# [time-nuts] Safely getting the electrical length of a connected antenna feedline

Didier Juges shalimr9 at gmail.com
Thu Aug 11 14:20:53 EDT 2016

```I used the PPS from a Thunderbolt (fast rise lime, low rep frequency, was handy) and a digital storage scope and a couple of resistors to make a reflectometer based on this experiment:

www.ko4bb.com/getsimple/index.php?I'd=coax-cable-impedance-matching

You can very clearly see a 50 ohm/75 ohm mismatch.

The biggest variable will be the velocity factor.

Didier KO4BB

On August 8, 2016 2:18:02 PM CDT, Bob Albert via time-nuts <time-nuts at febo.com> 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 <bob at evoria.net> 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 -
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