We propose a compact wideband planar quad-element multiple input, multiple output (MIMO) antenna, which can cover a wide bandwidth ranging from 2.2 to 30 GHz. Novel reversed S-shaped walls provide high isolation between antenna elements within an extremely closed space, with the edge-to-edge distance between elements being only 1 mm. The simulated and measured results with respect to S parameters and radiation patterns are in good agreement. The experimental results indicate that the quad-element MIMO antenna can provide wide bandwidth (2.2-30 GHz), high isolation (with the transmission coefficients below -19 dB), and low profile (only ~λ0/40) within a compact structure (32 mm ×32 mm×4.5 mm). This compact wideband quad-element MIMO antenna with high isolation and low profile has important applications in mobile devices or other small-scaled equipment in future 5G communication.
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