(K-411) A Miniatured Fractal Antenna for Wireless Body Area Network

Chronic diseases are on the rise across the world as a result of unhealthy lifestyles and an aging population. In particular, six out of ten adults in the USA have a chronic disease, and four out of ten have two or more of them [1]. Managing such chronic diseases often requires continuous monitoring. In this regard, several intensive studies have been done on wireless communication technology, such as galvanic coupling (GC) [2], capacitive coupling (CC) [3], and a mix of these mechanisms. However, wireless technology, in general, suffers from weak transmission due to the challenging nature of the human body as a medium [4, 5]. Moreover, antennas for such wireless technology require compact size, low specific absorption rate, flexibility, and wideband characteristics. In this abstract, we present a miniatured fractal antenna for magnetic induction (MI) communication, which can overcome the transmission limitation due to biological tissue. The architecture of the proposed antenna is inspired by the Koch loop antenna (Fig. 1). The proposed miniature antenna offers several advantages, including wide bandwidth, multiband capabilities, and higher gain with quasi-omnidirectional radiation patterns. To ensure compatibility with the human body, a thin copper layer on a polyimide substrate is employed, providing both insulation and flexibility. The performance analysis of the proposed antenna defines that the proposed antenna can be used for MI-mediated body area networks with future possibilities in reconfigurable, robust, and energy harvesting.

Fractal antennas use self-similar features like space-filling curves to enhance conductor perimeter and length within a limited region. Fractal geometry consists of Initiator (0th stage), and the Generator is a structure that replicates the pattern sequentially on the initiator in subsequent stages with varied dimensions [6]. The detailed configuration of the proposed antenna is shown in Fig. 1. The first stage at the 0^th iteration of proposed Koch loop-shaped fractal antenna is represented in Fig. 1(a). The 1^st iteration of 2^nd stage is designed by applying the Meander-like fractal structure to the entire side of the rectangle patch (Fig. 1(b)). The meandering part length is 1/3 scale of the side length of the rectangle patch. Likewise, the 3^rd iteration of the 3^rd stage is shown in Fig. 1(c). Lastly, the 4^th iteration of the 4^th stage is customized by introducing ‘+’ shape for improving input impedance and maintaining resonance at a small dimension (Fig. 1(d)). To maintain the antenna structure and the rational use of the limited space, the antenna is designed to be 6.5 × 6.5 × 0.035 mm³ on a copper material [7]. The main copper antenna body adheres to a polyimide substrate, which has a dielectric constant of 3.5 and a lossy tangent of 0.001. The selection of polyimide substrate is due to biocompatibility, impedance matching, and minimum reflection. After antenna design, the co-planar waveguide (CPW) is fed into the transmission line by adjusting the trace width (W) and ground plane spacing (S) for 50-ohm impedance matching.

Results and Discussion:

The designed miniature fractal antenna is simulated in CST Studio Suite 2023. Fig. 2 depicts the simulation results of return loss vs. frequency for each iteration design. Due to impedance mismatch, the 1st repetition has no resonance frequency. However, the impedance improves dramatically with the 0^th, 2^nd, and 3^rd repetitions. The 0th iteration resonates at 15.56 MHz with a bandwidth of 1.67%, whereas the 2nd iteration resonates at 10.2 MHz with bandwidths of 1.27%, and the 3^rd iteration resonates at our desired frequency of 13.91 MHz with -20.1 dB return loss with 78% efficiency. The simulative evaluation shows that antenna fractional bandwidth improves with iteration. Fig. 3 shows the radiation patterns of the proposed fractal antenna at 12 - 14 MHz resonant frequencies. The patterns represent the field strength of magnetic fields. At phi = 90 degrees, dipole-like characteristics are observed. The fractal antenna exhibits omnidirectional properties, making it suitable for e wireless body area network (WBAN) applications. Fig. 4 depicts the transient analysis when the antenna is in excitation condition: Fig. 4(a) shows surface current distributions. Fig. 4(b) and 4(c) show E-field and H-field, respectively. The majority amount of the current is concentrated at the edges of both the patch and the modified ground plane. This observation validates antenna design effectiveness on the overall performance. The gain in specific absorption rate (SAR) and return loss values also show minimal variation. At an input power of 1 W, the SAR value is 409.8 W/kg, which satisfies international standards for the wireless communication [8].

Conclusion:

The design and simulation of a proposed Koch loop-inspired fractal antenna demonstrate the capability to use for wireless body area network applications. Flexible substrates and materials are chosen to comply with human use. Despite the incorporation of various gain augmentation approaches, the antenna's SAR value remained virtually unchanged.

This research was supported by the National Science Foundation (NSF) CNS-2129659.

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