Spatial-Gradient Magnetic Antenna for Long-Distance Weak Magnetic Signal Transmission

Conventional magnetic-induction transmission is limited to short-range delivery, failing to meet the long-distance weak magnetic wake-up signal transmission demands of distributed off-grid sensors. Although extremely low frequency (ELF) fields feature low attenuation and theoretically support ultra-long propagation, existing ELF antennas suffer inherent limitations of mechanical abrasion, exclusive material dependence, and near-field-restricted coupling. This paper proposes a novel spatial-gradient transmitting antenna with four groups of coaxial, identically wound gradient coils in a tangential–parallel layout for long-distance weak magnetic wake-up signal transmission. A coaxial symmetric mutual coupling circuit model is established and analyzed via systematic parametric simulations. Field experiments demonstrate stable ELF signal transmission at 153 Hz over 650 m. The proposed fully static coil structure eliminates mechanical wear and material reliance and achieves extended coupling range using superimposed quasi-static magnetic fields. This low-complexity design fills the research gap in meter-to-kilometer-scale weak magnetic signal transmission, enabling reliable remote wake-up of ultra-low-power sensors powered by their on-board local batteries in complex propagation environments such as underground and conductive media.