The term "velocity factor" usually applies to a transmission line which
has two conductors. (I'll exclude the "G line" and variations from this
discussion.) Velocity factor describes the speed of propagation of the
differential field between the two conductors (including any fringing),
and is equal to 1/sqrt(keff) where keff is the effective dielectric
constant of the material in which the field is propagating. If some of
the field is in air and some in another dielectric, the effective
dielectric constant will be some value lower than that of the other
dielectric.

In a coaxial cable, the entire field is confined between the conductors
-- inner conductor and shield -- so the effective dielectric constant is
that of the insulating material. In the case of your coax, the
insulating material is a combination of air and plastic, with an overall
effective dielectric constant of 1/(0.83)^2 ~ 1.45. When the shield is
in place, this is the effective dielectric constant for the field, and
it determines the velocity factor.

When you remove the shield and excite a transmission-line mode of
propagation, the wire is one conductor of the line. The other is
effectively the Earth, another half of a dipole antenna, and/or other
nearby conductors. It should be apparent that the vast majority of the
field between the conductors is now air. Consequently, the effective
dielectric constant for the field is very nearly 1 -- that of air -- and
the velocity of propagation of the transmission line mode is very nearly
one. The actual value depends on the thickness and dielectric constant
of of the wire and dielectric and the spacing to other other conductors.
In practice, the in effective length between insulated and uninsulated
wire in an antenna ends up being around 2 - 3% for typical insulating
layers. EZNEC has the ability to include a thin insulating layer on a
wire, so you can use it to find the difference in resonant frequency of,
say, a dipole with and without the insulation and from that infer a
"velocity factor" change caused by the insulation. The use of that term
for propagation on a radiating wire is a bit outside the use generally
accepted in the professional community. It seems to be a common use
among amateurs, however. If you want to use that term in this context, a
representative typical value would then be 97 - 98%.

Roy Lewallen, W7EL

PaoloC wrote:
Hi,
I cannot come up with an answer.

Let's say I have a coax cable with 83% velocity factor. I want to use
its inner conductor (that is solid copper single conductor) so I remove
the outer sheath and the braid.

I am left with the inner conductor and what was the dielectric, now
performing as an insulator around my wire.

What is the resulting velocity factor?
* 83%, as the original cable
* 100%, copper's
* something else?

I have experienced that wire dipoles and verticals built with insulated
electric wire have a final length shorter than the theoretical value. I
"suspected" the PVC coating to vary the velocity factor. Am I wrong?

Thank you in advance for your hints.
Paolo IK1ZYW