Cutoff frequency

Summary

The cutoff frequency of a waveguide mode is the lowest frequency at which that mode can propagate. Above it, the mode travels along the guide; below it, the mode decays exponentially along the guide and carries no power over long distances. It is set by the size and shape of the cross-section, through an eigenvalue of the transverse Laplacian. In electronics the same term names the frequency at which a filter's response drops to half power.