Compact coplanar waveguide-fed Ultra-wideband MIMO antenna with half slot structure

Abstract

In this paper, a compact multiple-input-multiple-output (MIMO) antenna is proposed for Ultra-wideband (UWB) applications. The proposed MIMO antenna is comprised by two symmetrically placed half slot antenna elements which one side of the slot is open to the air. Two antenna elements share the same ground plane, and each opening of the half slot gapes to the opposite directions. For covering the entire UWB frequency band, coplanar waveguide-fed structure is utilized in the antenna. This feeding mechanism is prone to generating multiple resonant frequencies and realizing the compact construction. The impedance bandwidth is sensitive to the shape of the radiator. To realize good impedance matching, a fine-tuned ellipse-shaped radiator, which is with semi-major axis 4.8mm and semi-minor axis 3.74 mm, is obtained through numerous optimizations and experiments. The reflection coefficient decreases at the high frequency, because the currents are concentrated on the edge of the slot, and the dielectric loss leads to the loss of energy. To achieve good isolation performance at lower UWB frequency, a fork-shaped slot is etched on the ground, at the center of the bottom. The fork-shaped slot effectively suppresses the currents flowing on the same ground plane, and the mutual coupling of 3–5 GHz reduces from −8 dB to −15 dB below. The overall size of the proposed antenna is only 23 × 18mm2. The results show that the achieved impedance bandwidth (|S 11 |, |S 22 | < −10 dB) is from 3 to 12.6 GHz, and the mutual coupling is lower than −15 dB. The antenna also contains stable gain and omni-directional radiation patterns. The proposed compact MIMO antenna with effective UWB performance is competitive for portable UWB MIMO devices.