illustrated in fig1 a 2 a 3 d the dimensions of the design are provided in tables 1 and 2 where table 1 includes the predefined values and table 2 the values calculated by the design process in the design the thickness of the metal box 130 was not critical from a microwave design point of view but was set at 0 25 mm on all sidewalls and 0 15 mm on top and bottom surfaces the length of the posts 110 120 lenres was calculated to be electrically equal to one quarter of a wavelength at the mid band frequency of the filter 100 for the design where the dielectric was essentially air the mid band length lenres was calculated by the equation where u03bd p was the phase velocity of a wave propagating along the transmission line and u0192 o was the center frequency of the filter 39 s passband for the present design having posts 110 120 for a tem transverse electromagnetic mode wave with an air dielectric u03bd p was equal to the speed of light in a vacuum or 2 998 10 8 m s the center frequency of the filter 100 was 25 0 ghz making lenres 2 998 mm however the length was then adjusted in simulation to correct for non ideal effects to provide the value listed in table 2 leaving the design example and turning to other exemplary configurations of the present invention fig4 a 4 b schematically illustrate an isometric and cross sectional views respectively of a further.