Waveguides And Guided Waves

In a waveguide, the phase velocity is always greater than the speed of light . This is due to the waveguide boundary conditions altering wave propagation. The phase velocity is given by:
which is always greater than . However, this does not violate relativity since the group velocity remains below .

The dominant mode for a rectangular waveguide is the mode with the lowest cutoff frequency. For a rectangular waveguide, the cutoff frequency for TE modes is given by:
where and are the dimensions of the waveguide, and are the mode indices. The lowest mode is , as it has the lowest cutoff frequency.