Very wide stopbands can be achieved by cascading several waffle-iron filter units together. Each unit is designed for a stopband in different, but overlapping, ranges. The filter with the highest frequency stopband has the smallest, and greatest number, of teeth. The units are connected together with λ/4 impedance transformer sections of waveguide in order of progressively higher frequency operation. Since the impedance transformers are operating at different frequencies the ones connected to the units with smallest teeth are shorter than those connected to units with larger teeth. Matthaei gives an example of a three unit waffle-iron filter designed to stop all harmonics of the passband from the second to the tenth – a combined stopband of to with a rejection of .Matthaei et al., pp.938–941Sharp, p.111
