 The earliest waffle @-@ iron filters were designed with the image parameter method of filter design . Cohn 's original data for the corrugated filter could also be applied to the waffle @-@ iron with only a small adjustment of one parameter . An alternative approach to using Cohn 's empirical data , but still an image parameter design , is due to <unk> who used a waveguide T @-@ junction equivalent circuit to represent corrugations and this method was later extended by others to waffle @-@ irons . 
