Analytical expressions are developed for characterization of ridged / finned waveguides for use in the design of direct coupled waveguide filters with ridged and / or unequal width resonators. The analytical expressions are used to control the harmonic passbands and higher order modes. A survey is made for comparison of responses of waveguide filters using unequal width resonators and unequal ridge resonators. it is found that ridged resonators are more effective in suppressing the spurious passbands in the upper stopband. A simple and accurate approach is developed for the design of the filters. It is based on impedance scaling of each resonator of the inverter coupled prototype filter so that it can be equated to the desired half wavelength wave-guide resonator with selected cross-sectional dimensions. This approach is found to be more effective in controlling the band edge shifts seen in the classical susceptance slope parameter approach.