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@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 <b...@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 >_______________________________________________ >time-nuts mailing list -- time-nuts@febo.com >To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. > > > >_______________________________________________ >time-nuts mailing list -- time-nuts@febo.com >To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. -- Sent from my Moto-X wireless tracker while I do other things. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
