WAVEGUIDES only allow frequencies above cut-off frequency and do not pass below the cut-off frequencies.Hence it acts as a high pass filter.The cut off frequency is given as:\({{\rm{\lambda }}_{\rm{C}}} = \frac{2}{{\sqrt {{{\left( {\frac{{\rm{m}}}{{\rm{a}}}} \RIGHT)}^2} + {{\left( {\frac{{\rm{n}}}{{\rm{b}}}} \right)}^2}} }}\)Where a and b are the DIMENSIONS of the waveguide (a>b)m and n are mode numbers TEMN