Four-Port Compact Metamaterial MIMO Antenna with Stub-Based Bandwidth Improvement

This paper presents the design of a compact four-element MIMO antenna based on a metamaterial structure and a reactive load generated by an open-circuit stub. The radiator array, arranged in an axial symmetry configuration, provides high inter-element isolation despite a sub-millimeter separation. The design is optimized for 5G n77/n78 band applications and employs a metamaterial structure composed of embedded octagonal split-ring resonators (SRRs) integrated on a Duroid RT5880 0500 (ϵr=2.2,h=1.27 mm) substrate. This configuration achieves high miniaturization, with individual radiators of 19×9.53 mm2. Furthermore, through a stub-loading technique, the array is enhanced in two significant aspects: (a) it exhibits an increased impedance bandwidth, rising from a 23% fractional bandwidth in the stub-less design to 39% in the final architecture; and (b) a shift of the lower cut-off frequency toward lower values is obtained, resulting in a reduction of the radiator's electrical length, which translates into physical size diminution. The total array has a size of only 28.8×28.8 mm2 (0.24λ0×0.24λ0, considering the lower cut-off frequency). Despite the proximity between radiators and the absence of electromagnetic decoupling structures, the design ensures inter-element isolation exceeding 15 dB in the lower band and reaching values above 20 dB in the mid and upper bands. Diversity metric analysis confirms high performance, yielding an Envelope Correlation Coefficient (ECC) ≪0.005, Diversity Gain (DG) close to the ideal value (≥9.9), Total Active Reflection Coefficient (TARC) below -10 dB (converging in random phase analysis), and a Channel Capacity Loss (CCL) of less than 0.4 bits/s/Hz. Therefore, the proposed antenna stands as an ideal design for compact 5G communication devices.