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

Bob Albert bob91343 at yahoo.com
Mon Aug 8 15:18:02 EDT 2016

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.

    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 - AE6RV -----------------------------------------------------------------

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