I've found quite a variation in foamed dielectric cable velocity factor
from lot to lot of the same brand and type, even with major brands.
Apparently they don't control the density of the dielectric very well.
So if you're planning on using foamed dielectric cable in an application
where VF is important, I highly recommend that you measure the VF of a
sample from the same piece you'll be using.

Roy Lewallen, W7EL