[TowerTalk] Understanding Coax Loss Measurements

[Kirk Kleinschmidt]

Okay, gang,
Thanks for the advice. I will download the AC6LA info and see if I can improve my game. :)
This is a learning experience for me, as I haven't gotten into the nitty gritty of feed line measurements before, and I need to become more familiar with my RF tools and procedures.
My older MiniVNA -- at least with the Blue VNA software I use with it -- doesn't seem to have the specific calibration procedures that other devices have. It calibrates "short" on the transmission mode and "open" in the S11 reflection mode. but I'm not finding a 50-ohm calibration setting for the moment.
I also have a Rigol spectrum analyzer, but my expertise in using it doesn't get me very far for the moment. It may offer 50/75 ohm calibration. Will have to check.
Part of this process also involves learning how to prep and install various coaxial connectors (I was behind the times), so I have been testing compression, clamp, screw, and crimp connectors, for which I now have suitable tools.
Interesting: lengths of RG-11 measure exactly the same with three different connector styles. That's what I was hoping for (and expecting).
I have 2,000 feet of RG-11 quad shield on hand, so I'd like to use it while actually knowing whether it's working as expected. It seems to be.
Now, to the back yard before it starts snowing for real! I need 102 feet to reach my inverted-L and 88 feet to reach the tower box for my Death Ray Hexagonal Beam. It's mounted on the cutest 22-foot tilt-over tower you've ever seen, but it will have to do until spring when it will move to a 50-foot tower.
By then I hope to have the VNA/coax sweeping techniques down pat.
And for one commenter: I don't have the live sweep in front of me, but as I recall there were 5 or 6 "sine wave" peaks in the 1-35 MHz VNA sweep, so perhaps your comments about specific reflections explain "the wiggles" when sweeping 75-ohm coax.
Regards,
Kirk, NT0Z



My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com and on the Amazon Kindle (soon) 

    On Saturday, November 28, 2020, 1:08:38 PM CST, Jim Brown <jim at audiosystemsgroup.com> wrote:  
 
 On 11/28/2020 3:43 AM, Kirk Kleinschmidt via TowerTalk wrote:
> I'm trying to characterize some coaxial cables and various connectors, and the results from my two measuring methods aren't reconciling well. My understanding of the tools is also less than complete.

First, make sure that your VNA is well calibrated. For cailbration, you 
will need standard Open, Short, and 50 ohm parts.

Then, use it to make S11 measurements over the frequency range of 
interest. I'd go with 1-50 MHz. Then download a copy of AC6LA's ZPlots 
Excel spreadsheet, export your data to it, and follow his instructions 
for characterizing the cable.

This will give good results for coax and two wire line of any impedance.

73, Jim K9YC
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