Design and Analysis of a Low Profile Millimeter-Wave Band Vivaldi MIMO Antenna for Wearable WBAN Applications

The development of a reliable Wireless Body Area Network (WBAN) relies significantly on the quality of wearable antennas. Therefore, this paper proposes a low-profile four-element Multi-Input-Multi-Output (MIMO) antenna for wearable millimeter-wave (mm-wave) WBAN applications. The MIMO antenna structure incorporates a standard Vivaldi antenna and a frequency-selective surface that encompasses the 28 GHz and 30 GHz of the mm-wave band with a 36.44% fractional bandwidth. It offers inter-element isolation of less than -20 dB in a compact space of 16× 20 mm2. Conformability analysis, along with testing on Gustav's model chest, hand, and leg, was evaluated in terms of the antenna impedance bandwidth, gain, efficiency, and radiation pattern. The simulated characteristics of the MIMO antenna were tested through measurements in free space and on the human body using a prototype of the antenna. Furthermore, the MIMO antenna exhibits a low envelope correlation coefficient of less than 0.24, high diversity gain of greater than 9.95 dB, and an acceptable total active reflection coefficient of less than -10 dB. To ensure safety, the Specific Absorption Rate (SAR) analysis revealed acceptable levels of 0.397 and 0.267 (W/kg) at 28 GHz and 30 GHz, respectively. The proposed MIMO design is suitable for wearable WBAN applications owing to its small size, consistent gain, and compatibility with the human body in terms of a constant impedance bandwidth and end-fire radiation pattern.