Essentially, waffle-iron filters are waveguide low-pass filters with wide stop bands above their pass bands. The L-band waffle-iron filter discussed in this correspondence was originally developed at the Stanford Research Institute, Menlo Park, CA, USA. Subsequent reports and modifications have been presented by Young and Schiffman [1963]. This correspondence serves to supplement the existing information on waffle-iron filters, particularly as to their high power characteristics and the modifications required to achieve the high power characteristics.