Q:

Velocity factor in coax cables

I mainly see the term "velocity factor" used when referring to the velocity of propagation of an electromagnetic wave front in coax cables.  This velocity seems strongly dependent on the properties of the cable's dielectric.  Would coax cables stripped of their tubular shielding (but not their dielectric) have the same velocity factor as a normal coax line?  Or is the velocity affected by electromagnetic coupling between the core and shield? 

A:

Well, in vacuum the velocity of a wave is $c_0=3\cdot10^8$m/s. The speed in a medium is $\frac{e_0}{n}$ where $n$ is the refractive index of the material .The relationship between the refractive index and dielectric constant is $n=\sqrt{\epsilon_r\mu_r}$ where $\epsilon_r$ and $\mu_r$ is the relative permitivitty (dielectric constant) and relative permeability of the material.  
Thus the velocity of the wave is $\frac{c_0}{\sqrt{\epsilon_r\mu_r}}$
Stripped cable should show the same speed for the wave still confined in the dielectric.  You can however, if you want to, dig deeper into group and phase velocity and try to see where that math takes you.