Electrically-Shielded Coil-Enabled Battery-Free Wireless Sensing for Underwater Environmental Monitoring

Abstract

Battery-free wireless sensing in extreme environments, such as conductive solutions, is crucial for long-term, maintenance-free monitoring, eliminating the limitations of battery power and enhancing durability in hard-to-reach areas. However, in such environments, the efficiency of wireless power transfer via radio frequecny (RF) energy harvesting is heavily compromised by signal attenuation and environmental interference, which degrade antenna quality factors and detune resonance frequencies. These limitations create substantial challenges in wirelessly powering miniaturized sensor nodes for underwater environmental monitoring. To overcome these challenges, electrically-shielded coils with coaxially aligned dual-layer conductors are introduced that confine the electric field within the coil's inner capacitance. This configuration mitigates electric field interaction with the surrounding medium, making the coils ideal for use as near-field antennas in aquatic applications. Leveraging these electrically-shielded coils, a metamaterial-enhanced reader antenna was developed and a 3-axis sensor antenna for an near-field communication (NFC)-based system. The system demonstrated improved spectral stability, preserving resonance frequency and maintaining a high-quality factor. This advancement enabled the creation of a battery-free wireless sensing platform for real-time environmental monitoring in underwater environments, even in highly conductive saltwater with salinity levels of up to 3.5%.

Keywords: battery‐free; electrically‐shielded coil; near‐field antenna; underwater environmental monitoring; wireless sensing.

Figure 1

Illustration of the battery‐free wireless sensing system for underwater environmental monitoring.

Figure 2

Electrically‐shielded coils. a) Configuration of a TC‐CSR and its electric current profiles along inner and outer conductors’ surfaces at resonance mode. b) Simulated electric currents’ magnitudes of the TC‐CSR. c) Configuration of an OC‐CSR and its resonating electric current profiles. d) Simulated electric currents’ magnitudes of the OC‐CSR. e,f) Simulated electric field patterns on the cutting plane of the TC‐CSR (e) and OC‐CSR (f). g–j) The variations of the reflection spectra of a homemade coil resonator made of copper wire (g), a commercial spiral resonator loaded with a capacitor (h), a TC‐CSR (i), and an OC‐CSR (j).

Figure 3

Electromagnetic characterizations of the reader antenna. a) Configuration of the reader antenna. b) Reflection spectra of the feeding loop as the tuning capacitance is varied. The inset shows the configuration of the feeding loop, constructed from TC‐CSR, along with a matching and tuning circuit. c) The reflection spectrum of the metamaterial within the reader antenna. The inset illustrates the configuration of the metamaterial, composed of an OC‐CSR array. d) Magnetic field patterns on the cutting plane of the metamaterial at different resonance frequencies. e) Reflection spectra of the reader antenna while sweeping the tuning capacitance. f) The magnitude and direction of the magnetic field across the various resonating modes of the reader antenna. g) Reflection spectra of the reader antenna with various curvatures. h) Magnetic field strength decay rate for the reader antenna with various curvatures.

Figure 4

Magnetic field mapping in underwater environments. a) Illustration of the magnetic field mapping setup. b–e) Measured magnetic field strength on the cross‐section (indicated by the yellow grid in (a)) when the communication medium is air (b), DI water (c), 0.9% saltwater (d), and 3.5% saltwater (e). f) The decay rate of the magnetic field strength in the four communication medium as moving away from the reader antenna. g) Spectra of the magnetic field distribution at location (indicated by the pentagram in the inset figure) with separation distance from the reader antenna of 50 mm.

Figure 5

NFC sensor node design and experimental validation. a) The configuration of the NFC sensor circuit is integrated with a TC‐CSR‐enabled sensor antenna. b) The photo of the NFC sensor circuit. c–f) Variations of the effective resistance (c), inductance (d), capacitance (e), and quality factor (f) of the sensor antenna made by a TC‐CSR loaded with a capacitor as sweeping the length of the coaxial cable of the TC‐CSR. g) Illustration of NFC sensor node integrating with 3‐axis sensor antenna. h) The experimental setup for the battery‐free wireless sensing for underwater environmental monitoring. i) The measured water temperature as pouring hot water and ice into the container. j) The measured ambient light intensity by switching on and off of the light. k) Quantitative analysis for the power harvested by the sensor node.