Compact Fractal-Shaped Frequency Selective Surface for Distant Liquid Sensing With Multiple Sensitive Spots

This article introduces compact battery-less wireless radio frequency (RF) sensors using fractal-shaped electromagnetic surfaces. Fractal structures allow a compact total size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.75\lambda _{{0}} \times 0.75\lambda _{{0}}$ </tex-math></inline-formula> at 2.9 GHz. In conventional sensors based on a frequency selective surface (FSS), the material under test (MUT) needs to be placed in all the unit cells or play the role of a substrate. However, in the proposed sensors, the MUT covers only 3%–7% of the electromagnetic surface with a volume of 1 mL. In contrast with conventional RF sensors with one local sensitive spot, the proposed surface can provide multiple sensitive spots. This capability has been tested using several different liquids including isopropyl alcohol (IPA), methanol, ethanol, acetone, and deionized (DI) water. The proposed sensing platform exhibits remarkable sensitivity while the reader antenna is over 10 cm away from the FSS. Changing the MUT from air to IPA and DI water results in a 40 and 350 MHz shift in the resonant frequency, respectively. This sensor can tolerate polarization misalignment between the reader antenna and the FSS.