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

Tom Van Baak tvb at LeapSecond.com
Thu Aug 11 16:19:04 EDT 2016


FYI: In Didier's post below, the correct URL is:

    http://www.ko4bb.com/getsimple/index.php?id=coax-cable-impedance-matching

I'm guessing a mobile spell checker changed his " id= " to " I'd= " (even though it was part of a URL).

/tvb

----- Original Message ----- 
From: "Didier Juges" <shalimr9 at gmail.com>
To: "Bob Albert" <bob91343 at yahoo.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Thursday, August 11, 2016 11:20 AM
Subject: Re: [time-nuts] Safely getting the electrical length of a connectedantenna feedline


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 -
>AE6RV -----------------------------------------------------------------
>AE6RV.com
>
>GFS GPSDO list:
>groups.yahoo.com/neo/groups/GFS-GPSDOs/info
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-- 
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