The development of the use of EBG structures in recent years has shown that the use of UC-EBG is able to increase the isolation between microstrip antenna elements, so that it can be used as an alternative design to improve the performance of microstrip array antennas that have large and massive arrays. In its early development, the practical application of the EBG structure had difficulty in accommodating the large physical size because it had to be designed at half-wavelength at the stop band frequency [23]. However, in its development, various designs have been proposed by several researchers [12]-[26] showing improvements and sizing solutions for UC-EBG so that it can be applied in various applications.
In this paper, we observe the application of UC-EBG with a new configuration that surrounds the massive array element, then observe its effect on mutual coupling performance, ECC and also the effect on the massive array antenna gain. The UC-EBG design begins by calculating the capacitance (C) and inductance (L) parameters produced by the material gap between elements by following the following equation [12]:
$$C= \frac{W{{\epsilon }}_{0}(1+{{\epsilon }}_{0})}{\pi }{\text{cosh}}^{-1}\left(\frac{W+g}{g}\right)$$
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and,
with,
$${f}_{0}= \frac{1}{2\pi LC}$$
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\(h\) is substrate thickness, \({\mu }_{0}\) is the permeability of free space, \(W\) is EBG structure width, \({\epsilon }_{0}\) is permittivity of free space, and \(g\) is gap between two EBG cell.
By determining the frequency band in the EBG area, we can calculate the dimensions of the EBG structure (W) and the gap between them (g) as shown in Fig. 1.
Furthermore, after obtaining the dimensions of the EBG (W) and the gap value (g), the next step is to find the size of the strip line (ls) as shown in Fig. 1 by following Eq. (4):
$${l}_{s}= {\alpha }-\text{s}$$
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Where \(\alpha\) is the frequency periodicity and s is the inductance value which has been converted in mm.
The calculation results following Eq. (1)-(4) obtained the size of the UC-EBG structure at a frequency of 3.5 GHz as shown in Table 1 and Fig. 2.
Table 1
Dimension of UC-EBG Structure
No. | Parameter | Dimension (mm) |
1. | W | 5.00 |
2. | G | 1.00 |
3. | \({l}_{s}\) Horizontal | 3.05 |
4. | \({l}_{s}\) Vertical | 3.81 |
In this study, the massive MIMO antenna design uses 64 elements and the position of the UC-EBG unit cell is placed at a distance of 0.25λ or in the middle between the elements. Thus, it is necessary to adjust the UC-EBG strip line structure to make each UC-EBG unit cell parallel. Therefore, the length of the strip line structure on the UC-EBG between horizontal and vertical is different. The length of the horizontal strip line structure is 3.05 mm, while the length of the vertical strip line structure is adjusted and the length is 3.81 mm.
Design of MIMO antenna 8x8 (64 elements) as shown in Fig. 3. The distance between the elements is half a wavelength. The microstrip antenna element is designed following the standard procedure and the rectangular microstrip antenna equation [27]:
$${W}_{p}=\frac{c}{{2f}_{r}\sqrt{\frac{{{\epsilon }}_{r}+1}{2}}}$$
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$${{\epsilon }}_{eff}=\frac{{{\epsilon }}_{r}+1}{2}+\frac{{{\epsilon }}_{r}-1}{2}\left(\frac{1}{\sqrt{1+12\frac{h}{{W}_{p}}}}\right)$$
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$${L}_{eff}=\frac{c}{{2f}_{r}\sqrt{{{\epsilon }}_{eff}}}$$
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$$\varDelta L=0.412h\frac{({{\epsilon }}_{eff}+0.3)\left(\frac{{W}_{p}}{h}+0.264\right)}{({{\epsilon }}_{eff}-0.258)\left(\frac{{W}_{p}}{h}+0.8\right)}$$
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$${L}_{p}={L}_{eff}-2\varDelta L$$
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\({W}_{p}\) is width of patch antenna element, \({L}_{p}\) is length of patch antenna element and h is height of patch antenna element. \({\epsilon }_{r}\) is permittivity relative, \({W}_{g}\) is width of the ground plane and \({L}_{g}\) is length of the ground plane.
The results of the design according to the working frequency of 3.5 GHz as shown in Table 2 and the structure of the antenna elements as shown in Fig. 3.
The placement of the UC-EBG structure configuration on the 64-element massive MIMO antenna proposed in this study is shown in Fig. 4, where the UC-EBG structure limits the entire massive MIMO antenna element structure.
Table 2
Dimension of Massive MIMO Antenna
No. | Dimension | Size (mm) |
1. | \({W}_{p}\) | 19.60 |
2. | \({L}_{p}\) | 14.60 |
3. | \({W}_{g}\) | 321.79 |
4. | \({L}_{g}\) | 276.79 |
To determine the effect of the addition of the UC-EBG structure on the performance of the massive MIMO antenna, in this study we compare the performance of the massive MIMO 8x8 (64 elements) antenna without UC-EBG as shown in Fig. 3 with the performance of the massive MIMO antenna with the addition of the UC-EBG structure as shown in Fig. 4. The flowchart of the workflow process as shown in Fig. 5.