<1>¿¿This paper is devoted to mathematical simulation of the "Trimpi" effect on the base of the investigations of VLF field diffraction at a three-dimensional irregularity in the lower ionosphere. The problem of the field of a vertical electric dipole in a plain waveguide with homogeneous properties of the Earth's surface and inhomogeneous ionospheric wall is considered. The irregular impedance model of the waveguide is created using the known vertical profiles of the electron concentration and effective collision frequency. The modeled ionospheric irregularity is considered as a finite cylinder without any limitations to its shape and the dimensions of its base. An original asymptotic method using the apparatus of integral equations is used for solving the diffraction problem. On the basis of the numerical simulations performed, one can draw the following conclusions: not only the forward scatter of the field but the backscatter as well are observed; the irregularity impact depends on the propagation path orientation relative to the geomagnetic field, on the underlying surface properties, and the irregularity location and its geometric dimensions. The found field variations are of a significant character and can be detected experimentally.