Loosely Coupled Surface Wave Wireless Propagation Channel on Bulky Metallic Pipes: Measurement, Modeling and Analysis

To explore the effect of coupled distances on surface waves (SW) mode excitation in bulky conductor cases, whose diameter is comparable to or exceeding one-wavelength, we design the wirelessly coupled SW launchers for long, bulky conductors of varying diameters, aiming to establish a loosely coupled SW channel model for Industrial Internet-of-Things (IIoT) environment. A path loss (PL) model is proposed, incorporating both the coupled distance and transceiver distance. Unlike traditional SW wireless channels, which require a tightly coupled distance of less than half-wavelength, our results indicate that increasing the conductor diameter can extend the effective coupled distance to up to 2-wavelength. This loosely coupled configuration reduces the PL by approximately 17 dB compared to free-space propagation at the same transceiver distance, suppresses the first multi-path level by at least 5 dB, and yields a channel capacity gain of 1.5~14 Gbps. Furthermore, under a fixed loosely coupled distance, the SW wireless channel becomes increasingly directional as the conductor’s diameter grows. The proposed model is expected to significantly enhance the deployment of SW nodes in IIoT environments that utilize multiple bulky conductors.