#326: How to Measure the impedance of 'unknown' coax using a NanoVNA

This short tutorial video shows how to use the NanoVNA to measure the impedance of an unmarked piece of coax transmission line. It uses the unique impedance transformation that occurs with a quarter-wavelength transmission line. By measuring the resistive component of the line at a frequency at which the line is a quarter-wavelength long, the unknown line impedance is simply the square root of the product of the termination impedance and the measured value.
See this playlist of additional tutorials using the NanoVNA:
Notes from this video can be found here:

[barrykery]

I used my Rig Expert AA-600 to check feed lines to find out its impedance. I did not use the Smith Chart but still found the correct impedance. The way you just showed is so much easier and spot on. No hunting needed. Now I use my NanoVNA H 4. Super simple. Thanks for posting such a great informative video.
Barry, KU3X

[Testequip]

Some folks have a wonderful knack of conveying information. Alan is certainly an expert in the field of teaching difficult concepts. And to think he does this as a labor of love. Tektronix certainly has one heck of an employee working for them!!!
PS: I would love to see SignalPath, eevblog, and Alan team up for a live stream fireside chat.

[CH_Pechiar]

Alan, thank you for this followup video. It si mindblowingly (I hope I spelled that correctly) simple!

[johnwest]

Thanks. Your clear, concise videos save both my time and sanity.

[DonDegidio]

Hi Alan,
Didn't know it was that easy to determine the impedance using the NanoVNA. Stay safe. 73 WJ3U

[goddlediddles]

I've just done it with a short test length of 75 Ω coax. It works perfectly. But now I need to measure the Z₀ of a short length of twin flat-line. If I hook it up to two connectors, will it also work?
Update: I'm able to answer the question myself. Yes, it does work, and quite well, too. The only conditions for an accurate measurement, though, is to lay the short piece of twin flat-line out, away from metal objects, and also make sure it is not exposed to electric or magnetic fields. I measured the Z₀ of a 1.5 Metre long piece of common twin bell wire. BTW, it came out at 110 Ω.

[Yosemite_Sam]

😅Very well explained and to the point. Brevity is good coupled with the equations to work through ourselves. For those who are a bit lost redoing the exercise in person is a revelation.

[sailaab]

really educational and something even i (a non technical person) could undeestand or apply in some way.
.
Thank you so much, sir!

[Luappy]

Always learn something (usually many things actually) from your videos!

[tocsals]

Happy New Year, Alan! This should help people who don't have access to an O-scope, but have a VNA handy.

[gbennett]

I measured my Tektronix BNC 50 ohm cable with my NanoVNA and the Smith chart displayed 50 ohms across the entire frequency range. Another cable which I knew was bad showed a wide impedance range across the frequency range and never got remotely close to 50 ohms. Thanks for the info.

[arthurconstantin]

Your videos are great. You are concise and knowledgeable. Thank you sir.

[johnathanasiou]

Great video & very useful for us hams!.
Wishing you a very happy new year for you, family & loved ones including health & happiness

[AlanGarfield]

Ha! You're a few weeks too late. I had to figure that out myself when I found a bunch of unknown coax. I actually used your poor mans TDR with the 9V battery method to get the length of the coax. And use the NanoVNA similarly to work it out. Great video!

[IKXOO--Paolo]

Simply a great teacher !!! TNX & 73s.

[miketoreno]

Great video Alan. Thank you for your work.

[hmsilvaantonio]

Thanks Alan, this video was most usefull to me, as many others you created. :)

[kardeef]

Thank you for all your videos. Though I understand about half, that half gets larger every video, helping me understand previous videos and helping me be able to wrap my head around things I have read. Know your making a difference. I used to be a ham ke4wjz. Since being disabled about 20 years ago . Right before my divorce 15 years ago my ex either sold or destroyed my 2 meter radio. I have rediscovered electronics and enjoy learning about RF after my divorce. Friends and family give anything electronic headed for a landfill. If I can't fix it I scavenge most the discrete parts and IC's from. This hobbie has pulled me from a dark place and has put alittle purpose in my life. Thanks again for sharing your knowledge and experience!

[AmatorElektronik]

I needed that. Thank you very much. Great video.

[gfvt]

Another excellent presentation, well done. I must admit I was thinking along the lines of the TDR and a long cable terminated with a 100 ohm potentiometer. Maybe that would be an interesting experiment? Great to see the NanoVNA being used for more than SWR plots, thank you for your great videos.