A Compact Tri-Port MIMO Antenna With High Isolation for Broadside and Conical Radiation

Wearable Internet of Things (IoT) devices are widely used in many fields. Such as smart bracelets for health, smart watches for sports, smart safety helmets for industry, and so on. These devices make life easier and more efficient. Multiple-input-multiple-output (MIMO) antenna technology improves data transmission rates and communication reliability by using multiple antennas in a device, so as to meet the demand for real-time data transmission and multidevice connectivity in the IoT. A compact tri-port MIMO antenna with high isolation suitable for wearable applications is proposed. The antenna is designed to work in the frequency band of 2.45 GHz industrial, scientific, and medical (ISM). The antenna has two layers and three ports with low profile suitable for wearable applications. The upper layer is a circular patch with four shorting pins, which excites the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm { TM}}_{01}$ </tex-math></inline-formula> mode with a conical radiation pattern. The lower layer is a circular patch inlaid in an annular patch, which respectively excite the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm { TM}}_{11}$ </tex-math></inline-formula> mode and the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm { TM}}_{31}$ </tex-math></inline-formula> mode with a broadside radiation pattern. To achieve high isolation, characteristic mode analysis (CMA) was conducted on the circular patch, the circular patch with four shorting pins, and the annular patch. The positions of the three ports are reasonably arranged based on mode diversity. The measured bandwidth of the port for conical radiation is 35 MHz (2.43–2.465 GHz), while the measured bandwidths of the two ports for broadside radiation are basically the same which is 50 MHz (2.417–2.467 GHz). The measured overall isolation exceeds 50 dB.