Jim Kelley wrote:
Roy Lewallen wrote:

As others have pointed out, it's risky to treat c as a variable or
medium-dependent speed.

Roy -

The convenient thing about using medium dependent c in that equation is
that we can use things such as index of refraction or velocity factor to
convert from vacuum 'c' to 'c' in another medium. The fact that it
makes the results of the calculation more accurate tends to mitigate any
risk that might be encumbered when using it.

To require that only vacuum c be used in the equation to me seems overly
authoritarian. I wonder how you feel about the speed of sound? :-)

73, Jim AC6XG

What, the speed of sound in a vacuum? I'm afraid you'll have to ask
Cecil or Art about that -- I'm not qualified to comment.

I'm not trying to be authoritarian about the use of "c", just reporting
what I find in my textbooks. Grabbing just one for example, Kraus'
_Electromagnetics_, on p. 352 I find that he uses v as the phase
velocity, and says, "For free space (vacuum) the velocity is a
well-known constant (usually designated by c and usually called the
velocity of light)." and shows an equation for c. Then he gives an
equation for the "relative phase velocity" p, as v/c.

In the back of R.K. Moore's _Traveling-wave Engineering_, c is listed as
"Velocity of light in vacuum". He uses v-sub-p for phase velocity.

A number of authors avoid using c altogether, but those who do seem to
universally use it to mean the speed of light in a vacuum. What texts do
you have where it's used to mean the phase velocity in a medium other
than air?

Of course, you can always go ahead and interpret c any way you want,
even if it isn't what the author intended. Then you can progress from
there to any number of bizarre conclusions. They'd fit right in with the
ones being "debated" over and over on this newsgroup.

Roy Lewallen, W7EL