What is the Propagation Constant?

RF Basics 
What is the propagation constant in a transmission line?
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Editorial Team - everything RF

Jan 4, 2018

Propagation constant is a measure of changes in a sinusoidal electromagnetic wave in terms of amplitude and phase, while propagating through a medium. This can be a transmission line or free space. The Propagation constant is a dimensionless quantity.

It is represented by the following formula:

α = Attenuation constant, it causes the signal amplitude to decrease while propagating through a transmission line. It is always a positive number, if we get a negative attenuation constant that means we are violating the first law of thermodynamics. Its Unit is dB/meter.

β = Phase constant, it is the imaginary component of the propagation constant. It gives us the phase of the signal along a transmission line, at a constant time. Its unit is radians/meter, but we often convert it to degrees/meter.