 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 . 
