Array Mutual Coupling Reduction Using L-Loading E-Shaped Electromagnetic Band Gap Structures

A mutual coupling reduction method between microstrip antenna array elements is proposed by using periodic L-loading Eshaped electromagnetic band gap structures. Two identical microstrip patch antennas at 2.55GHz are settled together and used to analyze the performance of the designed two-element antenna array. The two antenna elements are settled with a distance of about 0.26λ. To reduce the mutual coupling, the L-loading E-shaped electromagnetic band gap structures are used between these antenna elements. The simulated and measured results show that the isolation of the antenna array reaches 38 dB, which has a mutual coupling reduction of 26 dB in comparison with the antenna array without the decoupling structures.


Introduction
Microstrip antenna and its array are very popular in wireless communication systems in recent decades.Moreover, with the demand of large scale antenna arrays and small size, the mutual coupling reduction between the antenna array elements becomes more serious.Therefore, the technique to reduce the coupling is more urgent to be boosted.There are many techniques that are effective in reducing mutual coupling in which three solutions are prominent.The first categorization is to add parasitic elements on each patch antenna, which can achieve a 21 dB mutual coupling reduction between the antenna array elements in the bandwidth of 2.2% [1,2].The second group is to put various defected ground structures on the ground to improve the isolation [3][4][5].As a result, a mutual coupling has been reduced to about 15 dB with only one defected ground structure.The third solution is to use unique band-stop structure to reduce the mutual coupling of coplanar antenna array such as electromagnetic band gap (EBG) structures.After that, various EBG structures have been put between the two antenna elements to reduce the coupling, including two EBG cells [6][7][8][9][10][11].However, these EBG structures may change the center frequency of the antenna resonances.
From the three solutions discussed above, the EBG structures are very popular and have been widely adopted to give a coupling reduction in antenna array design [12][13][14][15].There are two typical methods to carry out an antenna array.However, these coupling structures depend on whether the EBG structures and the antenna array are in the same plane or not [16].If the EBG structure and the patch antenna array are not in the same plane, a multilayer EBG [16][17][18] structure composed of high and low permittivity layers is utilized.
In this paper, a mutual coupling reduction method between microstrip antenna array elements is proposed by using periodic L-loading E-shaped EBG structures.The proposed EBGs are settled between the two antenna elements and are placed in the same plane with the two antenna elements.By using the proposed EBG structures, more than 26 dB mutual coupling reduction is achieved without affecting the radiation patterns on the proposed antenna array.

Initial Design
This paper presents periodic L-loading E-shaped electromagnetic band gap structure to reduce the mutual coupling between two identical microstrip antenna elements.The proposed periodic L-loading E-shaped EBG is installed on the same plane and set between the two antenna elements.A mutual coupling reduction of 26 dB has been achieved on the basis of the proposed EBG structure which has slight effects on the radiation patterns of the proposed two-element antenna array.

EBG Structure.
The configuration of the proposed EBG structure is shown in Figure 1.The two antennas are identical and are designed to operate at the center frequency of 2.55 GHz.The proposed two-element antenna array is printed on a substrate whose permittivity is 4.4.The thickness of the substrate is 1.6 mm, which is very popular in practical engineering applications [19][20][21].The antenna array is optimized based on HFSS which is a finite element method.The antenna is optimized to a size of 56 × 60 mm 2 , including the entire ground plane.In this design, the antenna element is a rectangle patch antenna fed by a probe and the optimal dimensions are  1 = 15 mm and  1 = 28 mm.In order to reduce the coupling between these two antennas, a periodic L-loading E-shaped EBG structure is integrated together with the antenna array.The periodic L-loading E-shaped EBG structure consists of three EBG cells and the distance between the adjacent EBGs is  2 = 18.7 mm.The two antenna elements are separated by a distance of 30 mm.The dimensions of the proposed EBG cell are shown in Figure 1(b).
The antenna coupling can be reduced by controlling the dimensions of the proposed EBG structure.The dispersion diagram of the proposed EBG is shown in Figure 2. It can be seen that the gray area indicates the complete stopband in which no wave propagates in any directions.Thus, we can use the proposed EBG structure to reduce the coupling between the two antenna elements.

