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

[Didier Juges]

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 <tvb at 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" <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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