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The excitation of the lowest TM surface wave in grounded dielectric slab by a terminated parallel plate waveguide is discussed. The ground plane is the continuation of the lower plate of the waveguide and the infinite dielectric slab is partially filling the waveguide. The thickness of the slab, the height of the parallel plate waveguide, and the frequency are such that only the lowest slow wave can propagate in the partially filled waveguide and the grounded dielectric slab. The Fourier transform of the field scattered by the termination of the upper plate of the waveguide is found by means of the Wiener-Hopf technique and the far fields obtained by the method of steepest descents. The percentage of power reflected back into the waveguide, of power transmitted to the surface wave in the slab, and of power radiated into the open space are plotted vs the thickness of the slab for different heights of the waveguide and . This method of excitation is found to be very efficient. If the dimensions of the waveguide and the slab remain within a considerably wide range, the efficiency obtained for a given frequency is very close to the optimum. Therefore, the adjustments for maximum efficiency are not critical.