A Microstrip MIMO Antenna with Enhanced Isolation for WiMAX Applications

 10.2
angular frequency of the parallel ! resonator, 50 is the 34 35.4 336B1 26
resonant frequency, and l is the cutoff angular frequency.
 2 F2 33.5 336B2 24.7
 Then, the resonant frequency can be calculated as
 1
 5A = √ . (3) antenna array is 80 × 102 mm. Table I lists the values of
 2c !
 some dimension parameters of the designed antenna array.
 Next, Figure 2 shows the model of our proposed DGS- These parameters are defined in Figure 2.
based antenna array. The antenna is designed to operate
at 3.5 GHz and it is printed on a single layer FR4 substrate
 2. Design of the MIMO Antenna
with a dielectric constant of 4.4, a thickness of 1.6 mm,
and a loss tangent of 0.02. The antenna array includes four The MIMO antenna consists of two symmetric antenna
radiation elements (2 × 2) and three power dividers on top arrays placed side by side with a separation of 4.3 mm
of the substrate. The ground plane with DGS is placed on from edge to edge as illustrated in Figure 3. There are
the bottom. Using the formula in [10], we can calculate many feeding techniques for antenna such as coaxial feed,
the size of each radiation element. Furthermore, we select coupled, and _/4 impedance transformer [10, 12]. We
equal dividers whose principle can be found in [11]. We choose the _/4 impedance transformer in this paper due
use the CST Studio software to optimize and obtain that to its simplicity in impedance matching. Based on a FR4
the dimension of each element is 23.6 × 20.8 mm and the substrate with a thickness of 1.6 mm, the MIMO antenna
distance between elements is approximately 0.4_ where _ has overall size of 144 × 99 × 1.6 mm while the size of one
is the wavelength in free space. The overall size of the patch is 23 × 20.125 mm.
 100
 Vol. 2019, No. 2, December
 (a) Top view (b) Bottom view
 Figure 3. Configuration of proposed MIMO antenna.
 TABLE II of 40.7 degrees. Moreover, the antenna remains a high
 DIMENSION PARAMETERS (DEFINED IN FIGURE 3) OF PROPOSED radiation efficiency of over 90%.
 MIMO ANTENNA
 b) MIMO Array
 Parameter Value (mm) Parameter Value (mm)
 , 144 ! 99 As mentioned in Section II.2, the MIMO antenna is in-
 30 4.3 336B3 41.14 tegrated with DGS to reduce the effect of mutual coupling.
 To better understand the effect of DGS on mutual coupling
 336B4 25 F36B4 10
 reduction, we compare the S-parameters of the proposed
 ;36B4 6 F36B3 1.5
 MIMO antenna with and without DGS. The results are
 ;36B3 12
 displayed in Figure 6. It can be observed that the isolation
 between antenna elements is significantly improved in the
 Table II lists the value of some parameters of the case of DGS. The mutual coupling is −15 dB without DGS
proposed MIMO antenna. These parameters are defined while with DGS, this figure is less than −40 dB. This can
in Figure 3. To reduce the mutual coupling between the be explained as follows. Normally, the current distribution
elements of the MIMO antenna, we integrate a DGS on in microstrip antenna is uniform without DGS. When there
the ground plane. The DGS consists of five cells (two is DGS, the current is redistributed according to the size
large cells and three small cells). To make the capacitance and position of DGS. By adjusting the size and position
() and inductance (!) variable, we use a compensation of DGS, we can distribute the current at a desired place
structure as shown in Figures 1 and 2. This makes a while limiting the current at other places. This helps us to
flexibility in optimization. achieve a high isolation of 40 dB for the proposed antenna.
 On top of that, using DGS also improves the bandwidth.
III. RESULTS AND DISCUSSIONS We can see that the antenna bandwidth with DGS includes
 two resonant modes while this value is only one without
1. Simulation Results
 DGS. As a result, the bandwidth with DGS is larger than
a) Antenna Array without DGS.
