And air is mostly nitrogen. The nitrogen is squeezed into the plastic as it is being melted. Cables made with nitrogen gas injection can have a Vp of 83 percent or even more. In the lab, they can get close to 90 percent Vp, but it is very hard to keep it consistent. And consistency is the name of the game.
Coax cable is called coaxial because everything is on the same axis.
The impedance of this cable can be determined by three numbers. There is a simple formula that takes those three numbers and generates the impedance of the cable. In contrast, one can see that changing any of these dimensions or numbers can alter the impedance of the cable. For instance, if you step on the cable, it may change the distance between center and shield. Or if the operator in the factory changes the percentage of bubbles, then the dielectric constant will be different and so will the impedance.