Radio signal propagation in a tunnel exhibits distinct near and far regions with quite different propagation characteristics. This paper proposes a model that can distinguish these propagation regions and predict their respective propagation losses in the tunnel. The model relies on a break point to separate the propagation regions and a hybrid technique to calculate the propagation losses. The location of the break point is determined with the solution of a novel tunnel-propagation equation for the first time. The solution shows that the location of the break point depends strongly upon frequency, antenna position, and tunnel transversal dimensions. The model is compared with data measured in various tunnels at different frequencies (900 MHz, 1.8 GHz, and 2.448 GHz). The results show reasonable agreement between predictions and measurements.