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Decoupling of Dual-Band Microstrip Antenna Array with Hybrid Resonant Structure

A novel hybrid resonant structure is proposed to decouple a dual-band microstrip antenna array. The decoupling structure is composed of two H-shaped strips, and the lower and upper ones respectively collaborate with an X-shaped slot to reduce mutual coupling at 4.5 GHz and 5.5 GHz. Two sub-patches of different sizes share a connection feeding line to construct the dual-band array element, which is arranged along H-plane with the edge-to-edge spacing 0.15λl and 0.24λh (λl and λh are the free-space wavelengths of 4.5 GHz and 5.5 GHz, respectively). Simulated and measured results indicate that through loading the hybrid resonant structure, 31.6 dB and 24.0 dB reductions of mutual coupling at two frequencies are obtained, while the levels of coupling coefficients are both below −30 dB in two operating bands. Moreover, the modified radiation patterns, improved diversity metrics, and weakened coupled current distributions further verify its superior decoupling capability. The proposed decoupling structure reveals its promise in being employed in communication system and multielement linearly antenna arrays.


INTRODUCTION
Rapid advancement in wireless communication system calls for antenna arrays with superior performances, where mutual coupling is a crucial concern once densely arranging array elements, as it can result in a series of problems including mismatched impedance, deviated radiation pattern, enlarged side lobe, and scan blindness [1]. Also, the sizes of highly integrated devices are usually small due to limited space, but mutual coupling can merely be ignored when the distance between array elements is much larger than half wavelength. Meanwhile, to satisfy the demand of working in dual frequencies, dual-band antenna is often chosen as the basic element. Thus, it is of great significance to reduce mutual coupling of dual-band antenna array. Until now, various decoupling technologies have been presented and can be divided into three general categories. The first type is artificial electromagnetic material, which contains electromagnetic bandgap [2], frequency selective surface [3], metasurface [4,5], defected ground structure [6][7][8][9], and array-antenna decoupling surface [10]. The second type is resonant structure [11,12], which is employed to decouple antenna arrays at certain frequencies based on its resonant characteristic. The last one is to introduce a decoupling network [13,14] to create extra currents to cancel out the coupled ones, and it is often adopted for dual-band cases [15][16][17]. However, most dual-band cases focus on decoupling monopole antenna arrays, and few hammer at microstrip antenna array. Also, researches on employing resonant structure for decoupling dual-band antenna arrays are scarcely reported.
In this research, a hybrid resonant structure, composed of an X-shaped slot and two H-shaped strips, is proposed to decouple dual-band microstrip antenna array at 4.5 GHz and 5.5 GHz. The array element combines two sub-patches of different sizes with a communal connection feeding line to generate two resonant frequencies. Then, the elements are arranged along H-plane, whose edge-to-edge distances are 0.15λ l and 0.24λ h at two frequencies. The lower H-shaped strip and the X-shaped slot work together to reduce mutual coupling at 4.5 GHz, while the upper one collaborates with the slot to function at 5.5 GHz. Finally, the results exhibit that mutual coupling at two frequencies is reduced by 31.6 dB and 24.0 dB, and the isolations are above 30 dB in the working bands. Also, the H-plane radiation patterns are modified, and the diversity metrics are improved. The current distributions further elucidate the operating mechanisms and decoupling capability. Figure 1 demonstrates the geometry and configuration of the proposed two-element dual-band microstrip antenna array. The array element consists of two sub-patches with different sizes and a connection line, which is fed by a coaxial probe. When the array works at 4.5 GHz, the large sub-patch with the size of l 1 × w 1 plays a major role, so does the small one with the size of l 2 × w 2 at 5.5 GHz. Thus, through concatenating them together with line w 3 , the single antenna element can simultaneously operate at dual bands. The microstrip antenna array elements, printed on an FR4 substrate with the total size of L g ×W g ×h and the relative permittivity of 4.3, are separated by the spacing d, making the edge-to-edge distances 0.15λ l and 0.24λ h at two frequencies, then the center-to-center distances are 0.45λ l and 0.55λ h , indicating relatively strong mutual coupling between array elements especially at low frequency. After being optimized in the simulation software HFSS, the detailed dimensions of the dual-band antenna array are listed in Table 1. Fig. 3 plots the simulated S-parameters of the coupled dual-band antenna   array. It is apparent that the proposed antenna array resonates at two frequencies, and the coupling coefficients reach −14.6 dB and −19.6 dB at 4.5 GHz and 5.5 GHz, respectively.

