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 (|S
₁₁
| ≤ -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 mm
²
, respectively) are the smallest to date while ensuring acceptable performance, specifically impedance matching, isolation, envelope correlation coefficient (<;0.005), and diversity gain (>9.99 dB).