A compact ultrawideband (UWB) multiple-input-multiple-output (MIMO) antenna with a small size of 22×36 mm2 is proposed for portable devices. The MIMO antenna consists of two symmetric slot antenna elements with back-to-back separation of 7 mm. Adjusting the open-ended stepped radiator and position of the microstrip line can realize UWB impedance matching. In order to achieve wideband and high isolation, a cross-shaped decoupling slot and connecting metal line are etched on the ground plane. The cross-shaped slot between the antenna elements is used to decrease the mutual coupling caused by near-field at middle and high bands. The connecting line can be interpreted as a neutralized line, which produces an additional current path for the coupling ground currents. Measured S-parameters show that the isolation is better than -16 dB across the UWB of 3.1-10.6 GHz. The radiation pattern, gain, and envelope correlation coefficient are also measured. The proposed antenna with a simple structure and compact size achieves good impedance matching and excellent port isolation simultaneously, and is a good candidate for UWB MIMO systems.
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