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

Bob wrote in
:

On Tue, 06 Apr 2010 00:16:56 GMT, Owen Duffy wrote:
....
I have had sucess with placing a balun of a string of ferrite cores
over the line.

That means literally threading some suitable ferrite toroidal cores over
the transmission line you are measuring.

If you add a separate balun between the analyser and the cable under test,
you introduce an unknown component that will probably disturb your
readings.

Owen

On Tue, 06 Apr 2010 01:26:16 GMT, Owen Duffy wrote:

Bob wrote in
:

On Tue, 06 Apr 2010 00:16:56 GMT, Owen Duffy wrote:
...
I have had sucess with placing a balun of a string of ferrite cores
over the line.

That means literally threading some suitable ferrite toroidal cores over
the transmission line you are measuring.

If you add a separate balun between the analyser and the cable under test,
you introduce an unknown component that will probably disturb your
readings.

Owen

Another question -- I'm thinking of cutting a 10-foot section of
balanced line to test. Should I count the bared pigtails of the line,
which I will attach to the analyzer's coax output, as part of the 10
foot length? Or just count that part of the line where all insulation
is in place?

Bob
k5qwg

Bob wrote:
On Tue, 06 Apr 2010 01:26:16 GMT, Owen Duffy wrote:

Bob wrote in
:

On Tue, 06 Apr 2010 00:16:56 GMT, Owen Duffy wrote:
...
I have had sucess with placing a balun of a string of ferrite cores
over the line.
That means literally threading some suitable ferrite toroidal cores over
the transmission line you are measuring.

If you add a separate balun between the analyser and the cable under test,
you introduce an unknown component that will probably disturb your
readings.

Owen

Another question -- I'm thinking of cutting a 10-foot section of
balanced line to test. Should I count the bared pigtails of the line,
which I will attach to the analyzer's coax output, as part of the 10
foot length? Or just count that part of the line where all insulation
is in place?


Aha.. you start to see the problems in precision RF measurement... Where
is the "reference plane"..and how do you calibrate out the "fixture".

One way to do it is to do two sets of measurements. Do one with your 10
foot length. Then, cut 5 feet off and do it again. Then, the
"difference" between the measurements is the result for the now missing
5 feet.

How much precision are you looking for, anyway. To a first order, think
about how long that fixture is. If it's an inch or so, that's less than
1% of the overall length of the line.

Bob wrote:

Another question -- I'm thinking of cutting a 10-foot section of
balanced line to test. Should I count the bared pigtails of the line,
which I will attach to the analyzer's coax output, as part of the 10
foot length? Or just count that part of the line where all insulation
is in place?

Bob
k5qwg

I think 10 feet is going to be too short to make a good measurement,
because the lengths of such things as the pigtails and the MFJ are a
substantial fraction of the overall length. I recommend using the whole
length of line you have. You might have to be a bit creative in keeping
it away from other conductors, but that'll give you the best results.

When you do make the measurement, maintain the integrity of the line to
as close to the impedance meter as you can. Then measure the line to the
impedance meter connector.

Roy Lewallen, W7EL

On Tue, 06 Apr 2010 10:11:20 -0700, Roy Lewallen
wrote:

Bob wrote:

Another question -- I'm thinking of cutting a 10-foot section of
balanced line to test. Should I count the bared pigtails of the line,
which I will attach to the analyzer's coax output, as part of the 10
foot length? Or just count that part of the line where all insulation
is in place?

Bob
k5qwg

I think 10 feet is going to be too short to make a good measurement,
because the lengths of such things as the pigtails and the MFJ are a
substantial fraction of the overall length. I recommend using the whole
length of line you have. You might have to be a bit creative in keeping
it away from other conductors, but that'll give you the best results.

When you do make the measurement, maintain the integrity of the line to
as close to the impedance meter as you can. Then measure the line to the
impedance meter connector.

Roy Lewallen, W7EL

I have 53-foot- and 122-foot-long lengths of the line. I might stretch
the 53-footer from the roof out toward the back fench, and measure
that.

Bob
k5qwg

Bob wrote in
:

....
Another question -- I'm thinking of cutting a 10-foot section of
balanced line to test. Should I count the bared pigtails of the line,
which I will attach to the analyzer's coax output, as part of the 10
foot length? Or just count that part of the line where all insulation
is in place?

What you have is two transmission line sections in cascade, one with bare
conductors, and one with the conductors immersed in insulation.

If you want to measure the effects only of the latter, you need to find
some way of minimising the contribution of the former.

The calibration of the MFJ269 is not that flash that you will pick a mm
or two. When I have used them for the test you are performing, I zip tie
the conductor to the external threads of the connector so that there is
as close to zero length of 'different' transmission line as possible. You
could also use a small stainless hose clamp, but in my experience, the
zip tie has been reliable.

You can zip tie a piece of PE irrigation pipe to the VFO knob so that you
hand doesn't need to be within half a meter of the instrument, use a
wooden table to support the instrument, use the balun I suggested, and
arrange the line to minimise radiation from residual common mode current.

I would try to measure a length of 10m or so. It is a compromise between
making end effects (tails, effect of the windows) insignificant, an
effective balun, and physically supporting the line for least radiation
and other external influences.

Some of my focus was on trying to get a valid measure of R as well as X,
R due to line losses alone.

Owen