Stripped off coax velocity factor
 

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

Stripped off coax velocity factor
 

What you have left after the stripping process is just a wire with a
thick insulating jacket... The fact that it was part of a coaxial
cable before the stripping has no more bearing on the answer than the
fact that the copper was previously greenish ore nodules buried in the
ground and the jacket was made from nasty smelling crude oil...

So, forget all about 'coax', it has nothing to do with the question...
If your new wire had a jacket of the usual thickness then the loss of
velocity factor would be on the order of 3% compared to the usual 983/F
or 300/F... Given that it is a thick jacket I would guess the VF to be
down by 5%... So take what ever formula you are working with to
determine antenna length and reduce that length by 5% for cutting your
wire to resonance...

denny / k8do

Stripped off coax velocity factor
 

PaoloC wrote:
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?

Something else. EZNEC will even model it for you.
The user can set the dielectric constant and
thickness of the dielectric in the "wires" menu.
For instance, 0.1" of polyethylene insulation
on #14 copper wire lowers the resonant frequency
of an 80m dipole by about 5%.
--
73, Cecil http://www.w5dxp.com

Stripped off coax velocity factor
 

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

Stripped off coax velocity factor
 

Hmm, that is new to me Roy... So what is the professional term for the
velocity of a wave propagating along an unterminated antenna wire?

As far as the G-string, I took great interest in their characteristic
velocity in my younger days...

denny

Stripped off coax velocity factor
 

Denny wrote:
Hmm, that is new to me Roy... So what is the professional term for the
velocity of a wave propagating along an unterminated antenna wire?

The velocity is simply the speed of light if the wire is uninsulated
(except, of course, for traveling wave antennas like a Beverage, where
transmission line type analysis and "velocity factor" are appropriate).
Analysis of antennas made from insulated wire is rare, and perhaps
"velocity factor" is used in that case.

When using a transmission line analogy to explain antenna operation
(more-or-less but not completely accurately), the model usually assumes
speed of light propagation along the line, then a load at the end which
effects the shortening of the resonant length.

. . .

Roy Lewallen, W7EL

Stripped off coax velocity factor
 

Roy,

That was an informative post and I want to say thanks for taking the
time and effort to write your reply.

Regards - Roger


Roy Lewallen wrote:
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

Stripped off coax velocity factor
 

Thank you Roy, Denny and Cecil for your very useful answers.

I'll go ahead, peel off that coax, and work on a dual-quad for 70cm.

Paolo IK1ZYW

Stripped off coax velocity factor
 

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?

As others have already mentioned, you have a bit of wire with a plastic
environmental sheath. You didn't mention the frequency, but for HF I have
each close to trees. The calculated length is about 4% longer than the the
actual length requred for resonance, even with more than 3 metres of nylon
cord to support the ends.

In other words, the velocity factor of a bit of wire in space is 1. Put it
near something and that VF falls a bit. Stuff it in a metal tube and it will
fall a LOT, maybe even approaching half (0.66).