Summary This research suggests a compact uniplanar multiple‐input multiple‐output (MIMO) with four ports for n79/n46/millimeter‐wave (mm‐wave) applications. The size of the quad MIMO is only 30 × 30 × 0.8 mm3. MIMO system consists of four identical Z‐shaped radiators and common ground on the same plane and no decoupling structures are used for isolation. The system covers the bandwidth of 1.9 GHz (4.4–6.3 GHz) with a mid‐frequency of 5.6 GHz and also covers the high‐band frequencies ranging from 18 to 30 GHz with a bandwidth of 12 GHz. The suggested quad MIMO is fabricated on an FR‐4 board, and the measured outcomes are well in line with the simulated results. An isolation value of −11 dB has been achieved for mid‐band frequency and −24 dB has been attained for mm‐wave bands. Through the value of DG = 10 dB, ECC < 0.07, TARC < −3 dB, MEG < −5 dB, and the ratio of MEG = 1 dB, uniplanar quad MIMO shows acceptable MIMO diversity performance. The entire system was evaluated for the users' hand specific absorption rate (SAR) impacts and is within the limits. After the complete analysis of the miniature quad MIMO antenna, an 8‐port, and a 16‐port uniplanar MIMO are simulated for smartphone‐sized dielectric substrates and the performances were examined. The suggested MIMO system provides an efficient single‐layer MIMO antenna to 5G smartphones with high bandwidth and low SAR. The proposed quad MIMO systems are suitable for both the sub‐6 GHz band and the mm‐wave band.

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