Evaluation of Grounding Loss for Very-Low-Frequency Monopole Antennas Based on Tide-Modulated Equivalent Conductivity

This paper addresses the challenge of accurately evaluating the grounding loss of coastal very-low-frequency (VLF) monopole antennas under tidal coverage. An equivalent conductivity calculation method based on a layered lossy transmission line model and Wait’s equivalent admittance of ground screens is proposed to obtain the local equivalent conductivity of the tidally covered area. A sector-ring grounding loss model is then established, incorporating tidal depth, coverage area, and ground screen structure, and the effects of tidal coverage extent, tidal depth, non-uniform radial ground screens, and diurnal tidal dynamics on the grounding loss resistance are analyzed. The proposed analytical model is validated against full-wave FEKO simulations over 20–25 kHz, with an average relative error of 6.67%. Results show that when tidal water covers the sparse outer region of the ground screen, the highly conductive seawater layer provides an additional low-resistance return path, reducing grounding loss. When the tide intrudes further into the high-density inner region, current redistribution among the seawater layer, soil, and metallic ground screen weakens the loss reduction or even causes a non-monotonic increase. This work provides a theoretical basis for the tide-adaptive design of grounding systems for coastal VLF antennas.