It was pointed out that the definition of energy velocity has clear meaning only for a ray of electromagnetic waves with definite frequency and wave vector. Then
some explanation and proof were provided. By calculating the average energy velocities and the phase velocities of electromagnetic waves in boundless linear media and typical transmission lines
it arrives at the conclusion that:when the electromagnetic wave propagates through linear media by TEM mode
the average energy velocity equals the phase velocity; if electromagnetic waves are transmitted by the TEmn mode or TMmn mode through a transmission line
the average energy velocity is variable on the wave front
and it is always lower than the phase velocity or the speed of light in the vacuum.