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

Bob Stewart bob at evoria.net
Mon Aug 8 16:23:02 EDT 2016

Hi Bob,
Correct me if I'm wrong, but for timing purposes, I think that what I want is the electrical length in nanoseconds, not the physical length.  I hadn't considered the fact that the distance between peaks would give me a halfwave.  But, that's a halfwave of what, exactly?  Is it the distance from the far end to the tee, or to the generator or to the measurement device (SA/scope/meter)?  And since nanoseconds are important, is it actually to some point within the measuring device?  Or, since it's only nanoseconds, maybe the last few inches inside the scope aren't that important?  It's starting to sound like what I really want to do the final measurement with is my old Voltohmyst with the RF probe at the tee if I really want to be accurate.  That would be using the 8640 and tuning for either null or peak, whichever is sharper.

As far as a TDR is concerned, I think that the 1PPS output from my GPSDO should be sufficient.  In this case, I think I'd send the 1PPS to a tee at the start input on my 5370 and lock start and stop together with the phase switches opposite for start and stop.  For this test, I could actually control the length of time the 1PPS is high so that it doesn't spoil the measurement.  But, once again, will I see any sort of spike on the other end with a 3.3V 1PPS pulse?



      From: Bob Albert <bob91343 at yahoo.com>
 To: Bob Stewart <bob at evoria.net>; Discussion of Precise Time and Frequency Measurement <time-nuts at febo.com>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Monday, August 8, 2016 2:18 PM
 Subject: Re: [time-nuts] Safely getting the electrical length of a connected antenna feedline
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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