The Fresnel zone theory is employed to determine the boundary to distinguish propagation regions for line-of sight topographies in tunnels.The formula for calculationg the break point distance is obtained.A hybrid propagation model combining the free space propagation model and the modified waveguide propagation model is proposed.The hybrid model and the break point distance formula are validated with our own and other published experimental data in various tunnels and at different frequencies.It is shown that the free space propagation model can be used to predict propagation loss in region prior to the break point

while the modified waveguide propagation model offers excellent prediction of propagation loss after break point.