Baron wrote:
Roy Lewallen Inscribed thus:

Bob wrote:
On Sun, 04 Apr 2010 20:38:35 +0100, Baron
wrote:

You could get a sample and measure it !
Well, I discarded that idea because I have no idea how.

But then, on a hunch, I checked the manual that came with my MFJ-269,
and sure enough, on page 34, it tells how to measure Velocity Factor,
utilizing the distance to fault mode. It'll take a day or so to
recharge the 269's batteries, and then I'll have at it.

Bob
k5qwg
Unfortunately, it's not really simple to make measurements with
symmetrical line. You'll be exciting a common mode current which will
travel with a different velocity factor and affect the measurement.

Roy Lewallen, W7EL

Please could you elaborate on how and why a common mode current has a
different VF on a balanced line.


I'll take a shot..

The VF, to a first order, depends on the dielectric constant
(permittivity) of the medium separating the conductors of the
transmission line. (more correctly, the medium containing the electric
and magnetic fields) For differential mode, it's the insulation between
the wires (for window line, a value somewhere between that of the
plastic and that of air)...

For common mode, it's more the two wires acting as one conductor against
the surroundings (e.g. earth) as the other conductor. The permittivity
of that tends to be lower than that of the medium between the wires, so
the velocity factor is "faster" for the common mode than the
differential mode.

It's not quite that simple, of course, because the field surrounds the
conductors in all directions, not just conveniently between them.

Another way to look at it is think of a balanced pair with distributed L
and C suspended above a ground plane. The C (per unit length) between
the pair is different than the C to ground, as is the L, for the common
mode vs differential mode.