 One of the performance issues addressed by the waffle @-@ iron filter is that in many waveguide filters the attenuation is dependent on the transmission modes of the incident signal and some filters can be quite bad at suppressing spurious modes which may be contained in this signal . For instance , with the corrugated waveguide filter , on which the waffle @-@ iron filter is based , attenuation of TEn0 modes in the stopband is strongly dependent on the mode number . This is not the case with the waffle @-@ iron filter which attenuates all TEn0 modes nearly equally up to a certain frequency . The limit is the frequency at which the distance between the metal teeth is greater than half the free @-@ space wavelength of the signal . The reason for the waffle @-@ iron design 's good performance in this respect is that the distance between the teeth is the same in both the longitudinal and transverse directions and nearly the same in all directions in between . This makes the waffle @-@ iron nearly isotropic to TEM waves in all these directions . Since any TEn0 mode wave can be decomposed into two TEM mode waves travelling in different diagonal directions , all TEn0 modes are affected nearly equally . 
