A compact multiple input multiple output (MIMO) antenna with WLAN band-notch filtering function for portable wireless ultra-wideband (UWB) systems is proposed in this paper. The overall size of the antenna is 26 x 40 x 0.8 mm3, and it is fabricated on a low-cost FR4 substrate. The antenna comprises two identical planar monopole antenna elements, namely PM1 and PM2, which are fed by a 50-ohm coplanar waveguide. The PM1 and PM2 are positioned perpendicular to each other to minimize the mutual coupling between them. To further reduce the mutual coupling and to increase the impedance bandwidth, a long rectangular strip is protruded from the ground plane between the PM1 and PM2. To create the band-notch characteristics at WLAN band from 5 to 5.9 GHz, an inverted U-shaped slot is etched on the feed line. The simulated and measured results show that the proposed antenna achieves good impedance bandwidth (S11 ≤ -10 dB) from 2.2 to 11.4 GHz and mutual coupling (S21) of < -20 dB. The measured peak gain of 2.4 to 7.5 dBi and radiation efficiency above 90% are obtained except at notch band. The measured envelope correlation coefficient (ECC) of 0.008 in the whole operating band and omnidirectional radiation characteristics demonstrate that the proposed MIMO antenna is a suitable candidate for portable UWB systems.
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