An Ultra-Wideband MIMO Antenna Based on Dual-Mode Transmission Line Feeding for Wireless Communication

An ultra-wideband (UWB) MIMO antenna based on dual mode transmission line feeding for wireless communication is proposed in this article. The general method of realizing a UWB MIMO antenna is using different shapes of monopoles acting as a MIMO antenna element, while the ultra-wideband character of the proposed MIMO antenna is mainly obtained by the use of a dualmode transmission line in the coplanar waveguide (CPW) feeding line, which offers a novel method. The proposed MIMO antenna element is a rose-shaped monopole fed by a CPW feeding line. Compared to the traditional monopole, a rose-shaped monopole can introduce several extra resonant frequencies, and the impedance bandwidth can be improved. Besides, a dual-mode transmission line (DMTL) is introduced by adding specific stubs to the CPW feeding line. Arranging the stubs at the half wavelengths of the desired frequencies, mode transformation can be accomplished, and additional resonant modes can be generated. As a result, the impedance bandwidth can be further broadened. Results show that the fractional impedance bandwidth of the proposed UWB antenna element is 165.5% (2.59 GHz to 26.61 GHz). Then, the UWB antenna is applied to design a 4-element MIMO antenna. By loading four u-typed decoupling structures at the center of the MIMO antenna, the port-to-port isolation of the MIMO antenna can be increased to 20 dB within a wide bandwidth, especially 25.3 dB at the higher band (14-25 GHz). The proposed UWB MIMO antenna is manufactured and tested. Experimental results show that the impedance bandwidth covers 2.40 GHz to 25 GHz (165%). The diversity gain (DG) of the antenna in the operating band is about 10; the envelope correlation coefficient (ECC) is less than 0.002; and the radiation efficiency ranges from 85% to 95% in the whole working band. The design is a preferable candidate for MIMO systems.