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. I
suggest making an approximate measurement, then doing final adjustments
of the MFJ kept as far as possible from conductive objects including
yourself. You'll have to adjust it, let go, back off and read the meter,
readjust, etc. And then it'll still be a bit off unless the length of
the MFJ meter is quite short relative to a wavelength. You'll also have
to keep the line well away from any conductors and avoid coiling it. Of
course, the same problems will exist when you install the line in
whatever system it'll be used for, unless you can get it very well balanced.

It'll be a good exercise in learning some basic measurement techniques.
Whether your results are adequately accurate depends on the application
you'll be using the line for.

I sometimes taught a class on TDR techniques, and I'd start by
connecting a foot or so of two-conductor ribbon cable -- just soldered
into and to the shell of an SMA connector -- to a high speed TDR. The
trace would show the large reflection from the open end, of course, but
a smaller reflection seemingly coming from a point about 1/4 of the way
from the end. I explained that ribbon cable isn't controlled for
impedance, so it obviously had a construction anomaly at that point, and
pinched the line, running my fingers along until the reflection from the
fingers was at the same point as the anomaly. Then I cut the line well
toward the TDR unit, discarding the portion with the anomaly. When the
audience saw the *new* reflection about 1/4 of the way from the end of
the shorter wire, I had their attention. And thus began a discussion of
differential and common mode waves.

Roy Lewallen, W7EL