A Miniaturised MIMO Antenna Array of Two Vertical Monopoles Embedded Inside a Planar Decoupling Network for the 2.4 GHz ISM Band

This study introduces a fundamentally new approach to suppress mutual coupling among two closely-spaced vertical monopole elements of a multiple-input multiple-output antenna array. The 40 × 40 × 1.27 mm decoupling and matching element consists of a single planar ring resonator acting as a stop-band filter along with two tuning strips printed on an ungrounded substrate surrounding a two-element co-planar waveguide-fed monopoles separated by 8 mm (λ 0/16) at 2.45 GHz. Measurements reveal 14.2% bandwidth centred at 2.395 GHz over which the decoupling is below 20 dB, and −10-dB impedance bandwidth of 19.5% centred at 2.36 GHz. An improvement of 43 dB in isolation is observed at a frequency of 2.38 GHz. It is shown that the decoupled array provides a total realised gain of 1.69 dBi with excellent diversity performance as demonstrated by the low envelope correlation coefficient and improved link efficiency in highly-correlated channel environments due to the spatially orthogonal 3D radiation patterns. The capacity offered by the decoupled array is enhanced by 50, 22, 18 and 12% at signal-to-noise ratios of 5, 20, 30, and 50 dB, respectively, as compared to the coupled array in an urban micro-channel spatial model.