Split-ring-resonator-loaded multiband frequency agile slot-based MIMO antenna system

In this work, a compact quad-element multiple-input-multiple-output (MIMO) antenna system is proposed. The design consists of four annular slot-line based MIMO elements, printed on a commercially available RO4350 substrate with a total size of 60 × 120 × 0.76 mm³. Each annular slot has a rectangular slot inside it to increase its electrical length to further bring down the resonance frequency. Furthermore, a T-shaped defected ground structure is utilised to increase the isolation between the closely spaced antenna elements. Each element is loaded with a unique split ring resonator (SRR), which results in size reduction as well as multi-band operation covering 1.7–2.28, 2.35–2.85 and 2.9–3.1 GHz. Moreover, by using only a single varactor diode per element, fine frequency tuning over these bands is achieved. A wide frequency sweep over multi-band and a peak gain of 2.95 dBi with 82% maximum radiation efficiency is achieved. Low envelope correlation coefficient values are observed throughout the operating band, thus offering good MIMO performance. The overall planar antenna design is compact, low-profile and can easily be integrated with radio frequency front ends in second generation cognitive radios among many other applications.