Group and phase velocity

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They are not the same.
it is discussed in the Cheng book (its title i doubt : electromagnetics).

But as a simple example , look at a DSB tone modulation. u have a high frequency (Fc) cosine wave and its ( wavelength/frequency) is considered phase frequency (the main velocity).
and your message that is a tone (with freq: Fm) is made the envelope of the previous cosine waveform and its ( wavelength/frequency) is group velocity.

regards
 

hello friend
there is a distinct difference between phase and group velocities...better than me explaining, i hope u will be pretty clear when you refer to Simon Haykin...he has explained the difference with an example.

Regards,
Arunacha
 

The group velocity is the velocity of information propagation and, loosely, of energy propagation. The velocity of propagation of an envelope produced when an electromagnetic wave is modulated by, or mixed with, other waves of different frequencies.

Phase velocity is the velocity of propagation of a uniform plane wave, given by (a) the product of the wavelength and the frequency divided by (b) the refractive index of the medium in which the wave is propagating
 

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