|Abstract:||Electromagnetic signals in the ultra-low frequency (ULF) range of 3-3000 Hz has low signal attenuation and high penetration depth, making them ideal for underwater and underground wireless communication systems. However, the large wavelength of ULF signals (10^2-10^5 km) necessitates conventional antennas spanning across several kilometers, rendering them impractical for mobile applications. Alternatively, low-frequency communication over short distances has been achieved using a quasi-static magnetic field generated by electromagnetic coils that are few meters in length. Nevertheless, if these coils are scaled down to be made man-portable, they require large input power even to generate a magnetic field that can be detected over a few meters.
In this thesis, I explore an alternative method to design portable low-power ULF transmitters using the periodic mechanical motion of permanent magnets. First, I analytically show that the physical oscillation of a permanent magnet creates a time-varying magnetic field that can be used for communication. Using this principle, I present a ULF magneto-mechanical transmitter design that uses an electrically-driven torsional resonator. In the resonator, an oscillating permanent magnet (rotor) generates the magnetic field, and additional fixed permanent magnets (stators) provide the torsional stiffness. The phenomenon of resonance helps the rotor magnet achieve a large oscillation amplitude with low input power.
In order to analyze the mechanical behavior of the resonator, I formulate a theoretical model using point dipole approximation, and compare the results with finite-element simulations. The analytical model brings out the nonlinear dependence of magnetic torsional stiffness on the rotor oscillation amplitude. Further, it shows that the resonance frequency depends on the dipole moment of the stator, and the separation between the magnets. However, due to close proximity between the magnets, the point dipole approximation fails to accurately calculate the magnitude of torsional stiffness. I demonstrate that the near-field effects can be fully captured by magnetostatic finite-element simulations; the numerical results are used to estimate the natural frequency of the resonator. Following this, I analyze the electrical behavior and power consumption of the transmitter using a lumped circuit model. Calculations indicate that the resonator behaves like a tank circuit, and helps reduce the input power required to operate at resonance.
Using the theoretical framework, I design and experimentally demonstrate transmitter prototypes that work up to 1000 Hz. With a maximum linear dimension less than 25 cm, the transmitters can generate an equivalent of 1 fT rms at 1 km distance. I present single-rotor transmitter designs with one oscillating magnet that can operate at frequencies <200 Hz. Experiments show that the power consumption scales directly with frequency. Higher transmission frequencies can be achieved by reducing the moment of inertia of the rotor. I implement this using a magnetically coupled array of multiple oscillating magnets resulting in a multi-degree-of-freedom resonator. Eigenmode analysis establishes the resonator configuration necessary to achieve synchronous motion of the rotors. Calculations show that the in-phase mode has the highest eigenfrequency, and optimum mode shape can be achieved by tuning the rotor-stator distance. Using these concepts, I design and demonstrate a range of multi-rotor prototypes that work in 400 - 1000 Hz range. In addition to generating a constant amplitude time-varying magnetic field, the transmitter can be used to send messages; amplitude modulation up to 20 bps using On-off keying is demonstrated. Finally, I show that the signal generated by the transmitter is detectable over 30 m, and it can be used for communication in outdoor environments.
Overall, the resonant magneto-mechanical transmitter is compact (< 25 cm) and consumes low power (< 20 W), offering a way to miniaturize ULF transmitters. Such portable transmitters greatly impact wireless communication in harsh electromagnetic environments.