Non-L-Loading E-Shaped EBG Structure.
From the design of EBG cell, we can see that our EBG cell is comprised of an L-stub and an E-shaped strip.The EBG cell is shorted to the ground plane via a hole [22][23][24][25].We use three EBG cells to design the coupling structure and they are assigned periodically in the centerline between the array elements.
The mutual coupling is evaluated by using  12 .Since the antenna array is symmetrical, we use  11 and  12 to discuss the performance of the proposed antenna.First, proposed EBG structure without L-loading is settled between these two antennas and the array is shown in Figure 3. Mutual coupling is investigated by the HFSS and the simulation results are shown in Figure 4.It can be seen that the proposed antenna with EBG and without EBG has nearly the same resonance frequency.Moreover, the proposed EBG structure makes the resonance frequency higher.Our proposed EBG structure effectively reduces the coupling of the antenna array.As a result, 10 dB coupling has been reduced.Thus, we can say that the EBG is useful to improve the isolation between the antenna elements.
The radiation patterns of the proposed antenna array with non-L-loading EBG structure are shown in Figure 5.It is found that the proposed has good unidirectional radiation patterns and the proposed non-L-loading EBG structure has a little effect.

L-Loading E-Shaped EBG Structure.
Based on the analysis mentioned above, we find that the proposed EBG structure is effective in reducing the mutual coupling.Next, two Lshaped stubs are integrated into the E-shaped EBG to form L-loading E-shaped EBG structure.Then, the L-loading Eshaped EBG structure is utilized to reduce the mutual coupling of the antenna array, which is the antenna array shown in Figure 1.Also, the performance of the proposed antenna array is evaluated by the HFSS and the results are shown in Figure 6.We can see that the EBG structure with and without L-loading has no effect on the resonance frequency of the proposed antenna array.However, the mutual coupling is reduced to about 10 dB.
To understand the principle of the decoupling, the current distributions of the antenna array are given in Figure 7.We can see that the current is effectively stopped by using the L-loading E-shaped EBG structure.In the proposed antenna array with L-loading E-shaped EBG structure, the current is mainly flowing on the left antenna element and the L-loading Figure 9: Effects on the mutual coupling reduction of the proposed antenna array with varying parameters.

Effects on the Key Parameters.
In this design, the mutual coupling reduction of the proposed L-loading E-shaped EBG structure is determined by the entire length of L-loading Eshaped EBG cell, which is denoted as ℎ 0 = 2 × ℎ 1 + 2 × ℎ 4 + 2 × ℎ 5 + ℎ 6 + .Also, the width of L-stub ℎ 2 and the width of E-shaped EBG ℎ 7 have an obvious on-the-antenna resonance frequency.The distance of the antenna array is 30 mm, which is about 0.26 according to its resonance frequency.Here, ℎ 4 , ℎ 6 , and ℎ 7 are selected to investigate the mutual coupling reduction of the antenna array and the simulation results are shown in Figure 9.
It can be seen from Figure 9(a) that the center frequency of the decoupling moves to low frequency and the mutual coupling is reduced.When ℎ 6 and ℎ 7 are increased, the decoupling center frequencies also shift from high frequency to low frequency, which is shown in Figures 9(b) and 9(c), respectively.Additionally, the decoupling strength becomes weak.Thus, we can adjust the dimensions of the proposed L-loading E-shaped EBG structure to properly select the resonance center frequency and decoupling to make it suitable for practical engineering applications.

Results and Discussions
In order to get better mutual coupling reduction performance, the proposed L-loading E-shaped EBG structure and   The measured results agree well with the simulation ones, which help to verify the effectiveness of the proposed decoupling structure.However, there is a difference between the measured and simulated -parameters, including  11 and  12 , which may be caused by the fabricated errors.The measured radiation patterns are shown in Figure 12.It can be seen that the measured radiations are similar to the simulated ones.>84 mm × 72.5 mm 17 dB [10] 60 mm × 57 mm 42 dB [11] 70 mm × 30 mm 27 dB Proposed 56 mm × 60 mm 26 dB Table 1 shows the comparisons of the proposed antenna array with previously mutual reduction of antenna arrays with respect to the size and the coupling reduction.

Conclusion
An antenna array with periodic L-loading E-shaped EBG structure has been proposed, and its design procedure has been introduced in detail.The performance of the proposed antenna array has been investigated both numerically and experimentally.The mutual coupling reduction has been achieved by using the proposed periodic L-loading E-shaped EBG which has little effects on its radiation patterns.The results demonstrated showed that the proposed antenna array can operate at 2.55 GHz and a 26 dB coupling reduction has been achieved.Thus, the proposed antenna array is promising for future narrowband wireless communication applications.

Figure 8 :
Figure 7: Current distribution of the proposed antenna array.

Figure 10 :
Figure 10: Prototype of the fabricated antenna array.

Figure 11 :
Figure 11: Measured -parameter results of the proposed antenna array.

Figure 12 :
Figure 12: Measured radiation patterns of the fabricated antenna array on -plane and -plane.

Table 1 :
Comparison of the sizes and the coupling reductions.