This paper proposes a model of Waveguide Photonic Band Gap (WPBG) structures, based on the Leaky Mode Propagation (LMP) method, to perform a complete analysis of electromagnetic (e.m.) wave propagation, including radiation loss, for a structure of finite extension. The LMP method allows for the detailed examination of the interaction between guided modes and radiative modes, providing insight into how energy leaks from the waveguide. This model enables the accurate prediction of both the confinement and loss characteristics of the WPBG structures, making it suitable for applications where both controlled wave propagation and minimized energy dissipation are crucial. Furthermore, we demonstrate the effectiveness of this approach through simulations and discuss its potential impact on the design of photonic devices.