Narrow slots in a waveguide surface are widely used in X-band techniques as coupling elements or radiators. The filling of waveguides with dielectric is utilised in order to miniaturise devices or slow down waveguide waves. The layered filling gives some additional opportunities to control output system parameters. Those are scattering matrix elements of a slot in a waveguide filled with layered dielectric. In order to obtain them it is necessary to solve an excitation problem for such a waveguide when a source is a magnetic current equivalent to the electric field in a slot. The main difficulty of solving this problem is due to the fact that in the description of magnetic current density the Dirac delta-function is present. The most traditional methods of solving this problem (the eigen waves method or Green's function one for the field) lead to the expressions for the H-field in the source region containing the series and the additional divergent term where the current density is present in an explicit form. As consequence of this a series which must compensate divergence of this additional term is also divergent. As a result the solution obtained in a such form is not suitable for calculations.

This situation occurs in the following cases. Firstly, when we use the eigen-waves method for finding the field excited by a longitudinal slot. Secndly, when the direction used as a longitudinal one in the process of the Green's function construction is parallel to the magnetic current of interest. So for the narrow transverse slots we can use the eigen waves method without any difficulties. But in the case of longitudinal slots it is necessary to construct the Green's function using the direction perpendicular to the line of the magnetic current as a longitudinal one. For longitudinal slots this direction is perpendicular to the boundaries of adjacent dielectric layers. Such Green's functions were built in [1] and in B. A. Panchenko's works. This way specifies a representation of z-dependence of the field in the Fourier integral form. The same representation was used in [2] in the process of direct solving the Maxwell equations for obtaining an admittance of a longitudinal slot. The both above mentioned ways lead to very tedious computation formulas. The numerical results for a longitudinal slot in a waveguide with dielectric slab are given only in [3]. Another way of solving this problem, proposed in [4], permits to obtain more compact solution. The main idea of this method is expanding the magnetic current density over a set of potential vector functions and afterwards uniting this series with ones over LE- and LM-waves. The validity of this way is caused by the fact that the current density term in the solution has solely potential nature [5]. We call this way as a modified eigen waves (MEW) method.

The numerical results for the transverse slots, obtained with the eigen waves method, were described in [6]. In this paper we represent the numerical results obtained with the MEW-method for longitudinal slots and experimental results for both types of slots.