Design and analysis of modified octagonal ring-shaped MIMO antenna with connected ground for 5G sub-6 GHz n79, Wi-Fi 5, and Wi-Fi 6E band applications.

The increasing demand for compact, high-isolation, and wideband antennas in modern wireless systems has created the need for efficient multiple-input multiple-output (MIMO) antenna designs suitable for 5G sub-6 GHz, Wi-Fi 5, and Wi-Fi 6E applications. MIMO antennas improve channel capacity, spectral efficiency, link reliability, and multipath performance, but maintaining compactness, wide bandwidth, low mutual coupling, and SAR compliance remains a major design challenge. This work proposes and experimentally validates a compact four-port modified octagonal ring-shaped MIMO antenna with a connected-ground structure for the 5G n79, Wi-Fi 5, and Wi-Fi 6E frequency bands. The antenna is designed on a low-cost FR4 substrate with dimensions of 60 × 60 mm². Four modified octagonal ring-shaped radiating elements are arranged orthogonally to improve polarization and spatial diversity. A connected-ground structure with multiple strips and slits is introduced to enhance impedance matching and reduce mutual coupling. The antenna is analyzed through design evolution, surface-current distribution, parametric optimization, S-parameters, far-field radiation characteristics, diversity parameters, and SAR evaluation using a human-head phantom model. The fabricated prototype is measured and compared with the simulated results. The proposed MIMO antenna achieves a wide impedance bandwidth of approximately 3.3-7.7 GHz, covering the 5G sub-6 GHz n79, Wi-Fi 5, and Wi-Fi 6E bands. Mutual coupling remains below - 17.5 dB across the operating band. The antenna exhibits stable radiation characteristics, with peak realized gain varying from about 4.3 dBi to 5.58 dBi and radiation efficiency between 80 and 96%. The measured and simulated results show good agreement. The diversity performance is satisfactory, with the envelope correlation coefficient close to zero, diversity gain nearly 10 dB, total active reflection coefficient below - 10 dB, channel capacity loss below the acceptable limit, and balanced MEG values. SAR values are also within the Federal Communications Commission (FCC) and International Commission on Non-Ionizing Radiation Protection (ICNIRP) safety limits for both 1 g and 10 g tissue models. The proposed connected-ground four-port MIMO antenna provides compact size, wide impedance bandwidth, high isolation, stable radiation performance, good diversity characteristics, and SAR compliance. Therefore, it is a promising candidate for 5G sub-6 GHz n79, Wi-Fi 5, Wi-Fi 6E, and other high-data-rate wireless communication systems.