Feedline Alterations for Optimization-Based Design of Compact Super-Wideband MIMO Antennas in Parallel Configuration

This letter presents a technique for size reduction of wideband multiple-input-multiple-output (MIMO) antennas. Our approach is a two-stage procedure. At the first stage, the antenna structure is modified to improve its impedance matching. This is achieved through incorporation of an n-section tapered feedline, followed by reoptimization of geometry parameters. Reducing the maximum in-band reflection well beyond the acceptance level of -10 dB creates room for size reduction, realized in the second stage as explicit footprint area minimization with the constraints imposed on the maximum in-band reflection level (|S11| ≤ -10 dB) and element isolation (|Sε| ≤ -20 dB). The technique is generic and can be applied to various monopole-based MIMO structures. It is validated using two antenna structures operating from 2.8 to 20 GHz. Various complexities of the tapered lines are also investigated, from n = 1 to 5. To the best of the authors' knowledge, the obtained footprints (452 and 546 mm2, respectively) are the smallest to date while ensuring acceptable performance, specifically impedance matching, isolation, envelope correlation coefficient (9.99 dB).