 Incident signals containing TM modes above a certain frequency can generate modes which propagate along the longitudinal slots with the slots themselves acting as waveguides. The point at which this can start to happen is the frequency at which the height of the slot is greater than half the free-space wavelength of the signal. If this frequency is above the required stopband of the filter the effect is of no consequence. Otherwise, steps outside the filter are needed to suppress these modes and can be incorporated into the end-matching sections.
