Propagation constant
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For an electromagnetic field mode varying sinusoidally with time at a given frequency, the propagation constant is 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, γ, is a complex quantity given by γ = α +iβ, where α, the real part, is the attenuation constant and β, the imaginary part, is the phase constant.