Coupling factor as a function of the normalized radial extent of the evanescent region in Type-a evanescent regions, as expected from the model. The cyan, blue, magenta and red colors represent the result for a typical range of values of the β parameter, i.e., β = 1.2,  1.3,  1.4, and 1.5, respectively. The dashed lines correspond to the predictions obtained with the formalism of Shibahashi (1979) according to Eqs. (19), (20) and (24). The solid lines correspond to the predictions computed in the formalism of Takata (2016a) according to Eqs. (13), (21), (24) and (26).