When a waveguide surrounded by vacuum enters an embedding material, guided waves are scattered at this entry point due to the change in the surface impedance. This paper is concerned with the numerical modelling of the entry reflection of the fundamental longitudinal mode, L(0,1), in cylindrical waveguides. An approach based on modal analysis is used to characterize this reflection. It is validated against Finite Element modeling, very good agreement being obtained. As an example application, the cure monitoring of epoxy resins is discussed. © 2003 American Institute of Physics