During this research, a slotted complementary split ring resonator is intended as Electric Negative metamaterial (SNG with ?<0), to suppress displacement current between two E-plane coupled stripline fed rectangular dielectric resonator antenna (RDRA) array. By placing two layers of stacked ENG MTM as isolation walls, electromagnetic coupling reduced greater than 14 dB at resonance frequency while impedance matching maintained well. Also, the gain increased and envelope correlation reduced.
Rectangular dielectric resonator antennas (RDRAs) are actually an eligible choice in wireless communication and MIMO applications with regards to their advantages including small size, high radiation efficiency, wider impedance bandwidth and low loss in comparison with traditional patch antennas 1,2.
The electromagnetic mutual coupling can degrade Performance of MIMO systems in antenna arrays. Mutual coupling attributable to near-field effect, that the level of S21 will fall by 12 dB or 18 dB when the distance between two elements is doubled, or it may be resulting from far-field coupling, that its value will fall by 6 dB when the distance is doubled 3. Two antennae may interact if they share a common substrate and/or ground plane through substrate-bound modes including slow waves and parallel plate modes 4.
Nowadays different strategies to reduce mutual coupling in DRA are actually studied, e.g. 5 where an artificial magnetic conductor (AMC ) ground plane investigated theoretically to suppress mutual coupling between two Cylindrical DRAs6, also employing ring-shaped DGS surrounding E-plane coupled probe fed Cylindrical DRA arrays have already been introduced7. Resonant slits or defects in ground planes 8, 9, EM band gap structures (EBG) in planar and low profile antennas 10 were used.
An alternate mechanism of suppressing coupling in array antennas is characteristics of single negative metamaterial 11-19. In 11-16 SNG magnetic metamaterial was applied between low profile planner antennas. In 14 a different well engineered single negative metamaterial insulator was designed and experimentally examined to suppress coughing in closely-spaced monopole antennas.
This paper organized the following: in section 2 we study mutual coupling in two element stripline fed RDRA arrays as the distance between elements changes. Section provide us with a theory about single negative electric metamaterial and its particular application as antenna isolator over its rejection band with emphasize on numerical extracted parameters from scattering parameters. Section 4 presents simulation results extracted from CST by insulating ENG between two RDRA elements as a decoupler, also the effectiveness of electric materials as antenna isolator is discussed. Conclusions are typically in section 5.
Two parallel element E-plane coupled microstrip-fed RDRA is utilized to form produce a two-element MIMO array as shown in figure 1. dimensions and material of RDRA were chosen like same that used in 20 (relative permittivity, ?r=35.9 and dimensions a×b×d=18×18×8.9 mm) to radiate near 2.5 GHz. Stripline feeding system optimized for impedance matching to 50? SMA coaxial cable connector. The antenna is excited to function at mode TE111 and the resonance frequency is 2.5GHz. The mutual coupling as an important factor may affect the performance of arrays for instance resonant frequency, radiation pattern, and bandwidth.
The simulated reflection coefficient (S11=S22) vs frequency outcomes for several element spacing are plotted in fig2. Mutual coupling (S21=S12, since the system is passive) is shown in fig3. By comparing the cross-coupling ratio at 40mm and 80mm distances, it is conclude that when doubling the distance between two antennas, the coupling value is reduced by about 18 decibels. Which means that the coupling is caused by the near-field effects.
The envelope correlation coefficient from scattering parameters is achieved using formula (1)21. Fig4 shows the correlation coefficient near the resonance frequency. The simulated realized gain of the antenna array is shown in fig5. The gain at the resonance frequency is under 4 which is not sufficient for MIMO wireless application.
2- Electric negative material as insulator
ENG metamaterial provides highly efficient band-stop rejection. Figure 6(a) shows the front view of a unit cell which featuring its slotted CSRR conductive layer backed by stripline conductive layer. These layers are etched on both sides of Rogers 6010 substrate (relative permittivity , ?r=10.2 , and the dielectric loss tangent tan?=0.0023) with the thickness of 1.27 mm. The layers are made of copper with a conductivity of 5.8*107smand thickness 0.035 mm. The ENG is based on slotted complementary split ring resonator 22, 23. They’re stacked in order to create insulation walls that provide a passive LC resonant behavior which blocks transmission of EM energy over operating frequency band. The ENG insulator proposed with this paper has features a broadband performance for characterization and design. The commercial finite integration technique field simulator (CST) is employed.
To characterize proposed ENG, Driven mode analyses of metamaterial studied. fig6(b) shows transmission and reflection coefficients of the slab. Referring the retrieval method in 24 effective permeability and permittivity of the structure have been extracted from s-parameters and plotted in fig 6(c).
