First, the nulls are sharper than the peaks. What you do is measure the
frequency of adjacent nulls and calculate what frequency makes both of those a
half wave or multiple thereof. Then you know the number of half waves to the
mismatch. It's the distance from the tee to the remote end.
It takes a little thought, or you can read the article which presents some
formulas to make for less confusion. I guess. The article focuses on shorted
or open lines but you can get readings from nearly any line, as long as it's
not a flat line. Of course, the closer you are to a good match, the harder it
is to make the measurement, which is why I suggest finding a frequency range
far from the design of the termination equipment.
Your TDR should work also.
Bob
On Monday, August 8, 2016 1:23 PM, Bob Albert via time-nuts
<[email protected]> wrote:
I host a group called something like HF Antennas. There I posted a link to an
article on how to measure coaxial cable. The easiest way is with a spectrum
analyzer and a tracking generator.
You connect the generator to the analyzer through a Tee that goes to the
unknown coax. You will see a group of peaks and nulls over the spectrum. The
spacing is a half wave of the cable. The match needs to not be good to see the
nulls best, and you will need to know the propagation constant of the cable.
Chances are, the match won't be good over the entire range so you are okay with
that. Propagation constant of most coaxial cable runs around 66%.
You can also use a TDR setup but you'll have to make one, with a pulser and a
'scope. I downloaded a circuit for a pulser that uses one IC. I have the
parts but haven't built it yet, as I am stalled by the problem of connecting to
a 14 pin SMD part. The IC uses one part as an oscillator and the other 5 in
parallel to drive 50 Ohms. Again, you use a Tee and measure the time for a
reflection, bearing in mind that the trip is two ways over the same cable and
the time shown will be double the time for the calculation.
Bob
On Monday, August 8, 2016 12:00 PM, Bob Stewart <[email protected]> wrote:
Earlier this year, with some help, I pulled the dish off of an old DishTV
antenna on the roof and put a 5V bullet antenna on the mast. I also pulled a
new cable through by attaching it to the old one. The problem is that I was
not able to measure the new cable. So, the question is, without going back on
the roof in this heat, how can I measure the electrical length of the line I
pulled?
I was thinking of using my 8640B signal generator and sending some RF back up
the line to get a quarter wavelength at the null. But that assumes a lot,
including that the other end is open at 3MHz, or whatever the frequency works
out to be, as well as that the high voltage on the antenna end won't be high
enough to blow the LNA.
So, how much RF I can safely send up the line? I've got an 8558B spectrum
analyzer, but it's not on the bench, and it would be easier to use my scope,
which sadly is a 70s vintage Tek 455. Do I put this all together with a lead
from the generator to a tee at the measuring device and tune for a null? My
experience at getting precise measurements on anything longer than a few inches
is effectively none, but I'd guess that I want less than 0.5V at the LNA during
this test. Oh, and I do have an 8444A tracking generator that can output -10
dbm as well as a 10 db attenuator within easy access. That could get a quick
spot on the null point.
Most importantly, of course is the question of whether this will even work.
Bob - AE6RV -----------------------------------------------------------------
AE6RV.com
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