Hybrid Decoupling Resonant Structure
It is conspicuous that a shorted microstrip line whose length is smaller than λ g /4 (λ g is the guided wavelength) usually acts as an inductor, while that with the length larger than λ g /4 can behave as a combination of an inductor and a capacitor [18,19]. Also, the slot can be deemed as a capacitor [20]. Therefore, by connecting two H-shaped strips and an X-shaped slot via four metallized holes, a novel hybrid decoupling resonant structure, displayed in Fig. 2, is proposed to reduce mutual coupling of the above dual-band antenna array. The length of the lower H-shaped strip is designed to be smaller than λ gl /4 (λ gl is the guided wavelength at 4.5 GHz) because of the limited space, while that of the upper one can be larger than λ gh /4 (λ gh is the guided wavelength at 5.5 GHz). Therefore, the low and high resonant frequencies, f l and f h , can be represented as

RESULTS AND DISCUSSION
To validate the effectiveness of the proposed hybrid decoupling resonant structure, the coupled and decoupled dual-band antenna arrays are fabricated, measured, and compared, whose prototypes are shown in Fig. 4.      Figure 6 plots the simulated and measured radiation patterns of the two antenna arrays. It should be mentioned that during simulation or measurement, only port 1 is excited, while port 2 is terminated to a 50 Ω load. Usually, the strong mutual coupling between antenna elements can deteriorate the radiation patterns, especially in the arrangement direction. From Fig. 6(a), since the coupled dual-band antenna array is arranged in H-plane, the main direction of the H-plane radiation pattern deviates to 24 • at 4.5 GHz. However, after the hybrid decoupling structure is loaded, the radiation pattern is modified to the broadside. Also, the simulated and measured gains respectively increase from 6.02 dBi and 5.73 dBi to 6.32 dBi and 5.96 dBi. Meanwhile, for the H-plane radiation pattern at 5.5 GHz, due to the relatively low mutual coupling, the coupled array can radiate to the broadside without any deflection, but the gain is a little improved. In detail, the simulated and measured gains respectively increase from 6.04 dBi and 5.82 dBi to 6.35 dBi and 6.17 dBi. Besides, the E-plane radiation patterns of the coupled and decoupled antenna arrays also deviate on account of the influence from relatively adjacent frequencies. On account of the asymmetry of the antenna element and array, the simulated and measured radiation patterns with two ports excited are also demonstrated in Fig. 7. The similar deviations exist in the E-plane radiation patterns, but the antenna array can radiate to the broadside in the H-plane. Moreover, the measured gains at 4.5 GHz and 5.5 GHz respectively increase from 8.32 dBi and 8.56 dBi to 8.97 dBi and 8.61 dBi. It is also evident that the backward radiation of the decoupled antenna array is enhanced, which exists in other references that adopt defected ground structures or slots [6][7][8] as decoupling structures, especially when the element spacing is small. However, compared to the superior decoupling capability of the hybrid decoupling structure, the drawbacks are inferior.

Diversity Metrics
For a multiple input multiple output (MIMO) system, diversity metrics, including envelop correlation coefficient (ECC), diversity gain (DG), total active reflection coefficient (TARC), etc., are always significant parameters to evaluate its communication performance. ECC (ρ e ) is a typical parameter that can reveal the correlation of the antenna array elements, so is DG. They can be represented as [9] ρ e = |S * Usually, a minimized ECC and a maximized DG are required to maintain a high channel capacity and a good diversity performance [21]. Fig. 8(a) plots the measured calculated ECC and DG of the coupled and decoupled dual-band antenna arrays. It is apparent that the ECCs at 4.5 GHz and 5.5 GHz are decreased from 3.1 × 10 −4 and 2 × 10 −5 to 2 × 10 −6 and 8.7 × 10 −6 , which indicates a better diversity performance after decoupling. One crucial index of the phased array is TARC, which takes the effect of the phases of the incoming signals into consideration. TARC is given by [22] Γ t a = where θ is the input feeding phase. Fig. 8(b) displays the measured TARC of the coupled and decoupled dual-band antenna arrays. At 4.5 GHz, TARC of the coupled array increases with θ varying, but that of the decoupled one remains below −20 dB. Meanwhile, the change of θ has little effect on TARCs at 5.5 GHz. Therefore, it is concluded that the hybrid decoupling structure can improve TARC and the scanning characteristic.

Current Distribution
To better intuitively understand the operating mechanisms of the dual-band antenna array and the hybrid decoupling resonant structure, the current distributions of the two antenna arrays are displayed in Fig. 9. Likewise, during simulation, port 1 is excited with port 2 connected to a 50 Ω load. From

CONCLUSION
In this research, a novel hybrid decoupling resonant structure is presented to decouple a dual-band microstrip antenna array, whose element consists of two sub-patches and one connection feeding line. The lower and upper H-shaped strips respectively collaborate with the X-shaped slot to reduce mutual coupling at 4.5 GHz and 5.5 GHz. Simulated and measured results coincide with each other, revealing the superior decoupling capability of the proposed structure. 31.6 dB and 24.0 dB mutual coupling reductions are achieved, and the levels of coupling coefficients are both below −30 dB during the operating bands. Moreover, the H-plane radiation patterns are modified to the broadside, and the gains at two frequencies are improved. The results indicate that the proposed hybrid decoupling resonant structure has potential in the application of communication system and multielement linearly arrays.