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

[Bob Camp]

Hi

An even more significant question:

Is it worth doing? 

More or less:

Do you know the delay numbers for your antenna?

Do you know the delay numbers for your GPS module? 

How close can you *guess* the length of the cable?

Knowing absolutely nothing at all about your setup, I’ll guess the cable is 50 feet long. Maybe I’m off by 20 or 30 feet. 
Call that +/-40 to 60 ns. I’d hope you can guess closer than that. Your antenna and module could easily have delays 
in the 40 ns range. It has no impact on a “frequency” GPSDO. It is one of a number of static offsets in a time transfer system. 

Even the NIST level outfits seem to have issues coming up with a purely mathematical answer to “what is the offset”.  
The standard answer is to bring in a calibrated receiver and see how it all measures out. 

None of that is to say you should *not* work out the line length. It’s just to say that there is only so much value to the 
measurement. 

Bob

> On Aug 8, 2016, at 2:18 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
> 
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