What is the unit of phase constant?

It is a dimensionless quantity with unit radians as it is an angle.

Phase constant

• The phase constant, also known as the phase change constant, parameter, or coefficient, is the imaginary part of the propagation constant for a plane wave in electromagnetic theory.
• It is equal to the real part of the angular wave number of the wave and reflects the change in phase per unit length along the direction travelled by the wave at any instant.