 The waffle @-@ iron filter was invented by Seymour B. Cohn at Stanford Research Institute in 1957 . The basis for the filter is the corrugated @-@ waveguide filter . This consists of a series of ridges , or corrugations , across the width of the filter . There are corrugations inside the waveguide on both the top and bottom surfaces . The rising and descending ridges are aligned with each other but do not meet in the middle ; there is a gap in between . In the waffle @-@ iron filter there are , in addition , slots cut through the ridges down the length of the waveguide . This leaves a matrix of square islands , or teeth , on the top and bottom surfaces . 
