Flexible and Wearable Pattern-Reconfigurable Printed Monopole Antenna for On-Body Communication
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Date
2021-07
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KNUST
Abstract
The fundamental feature of reconfigurable antenna (RA) systems is the ability to modify its functional operating characteristics compared to conventional antenna systems. As the size of electronic devices continues to shrink with stringent space constraints, reconfigurable antennas provide a low-cost approach for introducing adjustable properties on a single antenna element. This is to eliminate the need for multiple single-purpose antennas in diversity applications. Recently, reconfigurable antennas have gained popularity in wearable devices and finding applications in health care for the diagnosis and treatment of diseases such as stroke and cancer in tissues. Many applications of wearable antennas in personal communication devices, military gadgets and emergency apparatus are continuously growing. It is required that on-body antennas are unobtrusive and highly efficient with conformable features that can be easily suited to the configuration of the human body. However, due to the conductive nature of the human body, up to about 50% of the radiated energy from conventional printed monopole antennas can be absorbed. This effectively renders the omnidirectional antenna unidirectional when mounted on or close to the human body.
To this end, this current work proposes a simple technique to convert the conventional monopolar radiation pattern of a printed monopole antenna into a unidirectional radiation. This is shown among other advantages to significantly reduce specific absorption rate (SAR) and achieve circular polarization. The initial process begins with the design of a compact, low-profile, slotted-stepped planar monopole antenna (PMA) at the 2.45 GHz Industrial, Scientific and Medical (ISM) band. Further analysis presents an optimized reconfigurable extension of the antenna based on two switchable RF PIN diodes. This is incorporated in the antenna to asymmetrically steer the radiation pattern at different directions in specific operating modes. It is also shown that the frequency-radiation characteristics linkage that leads to shifts in the frequency while the radiation pattern is adjusted is fully decoupled in this design. Here, the switching effect of the radiation pattern does not cause any significant detuning in the operating frequency. This is further validated at varying antenna positions and excitation levels.
To present a conformable form of the reconfigurable antenna, a flexible substrate material is chosen for the design. Flexibility and wearability tests of the antenna are performed at various degrees of bending and subsequent placement on a human phantom model. The on-body effects on the antenna are characterized in simulation and measured on real human body. Simulated results show stable radiation patterns, reflection coefficient (𝑆11<−20 𝑑𝐵) and a circular polarization at 2.45 GHz. Meanwhile, low simulated peak SAR of 0.46 𝑊/𝑘𝑔 and 0.39 𝑊/𝑘𝑔 averaged over 1𝑔 and 10𝑔 of tissue respectively are recorded when the antenna is mounted directly on the human body. This is achieved without the use of conventional large and bulky cavity-backed structures. Hence, the overall size of the antenna measures only a compact square size of 0.235𝜆, making it a very suitable candidate for on-body applications.