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

Tom Miller tmiller11147 at verizon.net
Mon Aug 8 16:31:44 EDT 2016

If you have a function generator with a sync output, you can use that pulse 
into a T connector with your scope to do a TDR measurement of the cable. If 
you have a scrap of the new cable, you can use that to calibrate the setup.


----- Original Message ----- 
From: "Bob Stewart" <bob at evoria.net>
To: "Discussion of Precise Time and Frequency Measurement" 
<time-nuts at febo.com>
Sent: Monday, August 08, 2016 2:18 PM
Subject: [time-nuts] Safely getting the electrical length of a connected 
antenna feedline

> 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
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