The purpose of the present paper is to provide insight into the fundamental mechanisms involved in the trapping of energy which is propagated in an evaporation duct. The emphasis is upon the solution of the problem for simple meteorological models with the intent of not obscuring the important principles with involved mathematics or exhaustive computer studies. Simple exponential-type profiles are assumed, and the wave equation is solved for these profiles by the well-established methods of mode theory. The results allow for inclusion of a large number of modes but are restricted to trapped modes. The results indicated that, for cases in which multiple modes are trapped, considerable signal fades occur at two- to three-mile intervals for centimeter wavelengths and such fades are strongly dependent upon receiver or transmitter heights. Further, it was found that the duct height is not a critical parameter unless it is comparable with mode heights.