A Planar Multiport Multi-Circular Patch MIMO Antenna on Flexible Substrate with Defected Ground Structure for Sub-6 GHz

In this proposed work, a flexible planar six-port Multiple-Input Multiple-Output (MIMO) microstrip antenna is presented that is intended for sub-6 GHz Fifth Generation (5G), wearable, and biomedical wireless applications. The antenna employs a multi-circular radiating patch topology. It is integrated with a Partial Ground Plane (PGP) and a Defected Ground Structure (DGS) techniques to achieve wideband impedance matching, reduced mutual coupling, and improved diversity characteristics. The single antenna element occupies a small space of 38.40 × 24 × 0.508 mm³, whereas the complete six-port configuration is arranged on a 165 × 76 mm² flexible Kapton platform corresponding to the dimensions of a 6.67-inch smartphone. The practical measurements demonstrate a wide bandwidth of 3.0-6.6 GHz, which includes all available sub-6 GHz 5G mid-band frequencies. The measured inter-port isolation remains greater than 35 dB and reaches approximately 58 dB for selected port combinations. The Envelope Correlation Coefficient (ECC) is maintained below 0.5, with a minimum of 1.15 ×10-5 while the Diversity Gain (DG) remains close to 10 dB. The Mean Effective Gain (MEG) is approximately 3 dB for all ports, with inter-port differences close to 0 dB. Practical line-of-sight measurements produce received-power levels ranging from -37.24 to -22.92 dBm. The antenna also preserves stable operation during on-body placement and mechanical bending. Moreover, the maximum specific absorption rate values are 0.775 W/kg averaged over 1 g of tissue and 0.335 W/kg averaged over 10 g, remaining within the applicable exposure limits.