Abstract: Very high frequency (VHF, 30–300 MHz) beyond-line-of-sight (BLoS) radio propagation primarily involves three medium-based modes: ionospheric volume or surface scattering (including meteor trails), tropospheric volume or surface scattering, and terrain diffraction. Current research predominantly focuses on individual propagation modes independently, addressing propagation calculations, channel modeling, and link analysis, In fact, irregular structures with diverse refractivity gradients exist in both the ionosphere and troposphere, enabling five propagation mechanisms for BLoS links: refraction, reflection, scattering, diffraction, and continuous medium propagation. Simultaneously, spatiotemporal random evolution of refractivity gradient structures in ionospheric and tropospheric irregularities leads to coupling between intra-layer and inter-layer propagation modes, collectively influencing actual transmission links. Conventional calculation methods based on single-medium propagation modes are inadequate for guiding engineering computation and analysis effectively. This paper begins with the fundamental properties of refractivity gradient structures in the ionosphere and troposphere, analyzes the applicability of the five propagation mechanisms, and proposes a multi-mode coupled propagation calculation method for VHF BLoS links. For observed VHF BLoS phenomena inadequately explained by single-medium, single-mode, or single-effect models, a link loss calculation method based on refractivity gradient structures elucidates the mechanism of multi-mode coupled propagation, and has preliminarily demonstrated practical values.