in
 sizes of the fabricated single and MIMO antennas are
a wide frequency range (under 0.0025 from 5.15 GHz to
 72 × 72 × 1.575 mm3 and 140 × 76 × 1.575mm3, respec-
5.8 GHz). This shows that the isolation of the proposed
 tively. The measured and the CST computed results
antenna is quite high. Move to Figure 11, there are some
 for the fabricated MIMO and single array are given in
places that the energy flows are concentrated higher
 Figure 13.
other places (red color).
 As displayed in Figure 13(a), the measured
 impedance bandwidth for |S11| < −10 dB is from
B. Measurement Results 5.19 GHz to 5.6 GHz corresponding the bandwidth
 For verification, the prototypes of the MIMO array in percentage of approximately 7.4%. Switch to
antenna, as shown in Figure 12, are fabricated on Figure 13(b), the bandwidth of the MIMO antenna is
N. N. Lan: Mutual Coupling Reduction in Microstrip Antennas using Defected Ground Structure 43
 Table III
 The Comparison between the Previous Works and My Work
 References [18] [19] [20] [21] My work
 Frequency [GHz] 5.4 5.8 28/38 5.8 5.5
 Bandwidth [%] 38 4.7 14.3/5.26 7.7 9.1
 Isolation [dB] 19 22 20 34 20
 Efficiency [%] x x 73 53.7 90
 Gain [dBi] x x 7.5 5.3 8.2
 Size 1.98λ × 0.972λ x 2.4λ × 1.8λ 1.87λ × 0.54 λ 2.56λ × 1.39λ
 Figure 14. The measured gain of the proposed antenna.
 soldering can cause an impedance variation of antenna
 and this directly affects to impedance matching. As
 a result, with MIMO antenna, there are a shift in
 frequency (S11) and the significant change between
 simulated and measured S21. However, there is a better
 result in measurement with single antenna when the
 second resonant mode is very close the simulated
 result and as a result, this mode is also the resonant
 frequency of antenna. Therefore, the frequency band
 for operating of the antenna is still ensured and this
 result is acceptable.
 Figure 14 illustrates the measured results of gain
 of the proposed antenna. While the simulated results
 of the single array and MIMO array are 7.3 dBi and
 (b) 8.5 dBi, the measured ones for these figures are 7
 dBi and 8.2 dBi, respectively. The gain values of an-
Figure 13. Measured results of the S-parameters: (a) single array
antenna; (b) MIMO array antenna. tennas in measurement are lower than the figure in
 simulation. This cause may be due to insertion loss
 of SMA connectors. However, the difference is very
 small. The results in this work have also been compared
500 MHz (9.1%, from 5.16 to 5.66 GHz). In addition, with the previous works as shown in Table III. From
the mutual coupling of the antenna is under -20 dB Table III, we can see that although the isolation of
over a wide frequency range. In these cases, there are antenna [19] is quite high with of 22 dB, however,
differences between measured and simulated results. the bandwidth percentage is not high (under 5%). In
This difference can be attributed to the tolerances of addition, the parameters of efficiency and gain did
the fabricated antenna array. In addition, the SMA not show in these documents [18, 19]. This is similar
44 REV Journal on Electronics and Communications, Vol. 10, No. 1–2, January–June, 2020
in [20] when the percentage of bandwidth is only 5.26. MIMO WLAN applications,” IEEE Antennas and Wireless
Besides, the efficiency and gain of antenna in [20] are Propagation Letters, vol. 14, pp. 751–754, 2014.
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