For an electromagnetic field mode varying sinusoidally with time at a given frequency, the logarithmic rate of change, with respect to distance in a given direction, of the complex amplitude of any field component. Note: The propagation constant, , is a complex quantity given by = +i, where , the real part, is the attenuation constant and , the imaginary part, is the phase constant. 2. In a physical transmission medium, e.g., a coaxial cable or optical fiber, the velocity of an electrical or electromagnetic signal expressed as a decimal fraction of the speed of light in vacuuo.