 Figure 4 presents the reflection coefficients of the pro- Figure 7 shows the pattern of the proposed MIMO
posed antenna array over the frequency band from 2.5 GHz antenna. It can be seen that the main lobe direction of the
to 4.5 GHz. As can be seen, the bandwidth of the antenna antenna is 190 degrees while the angular width at 3 dB
is 330 MHz, corresponding to 9.4% of the resonant fre- is 40.7 degrees. Normally, the main lobe direction of a
quency 3.5 GHz. In addition, the antenna has a low return microstrip antenna is 0 degree (uniform current distribu-
loss of −30 dB at the resonant frequency. tion). In our case, utilizing DGS causes a distortion in
 Figure 5 shows the 3D and polar patterns of the proposed the current distribution, thus the main lobe direction can
antenna array. It can be observed that the the antenna be changed. This is a common tradeoff when using DGS.
has quite high directivity: it has an angular width (3 dB) However, given the gain in antenna isolation, this main
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Research and Development on Information and Communication Technology
 Figure 4. Reflection coefficients of proposed antenna array. Figure 6. S-parameters of proposed MIMO antenna.
 (a) 3D (b) Polar (a) 3D (b) Polar
 Figure 5. Pattern of proposed antenna array. Figure 7. Pattern of proposed MIMO antenna.
lobe direction change is acceptable. Besides, the gain and azimuth, respectively. Figure 8 presents the ECC of the
radiation of the MIMO antenna reach more than 7.5 dBi proposed MIMO antenna. It is clear that the ECC is smaller
and 90%, respectively. than 0.01 from 3.08 GHz to 3.84 GHz, corresponding
 Moreover, in MIMO systems, the independence between to a band of 760 MHz. This is suitable for devices with
radiation patterns of the antennas can be evaluated by the d4 ≤ 0.3 [15].
enveloped correlation coefficient (ECC), denoted by d4.
ECC can be calculated from the S-parameters as [13] 2. Measurement Results
 ∗ ∗
 (11(12 + (21(22 In order to confirm the simulation results by experimental
 d4 = . (4)
 r    measurements, the prototype of the proposed antenna as
 1 − |( |2 − |( |2 1 − |( |2 − |( |2 [ [
 11 21 22 12 1 2 shown in Figure 9 is implemented based on a FR4 sheet
It can also be calculated from the radiation patterns as [14] (ℎ = 1.6 mm, YA = 4.4 and tan X = 0.02) with a size
 of 80×102×1.6 mm for a single array and 144×99×1.6 mm
 ∬  (\, q)∗ (\, q)3Ω
 4c 1 2 for the MIMO antenna.
 d4 = , (5)
 ∬  (\, q)∗ (\, q)3Ω ∬  (\, q)∗ (\, q)3Ω
 4c 1 1 4c 2 2 In Figure 10, we compare the simulated results based
where 1 and 2 are the far-field radiation patterns, gen- on the CST Microwave software and the measurement
erated from ports 1 and 2 of the antenna while \ and q ones. As can be seen, the measured impedance bandwidths
represents the spherical angles, namely, the elevation and at −10 dB of a single array and the MIMO antenna
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 Vol. 2019, No. 2, December
 IV. CONCLUSION
 This paper presents a MIMO antenna with enhanced
 isolation for WiMAX applications. The antenna is realized
 at the central frequency of 3.5 GHz and it is built on a
 FR4 substrate with parameters: ℎ = 1.6 mm, YA = 4.4, and
 tan X = 0.02. The proposed MIMO antenna contains two
 sets of 2 × 2 elements which incorporates DGS to obtain a
 high isolation between the elements. From measurements,
 the antenna achieves approximately 30 dB in isolation
 and 90% in radiation efficiency. Moreover, the antenna has
 a gain of 7.5 dBi while the measured bandwidth is 145 MHz
 at −10 dB. The proposed antenna has a compact size, a
 planar structure, an easy fabrication, and a low cost, thus
 can be utilized in practice.
Figure 8. Enveloped correlation coefficient of proposed MIMO antenna.
 ACKNOWLEDGMENT
 TABLE III
 PERFORMANCE COMPARISON OF PROPOSED ANTENNA AND RECENT This work is carried out in the framework of the
 ANTENNAS
 project entitled “Research on methods for mutual cou-
 References [17] [18] [19] This paper pling reduction between array antenna elements in multi-
 Frequency [GHz] 2.4 3.5 2.6 3.5 antenna wireless communication system” under Grant
 Bandwidth [%] 3.3 6.2 14.6 4.15 number CS2019-38.
 Isolation [dB] 15 22 15 29
 Efficiency [%] 85 87 x 90
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 Figure 9. Prototype of fabricated single array (top) and MIMO (bottom) antennas
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