 TE0n modes should not , in theory , be excited in the waffle @-@ iron filter because of its vertical symmetry about the centre @-@ line . However , in practice they can be caused by poorly mating waveguide flanges or misaligned teeth . These spurious modes can be suppressed by fitting thin wires across the width of the filter in the space between the teeth on the vertical centre @-@ line of the waveguide . This can be a better solution than over @-@ engineering the components to high precision and results in a more robust design . 
