sidewall cavities may be derived by using the formulation set forth in r v snyder 34 the dual mode filter a realization 34 microwave journal dec 1974 it has been found that the location points for irises 81c d 67c d fig5 may be determined which will achieve a minimum coupling coefficient as the center frequency of the stop band is varied if the longitudinal length of the cavity is given the distance l the input iris 81c should be centered approximatly 0 67 l from the fixed end of the cavity viz base 73 of the cavity fig7 in addition the center of iris 67a should be located at a distance from the fixed end of approximately 0 63 l screws 82a b and 79 are located in positions between these values it will be understood that gang tuning can be accomplished by using the type of gear coupling shown in fig1 of u s pat no 3 936 775 a still further embodiment of the invention may have a physical form similar to that of fig4 with an important difference with respect to dimensioning of the coupling lengths of waveguide sections 43 45 such embodiment would be based on the rhodes 34 natural 34 elliptic prototype structure 90 shown in fig8 using filter structure 90 and a design prototyping model a stop band filter may be designed in which uncoupled dual modes are utilized in a manner similar to the minimum phase design previously described with respect to fig4 network synthesis for the uncoupled dual mode elliptic design is described in j d rhodes 34 waveguide bandstop elliptic function filter 34 trans mtt 20 nov 1972 coupling and.