Design and Measurement of a Compact Millimeter Wave Highly Flexible MIMO Antenna Loaded With Metamaterial Reflective Surface for Wearable Applications

This article presents a flexible four-element MIMO antenna designed on a thin polyimide substrate to operate at 30.50 GHz for wearable applications. The antenna element structure is a combination of circular rings combined with T-shape stubs. The MIMO antenna isolation is enhanced through a novel decoupling structure consisting of a rectangular stub, four open-ended horizontal strip lines, and two vertical strip lines connecting the ground plane. It provides 22.5 dB isolation, even with conformal orientation. A metamaterial (MTM) reflective surface of
$9\times9$
unit cells is designed and placed below the antenna to improve its radiation characteristics, bandwidth, gain, and specific absorption rate (SAR). The metamaterial unit cell provides a double negative property with a wide stop-band range of 26.27–36.49 GHz. With an MTM surface, the measured bandwidth is 25.2–33 GHz with a maximum broadside gain of 8.90 dBi. The resulting SAR is reduced from 1.71 W/kg to 0.86 W/kg by incorporating MTM reflective surface. The MIMO antenna with MTM improves the bandwidth by 56%, gain by 32.5%, and radiation from bidirectional to broadside, compared to the MIMO antenna without MTM. Also, the antenna is measured and validated for diversity parameters such as Envelope correlation coefficient (ECC), Diversity gain (DG), Channel coefficient loss (CCL), Total active reflection coefficient (TARC), and Mean effective gain (MEG). The reduced SAR and improved antenna performance suggest the proposed MIMO antenna with an MTM reflective surface is suitable for on-body wearable applications in IoT devices, smart watches, headwear, and footwear devices.