Thank you Tom and yes, the spell checker tends to be overbearing at times Didier
On August 11, 2016 3:19:04 PM CDT, Tom Van Baak <t...@leapsecond.com> wrote: >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" <shali...@gmail.com> >To: "Bob Albert" <bob91...@yahoo.com>; "Discussion of precise time and >frequency measurement" <time-nuts@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@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. >_______________________________________________ >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.
