what is the electrical length of a transmission line in degree?

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It depends on frequency, its physical length and on its propagation
velocity.

Velocity Factor (VF) = Actual velocity on line / Velocity of light.

It is sometimes better to refer to length in wavelengths rather than
degrees.

One wavelength = 360 degrees. 1/4-wave line length = 90 degrees.

I'll leave somebody else to write down the actual formula for the
number of degrees in a given length of line in metres, at a given
frequency in MHz and given VF. I'm too tired.
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Reg.