Characteristics mode analysis based wideband Sub-6 GHz flexible MIMO antenna using a unique hybrid decoupling structure for wearable applications

This article presents a compact flexible four-element multiple input multiple output (MIMO) antenna for Sub-6 GHz wearable applications. A wideband monopole antenna with a modified edge tapered radiator and a lowered ground plane is replicated to form a four-element antenna. The proposed antenna is fabricated on a flexible polyethylene terephthalate (PET) substrate and holds an overall dimension of 65 x 56 x 0.25 mm(3). The antenna has a measured bandwidth of 3.55-5.3 GHz with a peak gain of 4.9 dBi. A novel hybrid decoupling structure with a neutralization line in the radiator and a unique defective ground structure (DGS) suppresses the coupling current and provides isolation better than -24 dB throughout the bandwidth. The antenna design evolution is explained using characteristics mode analysis (CMA). The MIMO diversity performance is relatively good with MIMO diversity metrics showing envelope correlation coefficient (ECC) < 0.001, diversity gain > 9.99, total active reflection coefficient (TARC) < -10 dB, channel capacity loss (CCL) < 0.3 bps Hz(-1) and multiplexing efficiency (ME) < -0.5. Specific absorption rate (SAR) analysis of the antenna is performed to check the antenna's suitability in wearable applications and the proposed antenna exhibits 0.745 W kg(-1) and 0.326W kg(-1) for 1g and 10g of tissue respectively which is much less than the permissible international standards.