Modulating the Earth's Magnetic Field for Communication in RF Denied Environments

Abstract

Low-frequency magnetic-field communication is attractive for radio-frequency (RF)-denied environments, including underwater, underground, and through-the-earth (TTE) communication, because electromagnetic fields at ultra-low frequency (ULF) and very-low frequency (VLF) experience substantially lower attenuation in conductive media than conventional RF signals. However, the long wavelengths associated with ULF/VLF operation make conventional antennas inefficient, physically large, and power intensive. Recent approaches, including mechanically based antennas and rotating permanent-magnet transmitters, reduce transmitter size but introduce moving parts, mechanical reliability concerns, and scaling limitations. This dissertation investigates controlled permeability modulation as a non-mechanical method for generating low-frequency magnetic signals. The central idea is that a static magnetic field can be converted into a time-varying magnetic-field signal by electrically varying the permeability of a nearby ferromagnetic structure. Two implementations of this principle are studied. The first uses a stationary permanent magnet as the static magnetic-field source. The magnetic flux from the permanent magnet is modulated by changing the effective permeability of a surrounding Metglas shield using an electrically driven control coil. The transmitter is analyzed under both high-power and low-power excitation, and the dependence of the modulation behavior on shield saturation, magnet orientation, control-coil excitation, and material properties is investigated through simulation and measurement. Analytical models are developed to describe flux redistribution in single-layer and multilayer ferromagnetic shielding structures containing an internal magnetic dipole source. These models relate magnetic shielding behavior to permeability, shield geometry, interlayer spacing, and magnet dimensions. A multilayer Metglas shield is then introduced to improve modulation depth while reducing shield weight and control power. Prototype transmitters are fabricated and experimentally characterized using three-axis air-core receiver coils. The second and the main contribution eliminates the permanent magnet and uses the ambient geomagnetic field as the static magnetic bias source. A ferromagnetic structure aligned with the local magnetic north–south direction perturbs the local geomagnetic field, and modulation is achieved by varying the permeability of the structure with a control coil. Measurements verify the orientation dependence of the received signal and show second-harmonic modulation under sinusoidal excitation, consistent with cyclic permeability modulation of the ferromagnetic material. These results demonstrate the feasibility of compact, non-mechanical ULF magnetic-field transmitters based on controlled modulation of a static magnetic field, including the ambient geomagnetic field. The magnetic-field generation concepts developed in this dissertation also suggest broader applications beyond communication, including rotating-field transcranial magnetic stimulation (TMS) systems, which are treated as a related application in the appendix and are the subject of separate publications.