Velocity Factor: Measuring using the VNA Method (00e4)

How to measure the velocity factor of a piece of coax using a VNA.
One viewer rightly pointed out that I misspoke at 19:30. The value of 236.129" is equal to 3/4 wavelength of 37.4886305 MHz, not 1/4. Anyway the velocity factor result is right.
Thanks to my viewer for pointing that out.

[@ka2pbo]

Fantastic video! You are a great teacher. I find it real easy to follow along in all your videos. I am starting a notebook dedicated to the lessons in your videos. I just finished writing up the "FM Deviation- Bessel Null Method". Really enjoyed that one.I know I could give these notes to folks who haven't seen the videos and they would be able to follow right along. Thanks for doing this! 73 Rick KA2PBO

[@eie_for_you]

Thanks, man! I am so glad that I can produce videos that help dispel the mist. Great idea on the notes, too. :-)

[@toddanonymous5295]

Excellent video Ralph. Theory with practical examples . A+

[@eie_for_you]

Thank you, Todd!

[@rentacowisgoogle]

Thanks! This was really easy to follow along with on my nanoVNA, which I'm just learning to use. Such a cool, inexpensive device! Some coax I tested gave a Vf of around 61%, but I didn't take time to measure the length exactly. Just following the steps :)

[@eie_for_you]

Thank you for the encouragement. I am very happy that it was helpful to you.

[@twkolejofil]

Actually, the VNA connects graphically two points near ±180° because it misses the exact opposite phase. So, technically, it is not 0° phase there. There is ±180°, just like longitude in the middle of Pacific Ocean does not jump to Greenwich 😉

[@eie_for_you]

Technically correct. As you rightly said, the VNA gives us samples on either side of this perfect frequency where the swap occurs. In order to picture this whole thing, we *imagine* that there is a point between where the VNA gives us a point where the phase is *nearly* 180 degrees and where it is *nearly* -180 degrees where it is zero (obviously a fictitious point whose sole purpose is to help us see a point between the aforementioned points). So you are right and my rather loose description could have been better worded. 🙂

[@andrewdeweerd461]

Nice job. I think I'll have a look at your other videos.

[@eie_for_you]

Thanks! I am glad you found the video helpful! 🙂

[@HiusLighting]

Thank you Ralph! ;)

[@eie_for_you]

You are very welcome! 🙂

[@mikebloodworth9258]

Hi, great instructional video. My PC software for the nanoVNA only does Smith charts. What is yours and where do I find it. . Note my NanoVNA is the orig V1. Cheers

[@eie_for_you]

Well, Mike, I use two different programs. Both can be found here: nanorfe.com/nanovna-v2-software.html The "VNA View" program is somewhat basic and always connects well with my nanoVNA. I really like the VNA Saver program in many ways. It sometimes has an issue with connecting to my nanoVNA. If it does, then I connect with "VNA View," then close VNA View and open VNA Saver and then it connects just fine. Always use the "Calibration Assistant" for best results. Hope this helps.

[@mikebloodworth9258]

​@@eie_for_you Hi, NanoVNA-QT finds my device port ok but the hangs. I've read it's only compatible with V2 NanoVna and mine is the orig V1 running 0.1.1 firmware. Any firmware above this is for the V2. I am running NanoVNA Saver v 0.5.4 latest from github. I've just discovered i can add the phase via Display setup on VNA Saver so all good now. Thanks for your help. Cheers Mike

[@eie_for_you]

@@mikebloodworth9258 Glad I could help! That's what this channel is all about. 🙂

[@ok4rm]

How about measuring Z of the coax? Or whatever unbalanced or balanced feedline, for that matter?

[@eie_for_you]

I have done this. I used the Time Domain Reflectometry method (see this on measuring velocity factor using the TDR method: kzbin.info/www/bejne/iIPFkJyAjs-ba80). The difference is that I terminate the coax with a very small, *non-inductive*, multi-turn potentiometer. I adjust the potentiometer until the reflected pulse disappears. Remove the potentiometer, measure its value and this will be the impedance of the coax. You are right, though, this would be a great topic for a video.😁

[@eie_for_you]

I've added that to my list of videos to do. Thank you for the suggestion! 🙂

[@ok4rm]

@@eie_for_you That sounds like a great idea. Measuring Z of a non-HF line, such as two twisted wires, should theoretically depend on wire diameter and insulation thickness, but measuring true value would be very interesting.

[@eie_for_you]

@@ok4rm I learned that when I worked for a medical company that was having a cable manufacturer create some custom data cable for installation of their product. We had to determine if the data cable (shielded twisted pair) was the right impedance, per specification. So, being that I was the entire engineering department for the company (no pressure there) I had to discover how to do this. It worked quite slick. We eventually got cable that was the right impedance and worked well over 100 meters. NICE!

[@andreaboarino]

Good job Ralph. Thanks for sharing. I've studied it all. Incidentally, if I'm not wrong, at 19:30 the value of 236.129" is equal to 3/4 wavelenght of 37.4886305 MHz, not 1/4. Anyway the velocity factor result is right.

[@eie_for_you]

Oh my ... an oopsie! Thank you for pointing that out! You are, indeed, right!

[@eie_for_you]

By the way, I added this note to the description of the video!