On Tue, 06 Apr 2010 00:16:56 GMT, Owen Duffy wrote:

Bob wrote in
:

...

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.

As Roy has explained, you need to stop common mode current from
significantly altering your measurement.

I have had sucess with placing a balun of a string of ferrite cores over
the line. It is easy to observe the effectiveness using a VNA sweep, a
bit tricker with the MFJ269.

I do have a W2DU-style balun of ferrite beads on coax, if that is what
you mean.

I also have an MFJ gizmo, a tiny 1:1 current balun for antenna
analyzers, a coax fitting on one side, and balanced line fasteners on
the other side -- but I'm guessing then I'd be measuring the velocity
factor of the balun, in addition to the balanced line.

Bob
k5qwg


I have also found that stretching the line out straight causes the worst
common mode problems, but if you coil it, you have to keep adjacent turns
much further apart than the line's conductor separation.

All this has to be done with the line suspended in the air, well clear of
other dielectrics or conductors. (Hint: fishing line can be your friend!)

Before these analysers, we measured the resonant frequency of a line
section using a GDO. By very loosely coupling the GDO, and reading the
GDO frequency from a calibrated receiver, good results could be obtained.

Owen