Propagation Mode Analysis of Undersea Insulated Single-Wire Transmission Line with Open Lossy Boundary

Abstract

Insulated single wires have been employed for undersea wireless power and data transfer systems. The inner conductor, insulating material and seawater form a structure superficially analogous to a coaxial cable, enabling electromagnetic waves to propagate along the single wire over long distances. Underwater units can wirelessly acquire energy or data via couplers placed near the single wire. However, the semi-infinite open boundary of seawater fundamentally alters the electromagnetic propagation regime. Previous works have largely employed quasi-static approximations in the low-frequency limit or antenna-based models at higher frequencies, both assuming a TEM (or quasi-TEM) mode within the dielectric insulation. The TEM assumption underlying both approaches breaks down at electrically significant scales. Starting from Maxwell's equations under the semi-infinite open boundary of seawater, this paper derives the propagation modes of the single-wire transmission line. The analysis reveals that the structure does not support a standard TEM mode, but exclusively the TM0 mode. The theoretical predictions are validated by full-wave simulations in CST, using a 1 m long insulated wire model with an inner conductor radius of 2.5 mm and a 2 mm thick dielectric insulation layer, in the 1 MHz to 100 MHz range, and are further confirmed by a water tank experiment, in which the measured response contains a single propagating mode whose attenuation agrees with theory. These results provide theoretical support for high-speed underwater wireless communication via single-wire structures.