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

Bob Stewart bob at evoria.net
Mon Aug 8 16:52:07 EDT 2016

Hi Bob,
I believe I know the delay for the antenna and Ublox specifies the delay in the receiver.  I can't guess within 20ft of the physical length of the antenna.  But, this is time-nuts, so yeah I want to be able to work this out.  And besides...

I figured out how to output an accurate 1PPS with my unit.  So, it's no longer just a frequency standard.  Briefly: I take advantage of the fact that the sawtooth on the Ublox units is less 22ns wide (about +/- 10.5ns according to my data).  I send the 1PPS from the receiver through a 30ns delay line to trigger a d-flop in a 7474, which arms the other d-flop in the 7474.  The 30ns delay ensures that the 1PPS output of the Ublox, including the sawtooth, is always within the dead time between the pulses from the OCXO.  So, the next 10MHz pulse triggers the other d-flop which goes through a 125 gate to the 1PPS output.  1/16th of a second later (a convenient time for me), the PIC resets the two d-flops and so it goes.  So, yeah, that's 100ns late, but the Ublox has a userDelay field, which I set to 100ns.  That moves the PPS back to the proper time.  Yes, there is a delay of the 10MHz through the d-flop and the gate, but that's knowable.  And there's about a 5ns difference between the 1PPS output and the 10MHz output.  So, given that the delays are all knowable, I think it's usable as a timing unit now; given the limitations of a GPSDO for that purpose.  

Any discussion about this method of getting a 1PPS should probably be in a new thread.  I'd be happy to link to a schematic if wanted.



      From: Bob Camp <kb8tq at n1k.org>
 To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Monday, August 8, 2016 3:14 PM
 Subject: Re: [time-nuts] Safely getting the electrical length of a connected antenna feedline

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 


> 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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