Figure 6. Analysis of the proposed SNG MTM-resonator. (a) Simulation setup for driven mode analysis in CST, (rin = 4, s = 1, d= 0.5, g = 0.5 and L = 13.6 , all dimensions are in mm)., (b) EM Simulated S-parameters and (c) retrieved effective permittivity and permeability of the proposed SNG MTM-resonator.
3- Mutual coupling reduction of RDRA arrays
In an effort to suppress coupling between elements two stacked layers of proposed band rejection slotted CSRR is located between of array with separated distance D=0.33?=40 mm mm from each other. Once these decouplers are excited through an electromagnetic field with proper polarization, the structures would have negative effective permittivity over certain frequency band. In turns, these metamaterials block propagating electromagnetic energy radiated by one antenna to another near element within an array. Therefor ENG layers comprised of resonators behave as the decoupling wall. To demonstrate the efficient isolation effect of proposed stop band ENG decoupler in RDRA, the Model setup shown in fig7, continues to be simulated.
Figure 7. Analysis of the RDRA array using SNG MTM-insulator, (a) Topology of proposed structure, (b) Scattering parameters
Figure 8(a) shows the Results of simulation reflection coefficient of antenna array system (S11) with/without a spacer and fig8(b) shows the mutual coupling (S21) between two antenna element with/without insulator. Envelope correlation for antenna system computed using formula (1) presented in fig8(c).
Two coupled antenna system without having decoupler (air case) is reference state for comparison purposes. Artificial structures generally are dispersive and anisotropic. These structure considered during this paper are based on are based on a slotted filter. Such insulator provides enhanced permittivity only in the direction normal to structure.
By inserting MTM, small shift (50 MHz) at resonance frequency accurse. Referring fig 9 by placing ENG between two antenna array system, the mutual coupling has become efficiently reduced over a wide bandwidth and more than 40 dB at the resonant frequency, while at the same time impedance matching is maintained well. Mutual coupling has been reduced from -9.7dB for air case at 2.5GHz to -52dB at 2.55GHz when ENG used.
Figure 8. Comparisons between reference (air case) and SNG MTM-insulator-loaded antenna simulation (a) reflection , (b) transmission and (c) envelope correlation coefficients.
The performance of the antenna array system for MIMO application is investigated. The envelope correlation coefficient ?e is computed. Results plotted in fig 8(c) for ENG insertion case. From Fig. 8(c), the envelope correlation results obtained using (1) for the ENG case shows low correlation between the antenna elements below 0.01 over the antennas frequency band. As compared to the air case, that is almost 40dB much better than air case.
Far-field radiation patterns for RDRA with/without spacer are computed as a way to quantify the performance of the antenna. Fig. 9 shows realized gain vs frequency. Maximum gain has become increased from 5.58dB for air case at 2.5GHz to 7.038dB at 2.55GHz when ENG used. That is, the gain of antenna improved 1.45 dB with insulation.
Fig10 illustrates how the displacement current is eliminated using well engineer metamaterial. Four states have been shown to illustrate the potency of of the structure on the removing of displacement current.? In two cases with/without separators, the magnetic fields caused by due to simultaneous feeding of these two antennas are shown in Fig. 11.
Next one antenna is fed whilst the the other antenna is terminated to 50? load at the port. The CST solver outcomes are are shown for a couple antennas with spaces and a case by insulating ENG metamaterials between two antennas. Most of transmission of radiated energy is blocked when compared to air case with a strong mutual interaction between elements. ENG wall reduces mutual coupling and eliminated the displacement current.
Figure 10. Snapshots of the H-field in the two cross section of structure, (XY-plane and XZ-plane) for the RDRA antenna system with feeding two antennas simultaneously (a) air case in XY plane, (b) air case in XZ plane ENG metamaterials , (c) inserting ENG metamaterials in XY plane and (d) inserting ENG metamaterials in XY plane. (D=0.33?=40 mm)Figure 11. Snapshots of the H-field in the two cross section of structure, (XY_-plane and XZ_-plane) for the RDRA antenna system when one antenna is fed while the other antenna is terminated to 50? load at port (a) air case in XY plane, (b) air case in XZ plane ENG metamaterials , (c) inserting ENG metamaterials in XY plane and (d) inserting ENG metamaterials in XY plane. (D=0.33?=40 mm)4- Conclusion
Within this paper, noticeable reduction (almost 40 dB) in mutual coupling among RDRA array elements achieved by inserting SNG electric metamaterial within 10 dB matching band. By utilizing the the slotted CSRR the gain of RDRA array is increased and envelop correlation is decreased.