Oval-Shaped SSPP Low Pass Filter for Wearable RF Applications

This paper presents a novel double-layer Spoof Surface Plasmon Polaritons (SSPP) based Low Pass Filter (LPF) constructed on a denim substrate, with dimensions <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$70 \times 30 \times 1.6 ~\text{mm}^{3}$</tex>. The proposed SSPP LPF integrates a tapered transmission line with circular rings that converge at the tapering point, along with a resonating oval-shaped nine-cell structure, complemented by a ground plane on the backside. This results in tighter electromagnetic field, wide impedance matching, and improved performance across a wide passband range from 0 to 5.9 GHz. The simulated results reveal that the SSPP LPF maintains a reflection coefficient <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathbf{S}_{11})$</tex> −10 dB and an ultra-low insertion loss <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathbf{S}_{12})$</tex> of only <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{0. 1 ~ d B}$</tex> across a broad passband spanning 0 to 5.9 GHz. Additionally, it ensures strong stopband rejection, achieving −60 dB attenuation at 6.9 GHz. Furthermore, the specific absorption rate (SAR) analysis indicates values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.0116 ~\mathrm{W} / \text{kg}$</tex> for localized exposure (1 g) and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.0696 ~\mathrm{W} / \text{kg}$</tex> for average exposure (10 g), demonstrating compliance with safety standards. These results demonstrate the potential of SSPP technology for flexible electronics, balancing efficiency and comfort while enabling advancements in smart textiles and RF components.