the velocity vector at a point, in a propagation medium, of a signal which may be ideally represented by two superimposed sinusoidal waves of equal amplitude and slightly different frequencies approaching a common limiting value
NOTE 1 – The magnitude of the group velocity is equal to the derivative of frequency with respect to the reciprocal of the wavelength.
NOTE 2 – In an isotropic medium the group velocity equals the phase velocity if the phase constant is a linear function of the angular frequency.
NOTE 3 – Each waveguide mode has its own particular group velocity.
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