The present work investigates the coupling mechanism between a ridged and a vertical, tilted rectangular waveguide via a non-offset longitudinal resonant slot. The excitation of the non-offset slot is realized in the presence of an offset small capacitive cylindrical post, attached to the ridged waveguide broad wall. In the present work no radiation phenomena were considered. The focus was on the coupling mechanism triggered by the slot and the post, under the assumption that both waveguides are terminated at a matched load. For this reason a general theoretical model was developed that enables the computation of the field quantities produced by the interaction between arbitrarily located longitudinal slot and metallic capacitive post inside the ridged waveguide at the presence of a twisted, vertical rectangular waveguide. The system of equations for this model was derived with the aid of the appropriate Dyadic Green Functions. The next step was to determine the most suitable method for solving the resulting system of equations. The Method of Moments was favored and applied along with the appropriate expansion and weight functions. Issues of solution convergence were also investigated. The theoretical results were compared against the results of a simulated model under Agilent HFSS. Additionally a test structure consisting of a ridged waveguide and a capacitive post was constructed and measured in the lab of the University of Duisburg - Essen. The measured data were compared against the corresponding results obtained by the theoretical model. The theoretical model was deployed in order to determine the impact of the geometric dimensions on the coupling behavior of the waveguide junction. The results show that via fine-tuning of the slot and post dimensions this waveguide junction can exhibit coupling characteristics that meet a variety of requirements.