Figure 1-27A.Different frequencies in a waveguide.
Figure 1-27B.Different frequencies in a waveguide.
The velocity of propagation of a wave along a waveguide is less than its velocity through free space
(speed of light). This lower velocity is caused by the zigzag path taken by the wavefront. The
forward-progress velocity of the wavefront in a waveguide is called GROUP VELOCITY and is
somewhat slower than the speed of light.
The group velocity of energy in a waveguide is determined by the reflection angle of the wavefronts
off the "b" walls. The reflection angle is determined by the frequency of the input energy. This basic
principle is illustrated in figures 1-28A, 1-28B, and 1-28C. As frequency is decreased, the reflection angle
decreases causing the group velocity to decrease. The opposite is also true; increasing frequency increases
the group velocity.