  The results of the development of an approximate approach, which can be
considered as an analogue of the WKB method, are presented. This approach gives
possibility to divide the electromagnetic field in structured waveguides into
forward and backward components and simplify the analysis of the field
characteristics, especially the phase distribution. The accuracy of this method
was estimated by comparing the solution of the approximate system of equations
with the solution of the general system of equations. For this, a special code
was written that combines the proposed approach with the more accurate one
developed earlier. The results of this comparison showed that the accuracy of
the proposed approximate approach for sections with group velocity gradients,
similar to the SLAC section, is about 1%.
