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Background and Objectives: Multi-input multi-output (MIMO) antennas have been of interest in wireless communications in recent years. In these systems, many antennas are placed next to each other. The most important issue in the design of MIMO antennas is mutual coupling. Many methods have been proposed to reduce the mutual coupling of MIMO antennas. Many of these methods require an additional substrate on top or bottom of the antenna. In the reduction of mutual couplings electromagnetic band-gap (EBG) structures are preferred because they are coplanar with the antenna and can be compactly designed. In this paper, to reduce mutual coupling in MIMO antennas, a novel compact EBG structure based on the genetic algorithm optimization is proposed. Methods: The method proposed in this paper to design an optimal EBG structure is to use a genetic algorithm (GA). In this method, an EBG unit cell is designed by a binary code, and then the 7×2 EBG structure of the unit cell is placed between two antenna elements with ?/2 distance. The optimization algorithm tries to find the best unit cell to reduce the mutual coupling between two elements. After 70 generations in the genetic algorithm, the GA determines a compact structure of EBG elements which reduces mutual coupling significantly. Results: Two-element patch antennas with and without the proposed EBG structure are fabricated and the mutual couplings between array elements are measured at 5.68GHz in both cases. It is shown that the proposed compact EBG structure reduced the isolation of the two antennas by 27 dB. This decrease in mutual coupling is much higher than in the previous papers. The proposed EBG has little effect on other antenna radiation parameters such as S11 and radiation patterns. Conclusion: In general, in this paper, a compact and coplanar EBG structure is proposed to significantly reduce the mutual coupling in MIMO antennas. The method presented in this paper can be used for other MIMO antenna configurations at other frequencies and the proposed method will create a completely optimal structure to reduce mutual coupling. [ABSTRACT FROM AUTHOR]

Today, multimode antennas for MIMO (multiple-input multiple-output) systems have been used extensively to develop capacity or to improve the quality of WIFI. Due to the orthogonality of characteristic modes, the theory of characteristic modes used to achieve minimum interference between channels is common in this type of antennas. In this paper, a four-port antenna with symmetric structure is presented on a circular patch to be able to produce four orthogonal patterns with extremely low correlation coefficients (less than 0.001) and high gain in the frequency range of 4.4–5.2 GHz and SWR <1.22, which is highly efficient in creating independent and simultaneous communication paths in MIMO systems. To create this orthogonality and decrease the correlation between channels, the characteristic modes method is used. Circular polarization can be obtained by changing feeding structure in this antenna. [ABSTRACT FROM AUTHOR]

In this paper, a closed-form design method for a novel compact unequal filtering Wilkinson power divider, WPD, based on symmetric and asymmetric right/left-handed (CRLH) coupled-line circuit structures is proposed and investigated. The structure is composed of an input asymmetric CRLH coupled-line (A-CRLH) section along with two output symmetric CRLH coupled-line band-pass filter transformers to provide unequal filtering performance. The analytical design equations along with ideal closed-form expressions for scattering parameters are derived for the proposed structure by using in- and anti-phase (c/p-mode) decomposition method. To theoretically verify the proposed design approach, two filtering unequal WPDs with different high-power dividing ratios of 1:10 and 1:15 are designed and simulated. Finally, as a typical example, a practical filtering WPD with a power-splitting ratio of 1:15 operating at 1.4 GHz, is implemented with more than 55.5% size reduction as compared to conventional one. Good agreement between calculated, simulated and measured results verifies the effectiveness of the proposed power divider for miniaturized, large power dividing ratio and filtering applications. [ABSTRACT FROM AUTHOR]